Furthermore, studies have highlighted a link between tropomyosin isoform expression and the acquisition of metastatic properties. A study compared isoform expression between a low- and highly-metastatic Lewis lung carcinoma cell line. [ 36 ] [ 37 ]  The study found that as cells become more metastatic, there is a marked decrease in the expression of HMW tropomyosin 2 protein and mRNA levels.


These results have been confirmed in primary tumors and human models. Studies in colon and bladder cancer found increased expression of the LMW tropomyosin  Tropomyosin 5NM1 . [ 38 ] [ 39 ]  The elevated expression of this isoform has also been seen in transformed rat fibroblasts, and it is thought that this isoform is required for the motility of highly metastatic melanoma. [ 40 ]  In addition, elevated expression of Tropomyosin 4 has been linked with lymph node metastasis in breast cancer.


All of these studies suggest that changes in the expression and complement of tropomyosin isoforms are integral to cancer and cancer progression. The consensus is that, in general, cancer cells become more reliant on LMW tropomyosins as HMW tropomyosins disappear with increasing malignancy. [ 10 ]  This discovery has led to the development of novel anti-tropomyosin compounds as potential anti-cancer agents.


Tropomyosins have been implicated in the autoimmune disease  ulcerative colitis , a disease of the colon that is characterised by ulcers or open sores. The link between this disease and tropomyosin was first acknowledged in a study that found blood serum taken from 95% of patients with ulcerative colitis contained antibodies that reacted positively to tropomyosin. [ 41 ]  Additional studies have confirmed these results, but also identify Tropomyosin 5 and Tropomyosin 1 as the primary tropomyosins involved in the pathogenesis of ulcerative colitis. [ 42 ] [ 43 ]  Tropomyosin 5 has been related to the development of pouchitis in the ileal pouch following surgery for ulcerative colitis. The elevated number of IgG-producing cells in the colonic mucosa of ulcerative colitis patients is largely committed to producing IgG against Tropomyosin  5-related epitopes. Tropomyosin 5 is, therefore, capable of inducing a significant T-cell response. [ 44 ]   A physicochemical analysis of common structural motifs present in 109 human autoantigens revealed that tropomyosins have the highest number of such motifs, and thus a very high propensity to act as autoantigens. [ 45 ]


In addition to the role tropomyosins play in ulcerative colitis, tropomyosin antibodies have also been reported in acute  rheumatic fever [ 46 ]
and the inflammatory disorder  Behcet's syndrome . [ 47 ]  In both instances, it is unclear as to whether these antibodies play a direct role in the pathogenesis of these human conditions or reflect the high antigenicity of tropomyosins released from compromised cells.


Nemaline myopathy is a muscle disease that is characterised by the presence of electron-dense rod bodies in skeletal muscle fibers. These electron-dense rod bodies are composed mainly of α-actinin and actin. The disorder is often clinically categorized into several groups, including mild (typical), intermediate, severe, and adult-onset; however, these distinctions are somewhat ambiguous, as the categories frequently overlap. Causative mutations have been detected in skeletal α-actinin, tropomyosin,  nebulin , and troponin. Within humans, mutations in both the γ-Tropomyosin and β-Tropomyosin genes have been identified. No mutations in the α-Tropomyosin gene have been identified in this condition for humans.


Tropomyosin is the primary protein responsible for  shellfish allergies , including those to  crustaceans  and  molluscs . [ 48 ] [ 49 ] [ 50 ] [ 51 ]


Tropomyosin also causes some cases of  cockroach  allergy. [ 52 ]


Given the vast array of processes that this protein has been reported to be involved with, there is great interest in tropomyosin isoforms within the scientific community.


One way in which specific isoforms of this protein can be studied in detail is through the use of antibodies. These specific antibodies can be used in protein-blotting experiments and applied to cells or tissue sections and observed under a microscope. This allows researchers not only to determine the level or concentration of an isoform or a group of isoforms, but to also identify the cellular location of a particular isoform and associations with other cellular structures or proteins.


At the present time, there are many commercially available antibodies; however, many of these antibodies are sold with minimal information regarding the antigen used to raise the antibody and therefore, the isoform specificity; as such, some research groups develop their own antibodies. Before these antibodies can be used, they must be extensively characterised, a process whereby the specificity of the antibody is examined to ensure that the antibody does not cross-react with other tropomyosins or other proteins.


Troponin , or the  troponin complex , is a complex of three  regulatory proteins  ( troponin C ,  troponin I , and  troponin T ) that are integral to  muscle contraction [ 2 ]  in  skeletal muscle  and  cardiac muscle , but not  smooth muscle . Measurements of cardiac-specific troponins I and T are extensively used as diagnostic and prognostic indicators in the management of  myocarditis ,  myocardial infarction  and  acute coronary syndrome . [ 3 ]  Blood troponin levels may be used as a  diagnostic marker  for  stroke  or other myocardial injury that is ongoing, although the sensitivity of this measurement is low. [ 4 ] [ 5 ] [ 6 ]


Troponin is attached to the protein  tropomyosin  and lies within the groove between  actin  filaments in muscle tissue. In a relaxed muscle, tropomyosin blocks the attachment site for the  myosin   crossbridge , thus preventing contraction. When the muscle cell is stimulated to contract by an  action potential ,  calcium channels  open in the sarcoplasmic membrane and release calcium into the sarcoplasm. Some of this calcium attaches to troponin, which causes it to change shape, exposing binding sites for myosin (active sites) on the  actin   filaments .  Myosin 's binding to actin causes  crossbridge  formation, and contraction of the muscle begins. [ 7 ]


Troponin is found in both  skeletal muscle  and  cardiac muscle , but the specific versions of troponin differ between types of muscle. The main difference is that the TnC subunit of troponin in skeletal muscle has four calcium ion-binding sites, whereas in cardiac muscle there are only three. The actual amount of calcium that binds to troponin has not been definitively established. [ 8 ]


In both cardiac and skeletal muscles, muscular force production is controlled primarily by changes in intracellular  calcium  concentration. In general, when calcium rises, the muscles contract and, when calcium falls, the muscles relax. [ citation needed ]


Troponin is a component of thin filaments (along with  actin  and  tropomyosin ), and is the protein complex to which calcium binds to trigger the production of muscular force. Troponin has three subunits, TnC, TnI, and TnT, each playing a role in force regulation. [ citation needed ] . Under resting intracellular levels of calcium, tropomyosin covers the active actin sites to which  myosin  (a molecular motor organized in muscle thick filaments) binds in order to generate force. When calcium becomes bound to specific sites in the N-domain of TnC, a series of protein structural changes occurs, [ citation needed ]  such that  tropomyosin  is rolled away from myosin-binding sites on actin, allowing myosin to attach to the thin filament and produce force and shorten the  sarcomere . [ citation needed ]


Individual subunits serve different functions: [ citation needed ]


Smooth muscle does not have troponin. [ 9 ]


TnT is a tropomyosin-binding subunit which regulates the interaction of troponin complex with thin filaments; TnI inhibits ATP-ase activity of acto-myosin; TnC is a Ca 2+ -binding subunit, playing the main role in Ca 2+  dependent regulation of muscle contraction. [ 10 ]


TnT and TnI in cardiac muscle are presented by forms different from those in skeletal muscles. Two isoforms of TnI and two isoforms of TnT are expressed in human skeletal muscle tissue (skTnI and skTnT). Only one tissue-specific isoform of TnI is described for cardiac muscle tissue (cTnI), whereas the existence of several cardiac specific isoforms of TnT (cTnT) are described in the literature. No cardiac specific isoforms are known for human TnC. TnC in human cardiac muscle tissue is presented by an isoform typical for slow skeletal muscle. Another form of TnC, the fast skeletal TnC isoform, is more typical for fast skeletal muscles. [ 11 ]  cTnI is expressed only in myocardium. No examples of cTnI expression in healthy or injured skeletal muscle or in other tissue types are known. cTnT is probably less cardiac specific. The expression of cTnT in skeletal tissue of patients with chronic skeletal muscle injuries has been described. [ 12 ]


Inside the cardiac troponin complex the strongest interaction between molecules has been demonstrated for cTnI – TnC binary complex especially in the presence of Ca 2+  ( KA = 1.5  ×  10 −8  M −1 ). [ 13 ]  TnC, forming a complex with cTnI, changes the conformation of cTnI molecule and shields part of its surface. According to the latest data cTnI is released in the blood stream of the patient in the form of binary complex with TnC or ternary complex with cTnT and TnC. [ 14 ]  cTnI-TnC complex formation plays an important positive role in improving the stability of cTnI molecule. cTnI, which is extremely unstable in its free form, demonstrates significantly better stability in complex with TnC or in ternary cTnI-cTnT-TnC complex. It has been demonstrated that stability of cTnI in native complex is significantly better than stability of the purified form of the protein or the stability of cTnI in artificial troponin complexes combined from purified proteins. [ citation needed ]


Subtypes of troponin (cardiac  I  and  T ) are  sensitive and specific   indicators  of  heart  muscle damage ( myocardium ). They are measured in the  blood  to differentiate between unstable  angina  and  myocardial infarction  (heart attack) in people with  chest pain  or  acute coronary syndrome . A person who recently had a myocardial infarction has areas of damaged heart muscle and elevated cardiac troponin levels in the blood. [ 15 ]  This can also occur in people with  coronary vasospasm , a type of myocardial infarction involving severe constriction of the cardiac blood vessels. After a myocardial infarction troponins may remain high for up to 2 weeks. [ 16 ]


Cardiac troponins are a marker of all heart muscle damage, not just myocardial infarction, which is the most severe form of heart disorder. However, diagnostic criteria for raised troponin indicating myocardial infarction is currently set by the WHO at a threshold of 2 μg/L or higher. Critical levels of other cardiac biomarkers are also relevant, such as  creatine kinase . [ 17 ]  Other conditions that directly or indirectly lead to heart muscle damage and death can also increase troponin levels, such as  kidney failure . [ 18 ] [ 19 ]  Severe  tachycardia  (for example due to  supraventricular tachycardia ) in an individual with normal coronary arteries can also lead to increased troponins for example, it is presumed due to increased oxygen demand and inadequate supply to the heart muscle. [ citation needed ]


Coronary artery  stent  placement can also cause immediate post-procedure elevated serum troponin levels. This can be problematic in a clinical setting as troponin values and guidance protocols become subject to more thoughtful interpretation. Essentially, making it difficult for a clinician to use troponin elevation diagnostically in this patient group. After stenting and related PCI procedures, troponin levels do return to standard levels once the stent has 'settled' and is no longer causing localized cardiac muscle inflammation. [ 20 ]


In patients with non-severe asymptomatic  aortic valve stenosis  and no overt  coronary artery disease , the increased  troponin T  (above 14 pg/mL) was found associated with an increased 5-year event rate of ischemic cardiac events ( myocardial infarction ,  percutaneous coronary intervention , or  coronary artery bypass surgery ). [ 21 ]


Troponins are also increased in patients with  heart failure , where they also predict mortality and ventricular rhythm abnormalities. They can rise in inflammatory conditions such as  myocarditis  and  pericarditis  with heart muscle involvement (which is then termed myopericarditis). Troponins can also indicate several forms of  cardiomyopathy , such as  dilated cardiomyopathy ,  hypertrophic cardiomyopathy  or (left)  ventricular hypertrophy ,  peripartum cardiomyopathy ,  Takotsubo cardiomyopathy , or infiltrative disorders such as cardiac  amyloidosis . [ citation needed ]


Heart injury with increased troponins also occurs in  cardiac contusion ,  defibrillation  and internal or external  cardioversion . Troponins are commonly increased in several procedures such as  cardiac surgery  and  heart transplantation , closure of  atrial septal defects ,  percutaneous coronary intervention , or  radiofrequency ablation . [ citation needed ]


The distinction between cardiac and non-cardiac conditions is somewhat artificial; the conditions listed below are not primary heart diseases, but they exert indirect effects on the heart muscle.


Troponins are increased in around 40% of patients with  critical illnesses  such as  sepsis . There is an increased risk of mortality and length of stay in the intensive-care unit in these patients. [ 22 ]  In severe  gastrointestinal bleeding , there can also be a mismatch between oxygen demand and supply of the myocardium.


Chemotherapy agents can exert toxic effects on the heart (examples include  anthracycline ,  cyclophosphamide ,  5-fluorouracil , and  cisplatin ). Several toxins and venoms can also lead to heart muscle injury ( scorpion venom ,  snake venom , and venom from jellyfish and  centipedes ).  Carbon monoxide poisoning  or  cyanide poisoning  can also be accompanied by the release of troponins due to hypoxic cardiotoxic effects. Cardiac injury occurs in about one-third of severe CO poisoning cases, and troponin screening is appropriate in these patients. [ 23 ] [ 24 ]


In both primary  pulmonary hypertension ,  pulmonary embolism , and acute exacerbations of  chronic obstructive pulmonary disease  (COPD),  right ventricular  strain results in increased wall tension and may cause  ischemia . Of course, patients with COPD exacerbations might also have concurrent myocardial infarction or pulmonary embolism, so care has to be taken to attribute increased troponin levels to COPD.


People with  end-stage kidney disease  can have chronically elevated troponin T levels, which are linked to a poorer prognosis. [ 25 ] [ 26 ]  Troponin I is less likely to be falsely elevated. [ 25 ]


Strenuous endurance  exercise  such as  marathons  or  triathlons  can lead to increased troponin levels in up to one-third of subjects, but it is not linked to adverse health effects in these competitors. [ 27 ] [ 28 ] [ 29 ]
High troponin T levels have also been reported in patients with inflammatory muscle diseases such as  polymyositis  or  dermatomyositis . [ 30 ] [ 31 ]  Troponins are also increased in  rhabdomyolysis .


In hypertensive disorders of pregnancy such as  preeclampsia , elevated troponin levels indicate some degree of myofibrillary damage. [ 32 ] [ 33 ]


Cardiac troponin T and I can be used to monitor drug and toxin-induced cardiomyocyte toxicity. . [ 34 ]


In 2020, it was found that patients with severe COVID-19 had higher troponin I levels compared to those with milder disease. [ 35 ]


Elevated troponin levels are prognostically important in many of the conditions in which they are used for diagnosis. [ 36 ]


In a community-based cohort study indicating the importance of silent cardiac damage, troponin I has been shown to predict mortality and first coronary heart disease event in men free from cardiovascular disease at baseline. [ 37 ]  In people with stroke, elevated blood troponin levels are not a useful marker to detect the condition. [ 6 ]


First cTnI [ 38 ]  and later cTnT [ 39 ]  were originally used as markers for cardiac cell death. Both proteins are now widely used to diagnose acute myocardial infarction (AMI), unstable angina, post-surgery myocardium trauma and some other related diseases with cardiac muscle injury. Both markers can be detected in patient's blood 3–6 hours after onset of the chest pain, reaching peak level within 16–30 hours. Elevated concentration of cTnI and cTnT in blood samples can be detected even 5–8 days after onset of the symptoms, making both proteins useful also for the late diagnosis of AMI. [ 40 ]


Cardiac troponin T and I are measured by  immunoassay  methods. [ 41 ] [ 42 ]


Troponin elevation following cardiac cell necrosis starts within 2–3 hours, peaks in approx. 24 hours, and persists for 1–2 weeks. [ 44 ]


The  vascular lacuna  (Latin:  lacuna vasorum (retroinguinalis) ) is the medial compartment beneath the  inguinal ligament . [ 1 ]  It is separated from the lateral  muscular lacuna  by the  iliopectineal arch . [ 1 ] [ 2 ]  It gives passage to the  femoral vessels , [ 1 ]  lymph vessels and  lymph nodes .


The lacunar ligament can be a site of entrapment for femoral hernias. [ 2 ]


Its boundaries are the iliopectineal arch, the inguinal ligament, the  lacunar ligament , and the superior border of the pubis.


The structures found in the vascular lacuna, from medial to lateral, are:


Vulpian–Heidenhain–Sherrington phenomenon  is a term given for slow  contraction  of denervated  skeletal muscle  by stimulating the  autonomic   cholinergic  fibers  innervating  its  blood vessels . It is named after French  neurologist   Alfred Vulpian  (1826–87), German  physiologist   Rudolf Heidenhain  (1834–1897) and English  neurophysiologist   Charles Scott Sherrington  (1857–1952). [ 1 ]


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The  accessory soleus muscle  is an  accessory muscle  of the calf which is rarely present in humans; [ 1 ]  it is, however, the most common accessory muscle of the  ankle . [ 2 ]


The muscle inserts on the anterior aspect of the  soleus muscle  or on the posterior aspect of the  tibia  or the muscles of the deep posterior compartment. It lies anterior to the  calcaneal tendon  and terminates on the calcaneal tendon or the superior or medial aspect of the calcaneus via fleshy fibers or a distinct  tendon . [ 3 ]


Present in approximately 3% (or 10%) [ 4 ]  of people, this muscle usually appears as a distant belly, medial to the  Achilles tendon . Clinically, the accessory soleus may be associated with pain and edema during periods of prolonged exercise. [ 5 ]


The  anal fascia  is the inferior layer of the diaphragmatic part of the  pelvic fascia , which covers both surfaces of the  levatores ani . It is attached above to the  obturator fascia  along the line of origin of the  levator ani , while below it is continuous with the superior fascia of the  urogenital diaphragm , and with the fascia on the  sphincter ani internus .


The layer covering the upper surface of the pelvic diaphragm follows, above, the line of origin of the levator ani and is therefore somewhat variable.


In front it is attached to the back of the  pubic symphysis  about 2 cm. above its lower border.


It can then be traced laterally across the back of the  superior ramus of the   pubic bone  for a distance of about 1.25 cm, when it reaches the obturator fascia.


It is attached to this fascia along a line which pursues a somewhat irregular course to the  spine of the   ischium .


The irregularity of this line is because the origin of the levator ani, which in lower forms is from the pelvic brim, is in man lower down, on the obturator fascia.


Tendinous fibers of origin of the muscle are therefore often found extending up toward, and in some cases reaching, the pelvic brim, and on these the fascia is carried.


This article incorporates text in the  public domain  from  page 421  of the 20th edition of   Gray's Anatomy   (1918)


The  antebrachial fascia  ( antibrachial fascia  or  deep fascia of forearm ) continuous above with the  brachial fascia , is a dense, membranous investment, which forms a general sheath for the muscles in this region; it is attached, behind, to the  olecranon  and dorsal border of the  ulna , and gives off from its deep surface numerous  intermuscular septa , which enclose each muscle separately.


Over the  flexor muscles   tendons  as they approach the wrist it is especially thickened, and forms the  volar carpal ligament .


This is continuous with the  transverse carpal ligament , and forms a sheath for the tendon of the  palmaris longus  which passes over the  transverse carpal ligament  to be inserted into the  palmar aponeurosis .


Behind, near the wrist-joint, it is thickened by the addition of many transverse fibers, and forms the  dorsal carpal ligament .


It is much thicker on the dorsal than on the volar surface, and at the lower than at the upper part of the forearm, and is strengthened above by tendinous fibers derived from the  biceps brachii  in front, and from the  triceps brachii  behind.


It gives origin to muscular fibers, especially at the upper part of the medial and lateral sides of the forearm, and forms the boundaries of a series of cone-shaped cavities, in which the muscles are contained.


Besides the vertical septa separating the individual muscles, transverse septa are given off both on the volar and dorsal surfaces of the forearm, separating the deep from the superficial layers of muscles.


Apertures exist in the fascia for the passage of vessels and nerves; one of these apertures of large size, situated at the front of the  elbow , serves for the passage of a communicating branch between the superficial and deep veins.


This article incorporates text in the  public domain  from  page 445  of the 20th edition of   Gray's Anatomy   (1918)


The  antitragicus  is an  intrinsic muscle  of the  outer ear .


In  human anatomy , the antitragicus arises from the outer part of the  antitragus , and is inserted into the  cauda helicis  (or tail of the  helix ) and  antihelix . [ 1 ] [ 2 ]


The function of the muscle is to adjusts the shape of the  ear  by pulling the antitragus and cauda helicis towards each other. While the muscle modifies the auricular shape only minimally in the majority of individuals, this action could increase the opening into the  external acoustic meatus  in some. [ 1 ]


The helicis minor is developmentally derived from the  second   pharyngeal arch . [ 1 ]


This article incorporates text in the  public domain  from  page 1035  of the 20th edition of   Gray's Anatomy   (1918)


The  aponeurosis of the  abdominal external oblique muscle  is a thin but strong membranous structure, the fibers of which are directed downward and medially.


It is joined with that of the opposite muscle along the middle line, and covers the whole of the front of the abdomen; above, it is covered by and gives origin to the lower fibers of the  pectoralis major ; below, its fibers are closely aggregated together, and extend obliquely across from the  anterior superior iliac spine  to the  pubic tubercle  and the  pectineal line  to form the inguinal ligament.


In the middle line, it interlaces with the  aponeurosis  of the opposite muscle, forming the  linea alba , which extends from the  xiphoid process  to the  pubic symphysis .


That portion of the aponeurosis which extends between the anterior superior iliac spine and the pubic tubercle is a thick band, folded inward, and continuous below with the fascia lata; it is called the  inguinal ligament .


The portion which is reflected from the inguinal ligament at the pubic tubercle is attached to the pectineal line and is called the  lacunar ligament .


From the point of attachment of the latter to the pectineal line, a few fibers pass upward and medialward, behind the medial crus of the  superficial inguinal ring , to the linea alba; they diverge as they ascend, and form a thin triangular fibrous band which is called the  reflected inguinal ligament .


In the aponeurosis of the external oblique, immediately above the  pubic crest , is a triangular opening, the superficial inguinal ring, formed by a separation of the fibers of the aponeurosis in this situation.


This article incorporates text in the  public domain  from  page 410  of the 20th edition of   Gray's Anatomy   (1918)


Arcade of Frohse , sometimes called the  supinator arch , [ 1 ]  is the most superior part of the superficial layer of the  supinator muscle , and is a fibrous arch over the  posterior interosseous nerve .


The arcade of Frohse is a site of interosseous posterior nerve entrapment, [ 2 ]  and is believed to play a role in causing progressive  paralysis  of the posterior interosseous nerve, both with and without injury.


The arcade of Frohse was named after German  anatomist , Fritz Frohse (1871–1916). [ citation needed ]


The  aryepiglottic muscle  or  aryepiglotticus muscle , often considered the  aryepiglottic part of oblique arytenoid muscle , is an  intrinsic muscle of the larynx . [ 1 ]  It is a direct continuation of a portion of the fibers of the  oblique arytenoid  muscle, sharing its  innervation  and blood supply, after these select fibers travel laterally around the arytenoid apex to the  aryepiglottic fold . [ 2 ]


The aryepiglottic muscle is innervated by the inferior laryngeal nerve, a branch of the  recurrent laryngeal nerve  (itself a branch of the  vagus nerve ). [ 2 ]  Together with the oblique arytenoid muscle, it helps to act as a  sphincter  and weak  adductor  of the  laryngeal inlet . [ 2 ]


In  electrocardiography , the  atrial action potential  are  action potentials  that occur in the  heart atrium . They are similar to  ventricular action potential  with the exception of having a more narrow phase 2 (plateau phase) due to a smaller calcium influx. Also, in comparison to the ventricular action potential, atrial action potentials have a more gradual  repolarization  period. This indicates that the atria's repolarization currents are not very large and they do not undergo a large repolarization peak. [ 1 ]


Cardiac action potential


The  brachial fascia  ( deep fascia  of the arm ) is continuous with that covering the  deltoideus  and the  pectoralis major muscle , by means of which it is attached, above, to the  clavicle ,  acromion , and  spine of the scapula ; it forms a thin, loose, membranous sheath for the muscles of the  arm , and sends septa between them; it is composed of fibers disposed in a circular or spiral direction, and connected together by vertical and oblique fibers.


It differs in thickness at different parts, being thin over the  biceps brachii , but thicker where it covers the  triceps brachii , and over the  epicondyles  of the  humerus : it is strengthened by fibrous aponeuroses, derived from the pectoralis major and  latissimus dorsi  medially, and from the deltoideus laterally.


On either side it gives off a strong  intermuscular septum , which is attached to the corresponding  supracondylar ridge  and  epicondyle  of the humerus.


This article incorporates text in the  public domain  from  page 442  of the 20th edition of   Gray's Anatomy   (1918)


The  caudofemoralis  (from the Latin  cauda , tail and  femur , thighbone) is a  muscle  found in the pelvic limb of mostly all animals possessing a tail. It is thus found in nearly all  tetrapods . [ 1 ]


The caudofemoralis spans  plesiomorphically  between femur (thigh) and tail; in mammals it is reduced and found directly  posterior / caudal  to the  gluteus maximus  and directly  anterior / cranial  to the  biceps femoris .


The caudofemoralis originates from the transverse processes of the second, third and fourth caudal  vertebrae .
The caudodistal portion of the muscle in mammals lies deep to the proximocranial portion of the Biceps femoris; near the middle of the  thigh , the caudofemoralis gives rise to a long, thin, and narrow  tendon  that passes distally to the knee joint and inserts into the  fascia lata  that is anchored to the lateral border of the  patella . [ 1 ]


Among  archosaurians , the caudofemoralis is divided in a  pars pelvica/brevis  (characterized by a pelvic origin) and a  pars caudalis/longa  (caudal origin), and the insertion on the femur is marked by the  fourth trochanter  (but this becomes reduced in  maniraptorans  and absent in  birds ).


In  mammals the caudofemoralis acts to flex the tail laterally to its corresponding side when the pelvic limb is bearing weight. When the pelvic limb is lifted off the ground, contraction of the caudofemoralis causes the limb to abduct and the shank to extend by extending the  hip  joint (acetabulofemoral or coxofemoral joint). In other tetrapods contraction of the caudofemoralis retracts the hindlimb.


The  corrugator cutis ani  is a  muscle  of the  human body , also known as the  Ellis' muscle , after the anatomist  George Viner Ellis . [ 1 ]


Around the  anus  is a thin stratum of involuntary muscle fiber, which radiates from the orifice. Medially the fibers fade off into the submucous tissue, while laterally they blend with the true  skin . By its contraction it raises the skin into ridges around the margin of the anus. The name of this muscle is primarily limited to older texts.


The  cremasteric fascia  is a  fascia  in the  scrotum . As the  cremaster  descends, it forms a series of loops which differ in thickness and length in different subjects. At the upper part of the cord the loops are short, but they become in succession longer and longer, the longest reaching down as low as the  testis , where a few are inserted into the  tunica vaginalis . These loops are united together by  areolar tissue , and form a thin covering over the cord and testis, the cremasteric fascia.


The cremasteric fascia lies between the more superficial  external spermatic fascia  and the deeper  internal spermatic fascia . [ 1 ]  It is a continuation of the  aponeurosis  of the  abdominal internal oblique muscle . [ 2 ] [ 3 ]


This article incorporates text in the  public domain  from  page 414  of the 20th edition of   Gray's Anatomy   (1918)


This article related to the  genitourinary system  is a  stub . You can help Wikipedia by  expanding it .


Cricoarytenoid muscles  are muscles that connect the  cricoid cartilage  and  arytenoid cartilage .


More specifically, it can refer to:


The  depressor supercilii  is an  eye   muscle  of the  human body . The nature of this muscle is in some dispute. Few printed anatomies include it (Netter, et al.) and many authorities consider it to be part of the  orbicularis oculi  muscle. [ 1 ]


On the other hand, many  dermatologists ,  ophthalmologists , and  plastic surgeons [ 2 ]  hold that the depressor supercilii is a distinct muscle and has a definite, individual effect on the movement of the eyebrow and skin of the  glabella . [ 2 ]


The depressor supercilii originates on the medial orbital rim, near the  lacrimal bone , and inserts on the medial aspect of the bony orbit, inferior to the  corrugator supercilii . In some specimens it exhibits two heads and in others, only one. [ citation needed ]


The  external intercostal muscles   or  external intercostals  ( intercostales externi ) are eleven in number on both sides.


The muscles extend from the tubercles of the  ribs  behind, to the cartilages of the ribs in front, where they end in thin membranes, the  external intercostal membranes , which are continued forward to the  sternum .
These muscles work in unison when  inhalation  occurs. The  internal intercostal muscles  relax while the external muscles contract causing the expansion of the chest cavity and an influx of air into the lungs.


Each arises from the lower border of a rib, and is inserted into the upper border of the rib below. In the two lower spaces they extend to the ends of the cartilages, and in the upper two or three spaces they do not quite reach the ends of the ribs.


They are thicker than the internal intercostals, and their fibers are directed  obliquely  downward and laterally on the back of the  thorax , and downward, forward, and medially on the front.


Continuation with the  external oblique  or  serratus anterior : A  supracostalis  muscle, from the anterior end of the first rib down to the second, third or fourth ribs occasionally occurs.


This article incorporates text in the  public domain  from  page 403  of the 20th edition of   Gray's Anatomy   (1918)


The  external sphincter muscle of the female urethra  is a muscle which controls  urination  in females. The muscle fibers arise on either side from the margin of the inferior ramus of the pubis. They are directed across the pubic arch in front of the urethra, and pass around it to blend with the muscular fibers of the opposite side, between the urethra and vagina.


The term "urethrovaginal sphincter" ("sphincter urethrovaginalis") is sometimes used to describe the component adjacent to the  vagina . [ 1 ] [ 2 ] [ 3 ] [ 4 ] [ 5 ]


The "compressor urethrae" is also considered a distinct, adjacent muscle by some sources, [ 6 ] [ 7 ] [ 8 ] [ 9 ]


The muscle helps maintain  continence  of  urine  along with the  internal urethral sphincter  which is under control of the  autonomic nervous system . The external sphincter muscle prevents urine leakage as the muscle is tonically contracted via  somatic  fibers that originate in  Onuf's nucleus  and pass through  sacral spinal nerves  S2-S4 then the  pudendal nerve  to  synapse  on the muscle. [ 7 ] [ 10 ]


Voiding urine begins with voluntary relaxation of the external urethral sphincter. This is facilitated by inhibition of the somatic neurons in Onuf's nucleus via signals arising in the  pontine micturition center  and traveling through the  descending reticulospinal tracts .


This article incorporates text in the  public domain  from  page 431  of the 20th edition of   Gray's Anatomy   (1918)


The  external sphincter muscle of the male urethra , also  sphincter urethrae membranaceae ,  sphincter urethrae externus , surrounds the whole length of the  membranous urethra , and is enclosed in the  fascia  of the  urogenital diaphragm .


Its external fibers arise from the junction of the  inferior pubic ramus  and  ischium  to the extent of 1.25 to 2 cm., and from the neighboring fascia.


They arch across the front of the urethra and  bulbourethral  glands, pass around the urethra, and behind it unite with the muscle of the opposite side, by means of a tendinous  raphe .


Its innermost fibers form a continuous circular investment for the membranous urethra.


The muscle helps maintain  continence  of  urine  along with the  internal urethral sphincter  which is under control of the  autonomic nervous system . The external sphincter muscle prevents urine leakage as the muscle is tonically contracted via  somatic  fibers that originate in  Onuf's nucleus  and pass through  sacral spinal nerves  S2-S4 then the  pudendal nerve  to  synapse  on the muscle. [ 1 ] [ 2 ]


Voiding urine begins with voluntary relaxation of the external urethral sphincter. This is facilitated by inhibition of the somatic neurons in Onuf's nucleus via signals arising in the  pontine micturition center  and traveling through the  descending reticulospinal tracts . During ejaculation, the external sphincter opens and the internal sphincter closes. [ 3 ]


This article incorporates text in the  public domain  from  page 429  of the 20th edition of   Gray's Anatomy   (1918)


In  anatomy  for  cardiac muscle ,  fascia adherens  are ribbon-like structures that stabilize non-epithelial tissue. They are similar in function and structure to the zonula adherens or  adherens junction  of  epithelial cells . It is a broad intercellular junction in the transversal sections of an  intercalated disc  of cardiac muscle anchoring  actin  filaments. It helps to transmit contractile forces. [ 1 ]


This  cardiovascular system  article is a  stub . You can help Wikipedia by  expanding it .


The  membranous layer of the superficial fascia of the perineum  ( Colles' fascia ) is the deeper layer ( membranous layer ) of the  superficial perineal fascia . It is thin,  aponeurotic  in structure, and of considerable strength, serving to bind down the muscles of the  root of the penis . Colles' fascia emerges from the  perineal membrane , which divides the base of the penis from the prostate.  Colles' fascia emerges from the inferior side of the perineal membrane and continues along the ventral (inferior) penis without covering the  scrotum . It separates the  skin  and subcutaneous fat from the  superficial perineal pouch .


In front , it is continuous with the  dartos fascia  of the penis and  Scarpa's fascia  upon the  anterior wall of the abdomen ;


On either side  it is firmly attached to the margins of the  rami of the pubis and ischium , lateral to the  crus penis  and as far back as the  tuberosity of the   ischium .


Posteriorly , it curves around the  superficial transverse perineal muscle  to join the lower margin of the  inferior fascia of the urogenital diaphragm .


In the middle line , it is connected with the  superficial fascia  and with the median septum of the  bulbospongiosus muscle .


This fascia not only covers the muscles in this region, but at its back part sends upward a vertical septum from its deep surface, which separates the posterior portion of the subjacent space into two.


This article incorporates text in the  public domain  from  page 426  of the 20th edition of   Gray's Anatomy   (1918)


The  fascial compartments of the forearm  are the  posterior compartment of the forearm  and the  anterior compartment of the forearm . [ 1 ]


The  flexor digiti minimi brevis  ( flexor brevis minimi digiti ,  flexor digiti quinti brevis ) lies under the  metatarsal  bone on the little toe, and resembles one of the  interossei .


It arises from the base of the  fifth metatarsal bone , and from the sheath of the  fibularis longus ; its tendon is inserted into the lateral side of the base of the first  phalanx  of the fifth toe.


Occasionally a few of the deeper fibers are inserted into the lateral part of the distal half of the fifth metatarsal bone; these are described by some as a distinct muscle, the opponens digiti quinti.


This article incorporates text in the  public domain  from  page 494  of the 20th edition of   Gray's Anatomy   (1918) .


The  gluteal aponeurosis  is a fibrous  membrane , from the fascia lata, that lies between the  iliac crest  and the superior border of the  gluteus maximus . A part of the  gluteus medius  arises from this membrane.


The  helicis major  (or  large muscle of helix [ 1 ] ) is an  intrinsic muscle  of the  outer ear .


In  human anatomy , it is the form of a narrow vertical band situated upon the anterior margin of the  helix , at the point where the helix becomes transverse. [ 1 ]


It arises below, from the  spina helicis , and is inserted into the anterior border of the helix, just where it is about to curve backward. [ 2 ]


The function of the muscle is to adjust the shape of the  ear  by depressing the anterior margin of the ear cartilage. While the muscle modifies the auricular shape only minimally in the majority of individuals, this action could increase the opening into the  external acoustic meatus  in some. [ 2 ]


The helicis major is developmentally derived from the  second   pharyngeal arch . [ 2 ]  It seem that only in  primates  is the  helicis minor  and major two distinctive muscles. [ 3 ]


The  Helicis minor  ( musculus helicis minor  or  smaller muscle of helix ) is a small  skeletal   muscle . The helicis minor is an  intrinsic muscle  of the  outer ear . The muscle runs obliques and covers the  helical crus , part of the  helix  located just above the  tragus .


The helicis minor originates from the base of the helical crus, runs obliques and inserts at the anterior aspect of the helical crus where it curves upward above the tragus. [ 1 ]


The function of the muscle is to assist in adjusting the shape of the anterior margin of the ear cartilage. While this is a potential action in some individuals, in the majority of individuals the muscle modifies auricular shape to a minimal degree. [ 1 ]


The helicis minor is developmentally derived from the  second   pharyngeal arch [ 1 ]  It seem that only in  primates  is the  helicis major  and minor two distinctive muscles. [ 2 ]


Iliocostalis muscle  is the muscle immediately lateral to the  longissimus  that is the nearest to the furrow that separates the  epaxial  muscles from the  hypaxial . It lies very deep to the fleshy portion of the  serratus posterior muscle . It laterally  flexes  the  vertebral column  to the same side.


Iliocostalis muscle has a common origin from the  iliac crest , the  sacrum , the  thoracolumbar fascia , and the  spinous processes  of the  vertebrae  from  T11  to  L5 . [ 1 ]


Iliocostalis cervicis  (cervicalis ascendens) arises from the angles of the third, fourth, fifth, and sixth ribs, and is inserted into the  posterior tubercles  of the transverse processes of the fourth, fifth, and sixth cervical vertebrae.


Iliocostalis thoracis  (musculus accessorius; iliocostalis thoracis) arises by flattened tendons from the upper borders of the angles of the lower six ribs medial to the tendons of insertion of the iliocostalis lumborum; these become muscular, and are inserted into the upper borders of the angles of the upper six ribs and into the back of the  transverse process  of the seventh  cervical vertebra .


Iliocostalis lumborum  (iliocostalis muscle; sacrolumbalis muscle) is inserted, by flattened  tendons , into the inferior borders of the angles of the lower six to nine  ribs . [ 1 ]


Iliocostalis muscle is supplied by the  dorsal rami  of  spinal nerves . [ 1 ]


Iliocostalis muscle laterally  flexes  the  vertebral column  to the same side. [ 1 ]  It bilaterally extends the vertebral column. [ 1 ]


This article incorporates text in the  public domain  from  page 399  of the 20th edition of   Gray's Anatomy   (1918)


The  iliopubic tract  is a thickened band of fibers curving over the  external iliac vessels , at the spot where they become femoral, on the abdominal side of the  inguinal ligaments  and loosely connected with it. It is apparently a thickening of the  transversalis fascia  joined laterally to the iliac crest, and arching across the front of the  femoral sheath  to be inserted by a broad attachment into the  pubic tubercle  and  pectineal line , behind the  conjoint tendon . In some subjects this structure is not very prominently marked, and not infrequently it is altogether wanting. It can be of clinical significance in  hernia repair . [ 1 ]


This article incorporates text in the  public domain  from  page 419  of the 20th edition of   Gray's Anatomy   (1918)


The  inferior longitudinal muscle of tongue  is an intrinsic muscle of the  tongue . [ 1 ]  It is situated on the under surface of the  tongue  between the  genioglossus  and  hyoglossus . [ citation needed ]  It is innervated by the  hypoglossal nerve (cranial nerve XII) . Its contraction shortens and thickens the tongue.


The inferior longitudinal muscle of the tongue is an  intrinsic muscle  of the  tongue . [ 1 ]  It is thin and oval in cross-section. It is situated between the paramedian septum, and the lateral septum. [ 2 ]  It extends from the root to the apex of the tongue. Posteriorly, some of its fibers attach onto the body of the  hyoid bone . [ citation needed ]  Anteriorly, its fibres blend with those of the  styloglossus ,  hyoglossus , and  genioglossus  to form the ventral area of the tip of the tongue. [ 2 ]


The inferior longitudinal muscle of the tongue is supplied by the  hypoglossal nerve (CN XII) . [ 3 ]


Contraciton of the inferior longitudinal muscle of the tongue shortens and thickens the tongue. [ 2 ]


This article incorporates text in the  public domain  from  page 1130  of the 20th edition of   Gray's Anatomy   (1918)


The  innermost intercostal muscle  is a layer of  intercostal muscles . [ 1 ]  It may also be called the  intima of the internal intercostal muscles . [ 2 ]  It is the deepest muscular layer of the  thorax , [ 3 ]  with  muscle fibres  running vertically (in parallel with the  internal intercostal muscles ). [ 2 ]  It is present only in the middle of each  intercostal space , and often not present higher up the  rib cage . [ 3 ]  It lies deep to the plane that contains the  intercostal nerves  and  intercostal vessels , and the  internal intercostal muscles . [ 3 ] [ 4 ]  The  diaphragm  is continuous with the innermost intercostal muscle. [ 5 ]


The  interosseous muscles of the foot  are muscles found near the  metatarsal  bones that help to control the toes. They are considered voluntary muscles.


They are generally divided into two sets:


The axial line goes down the middle of the 2nd digit, towards the sole of the foot (it's an imaginary line).


Both sets of muscles are innervated by the Lateral plantar nerve. [ 1 ]


The  interosseous muscles of the hand  are muscles found near the  metacarpal  bones that help to control the fingers. They are considered voluntary muscles.


This is often remembered by the mnemonic  PAD-DAB , as the  P almar interosseous muscles  AD duct, and the  D orsal interosseous muscles  AB duct. The axial line goes down the middle of the 3rd digit, towards the palm of the hand (it's an imaginary line).


Both sets of muscles are innervated by the deep branch of the ulnar nerve. [ 1 ]


The  interspinales  are short  muscle fascicles , found in pairs between the spinous processes of the contiguous  vertebrae , one on either side of the  interspinal ligament .


This article incorporates text in the  public domain  from  page 400  of the 20th edition of   Gray's Anatomy   (1918)


The  lateral cricoarytenoid  (also  anterior cricoarytenoid ) is an intrinsic muscle of the  larynx . It attaches at the  cricoid cartilage  anteriorly, and at the  arytenoid cartilage  of the same side posteriorly. It is innervated by the  recurrent laryngeal nerve . It acts to close the  rima glottidis , thus closing the  airway .


The muscle is directed obliquely superoposteriorly from its anterior attachment to its posterior attachment. [ 1 ]


The muscle's anterior attachment is onto the superior border of the arch of the  cricoid cartilage . [ 1 ]


Its posterior attachment is onto the anterior aspect of the  muscular process  of the ipsilateral  arytenoid cartilage . [ 2 ]


The muscle receives motor innervation from (branches of the anterior terminal division of) the  recurrent laryngeal nerve [ 3 ]  (which is in turn a branch of a  vagus nerve (CN X) ).


The muscle rotates the arytenoid cartilage medially (it thus acts as antagonist to the  posterior cricoarytenoid muscle  which rotates the cartilage laterally). [ 1 ]


The muscle closes the  rima glottidis , adducting (approximating) the apices of the  vocal process  to close the ligamentous part of rima glottidis (in which it is synergystic with the  oblique arytenoid muscles  and  transverse arytenoid muscle ). [ 1 ]  It thus functions to close the  airway . [ citation needed ]


It also shortens and slackens the vocal cords. [ 1 ]


The  levator anguli oris  ( caninus ) is a  facial muscle  of the  mouth  arising from the  canine fossa , immediately below the  infraorbital foramen . It  elevates  angle of mouth medially. Its fibers are inserted into the angle of the mouth, intermingling with those of the  zygomaticus ,  triangularis , and  orbicularis oris . Specifically, the levator anguli oris is innervated by the  buccal branches of the facial nerve .


This article incorporates text in the  public domain  from  page 383  of the 20th edition of   Gray's Anatomy   (1918)


A fibrous or muscular band is sometimes found attached, above, to the body of the  hyoid bone , and below to the  thyroid isthmus , or its pyramidal lobe. When muscular, it is termed the  Levator muscle of thyroid gland .


This article incorporates text in the  public domain  from  page 1270  of the 20th edition of   Gray's Anatomy   (1918)


The  levatores costarum  ( / ˌ l ɛ v ə ˈ t ɔːr iː z   k ə ˈ s t ɛər ə m / ), twelve in number on either side, are small tendinous and fleshy bundles, which arise from the ends of the transverse processes of the seventh  cervical  and upper eleven  thoracic vertebrae


They pass obliquely downward and laterally, like the fibers of the  Intercostales externi , and each is inserted into the outer surface of the  rib  immediately below the  vertebra  from which it takes origin, between the tubercle and the  angle  ( Levatores costarum breves ).


Each of the four lower muscles divides into two fasciculi, one of which is inserted as above described; the other passes down to the second rib below its origin ( Levatores costarum longi ).


They have a role in  forceful inspiration . [ 1 ]


The  longus capitis muscle  (Latin for  long muscle of the head , alternatively  rectus capitis anticus major ) is broad and thick above, narrow below, and arises by four tendinous slips, from the anterior tubercles of the  transverse processes  of the third, fourth, fifth, and sixth  cervical vertebræ , and ascends, converging toward its fellow of the opposite side, to be inserted into the inferior surface of the  basilar  part of the  occipital bone .


It is innervated by a branch of cervical plexus.


Longus capitis has several actions:


acting unilaterally, to:


acting bilaterally:


This article incorporates text in the  public domain  from  page 395  of the 20th edition of   Gray's Anatomy   (1918)


The  lumbricals  are four small  skeletal muscles , accessory to the tendons of the  flexor digitorum longus muscle . They are numbered from the medial side of the foot. [ 1 ]


The lumbricals arise from the tendons of the  flexor digitorum longus muscle , [ 1 ]  as far back as their angles of division, each springing from two tendons, except the first. The first lumbrical is unipennate, while the second, third and fourth are bipennate.


The muscles end in  tendons , which pass forward on the medial sides of the four lesser toes, and are inserted into the expansions of the tendons of the extensor digitorum longus muscle on the dorsal surfaces of the proximal  phalanges . [ 1 ]  All four lumbricals insert into extensor hoods of the phalanges, thus creating extension at the inter-phalangeal (PIP and DIP) joints. However, as the tendons also pass inferior to the metatarsal phalangeal (MTP) joints it creates flexion at this joint.


The most medial lumbrical is innervated by the  medial plantar nerve  while the remaining three lumbricals are supplied by the  lateral plantar nerve .


Absence of one or more; doubling of the third or fourth even the fifth. Insertion partly or wholly into the first phalanges.


The term "lumbrical" comes from the  Latin , meaning " worm ". [ 1 ]


This article incorporates text in the  public domain  from  page 493  of the 20th edition of   Gray's Anatomy   (1918)


Muscular defense  is a  reflex  of the  abdominal muscles  to contract upon mechanical force to the  abdomen , and serves as protection.


It is a  visceromotor reflex , since the  parietal peritoneum  and  viscera  are involved in generating the reflex. [ 1 ]


Myology  is the study of the  muscular system , including the study of the structure, function and  diseases  of  muscle . [ 1 ]  The muscular system consists of  skeletal muscle , which contracts to move or position parts of the body (e.g., the bones that articulate at joints),  smooth  and  cardiac muscle  that propels, expels or controls the flow of fluids and contained substance.


The  oblique arytenoid  is bilaterally paired intrinsic muscle of the  larynx . It is superficial to the  transverse arytenoid ; the oblique and transverse arytenoids are often considered two parts of a single muscle - the  interarytenoid muscle  (which is then said to have an oblique part and a transverse part). [ 1 ]


Each oblique arytenoid muscle attaches to both  arytenoid cartilages ; the two oblique arytenoids thus cross each other. [ 1 ]


Some muscle fibres of the oblique arytenoid muscle proceed to extend laterally around the apex of arythenoid cartilage into the  aryepiglottic fold , thus forming the  aryepiglottic muscle . [ 1 ]


Each oblique arytenoid muscle attaches to the posterior aspect of the  muscular process of arytenoid cartilage  of one side, and to the apex of the  arytenoid cartilage  of the opposite side; the two oblique arytenoid muscles thus cross each other obliquely. [ 1 ]


Arterial supply is provided by laryngeal branches of  superior thyroid arteries  and  inferior thyroid arteries . [ 1 ]


The muscle receives motor innervation from the  recurrent laryngeal nerve  (a branch of the  vagus nerve (CN X) ) [ 1 ]  (as is the case with all intrinsic muscles of the larynx except the cricothyroid muscle). [ citation needed ]


Together with the  aryepiglottic muscles , the two oblique arytenoid muscles function as a sphincter of the  laryngeal inlet  by adducting aryepiglottic folds, and approximating the arytenoid cartilages and the tubercle of epiglottis. [ 1 ]


The muscles act during swallowing or coughing. [ citation needed ]


The  oblique muscle of auricle  ( oblique auricular muscle  or  Tod muscle [ 1 ] ) is an  intrinsic muscle  of the  outer ear .


The oblique muscle of auricle is placed on the cranial surface of the  pinna . It consists of a few fibers extending from the upper and back part of the  concha  to the convexity immediately above it.


The  opponens digiti minimi  ( opponens digiti quinti  in older texts) is a  muscle  in the  hand . It is of a triangular form, and placed immediately beneath the  palmaris brevis ,  abductor digiti minimi  and  flexor digiti minimi brevis . It is one of the three  hypothenar muscles  that control the  little finger . [ 1 ]


It arises from the convexity of the  hamulus  of the  hamate bone  and the contiguous portion of the  transverse carpal ligament ; it is inserted into the whole length of the  metacarpal bone  of the little finger, along its ulnar margin.


The opponens digiti minimi muscle serves to flex and laterally rotate the 5th  metacarpal  about the 5th  carpometacarpal joint , as when bringing the little finger and thumb into opposition. It is innervated by the  deep branch of the ulnar nerve .


This article incorporates text in the  public domain  from  page 464  of the 20th edition of   Gray's Anatomy   (1918)


The  pharyngeal muscles  are a group of muscles that form the  pharynx , which is posterior to the oral cavity, determining the shape of its  lumen , and affecting its sound properties as the primary resonating cavity.


The pharyngeal muscles (involuntary skeletal) push food into the  esophagus . There are two muscular layers of the  pharynx : the outer circular layer and the inner longitudinal layer.


The outer circular layer includes:


During  swallowing , these muscles constrict to propel a  bolus  downwards (an involuntary process).


The inner longitudinal layer includes:


During swallowing, these muscles act to shorten and widen the pharynx.


They are innervated by the  pharyngeal branch  of the  vagus nerve  (CN X) with the exception of the stylopharyngeus muscle which is innervated by the  glossopharyngeal nerve  (CN IX). [ 1 ]


They are primarily supplied by branches of the  facial artery . Other blood supply includes the  ascending pharyngeal artery ,  lingual artery , and ascending and descending palatine arteries.


The  pubovaginal muscle  is a  pelvic floor   muscle  that attaches to the muscles of lateral walls of the midsection of the vagina and the  pubis . [ 1 ] [ 2 ] [ 3 ] [ 4 ] [ 5 ] [ 6 ]  It is relatively short compared to the other  levator ani muscles  and extends between the  pubic bones  and the  vagina . [ 7 ] [ 8 ] [ 9 ]   Other muscles that are part of the levator ani are: the  pubococcygeus muscle  which is made up of the  puboperineal , pubovaginal, and  puboanal  muscles; the  puborectal muscle ; and the  iliococcygeal muscle . [ 9 ] [ 10 ]  The pubovaginal muscle was identified by  anatomists  as early as 1912. [ 11 ]


The  quadratus plantae  ( flexor accessorius ) is separated from the muscles of the first layer by the lateral plantar vessels and nerve. It acts to aid in flexing the 2nd to 5th toes (offsetting the oblique pull of the  flexor digitorum longus ) and is one of the few muscles in the foot with no homolog in the hand.


It arises by two heads, which are separated from each other by the  long plantar ligament : the medial or larger head is muscular, and is attached to the medial concave surface of the  calcaneus , below the groove which lodges the tendon of the  flexor hallucis longus ; the lateral head, flat and tendinous, arises from the lateral border of the inferior surface of the calcaneus, in front of the lateral process of its tuberosity, and from the  long plantar ligament .


The two portions join at an acute angle, and end in a flattened band which is inserted into the lateral margin and upper and under surfaces of the tendon of the  flexor digitorum longus , forming a kind of groove, in which the tendon is lodged. It usually sends slips to those tendons of the flexor digitorum longus which pass to the second, third, and fourth toes.


Lateral head often wanting; entire muscle absent. Variation in the number of digital tendons to which fibers can be traced. Most frequent offsets are sent to the second, third and fourth toes; in many cases to the fifth as well; occasionally to two toes only.


The  rectus capitis anterior  ( rectus capitis anticus minor ) is a short, flat muscle, situated immediately behind the upper part of the  Longus capitis .


It arises from the anterior surface of the  lateral mass  of the  atlas , and from the root of its  transverse process , and passing obliquely upward and medialward, is inserted into the inferior surface of the  basilar part  of the  occipital bone  immediately in front of the  foramen magnum .


action:  aids in flexion of the head and the neck;
 nerve supply:  C1, C2. [ 1 ]


The  rectus capitis lateralis , a short, flat muscle, arises from the upper surface of the  transverse process  of the  atlas , and is inserted into the under surface of the  jugular process  of the  occipital bone .


The  rectus capitis posterior major  (or  rectus capitis posticus major [ citation needed ] ) is a muscle in the upper back part of the neck. It is one of the  suboccipital muscles . Its inferior attachment is at the  spinous process  of the  axis (Second cervical vertebra) ; its superior attachment is onto the outer surface of the  occipital bone  on and around the side part of the  inferior nuchal line . The muscle is innervated by the  suboccipital nerve  (the  posterior ramus  of  cervical spinal nerve C1 ). The muscle acts to extend the head and rotate the head to its side.


The rectus capitis posterior major muscle is one of the  suboccipital muscles . It forms the superomedial boundary of the  suboccipital triangle . [ 1 ]


The muscle extends obliquely [ 2 ]  superiolaterally from its inferior attachment to its superior attachment. [ 1 ] [ 2 ]  It becomes broader superiorly. [ 1 ]


Its inferior attachment is (via a pointed tendon [ 1 ] ) at (the external aspect of) the (bifid) [ 2 ]   spinous process  of the  axis (cervical vertebra C2) . [ 1 ]


Its superior attachment is at (the lateral portion of [ 1 ] [ 2 ] ) the  inferior nuchal line [ 1 ]  and the surface of the occipital bone just inferior to this line. [ 1 ] [ 2 ]


The muscle receives motor innervation from the  suboccipital nerve  (the  posterior ramus  of  cervical spinal nerve C1 ). [ 2 ] [ 1 ]


Superiorly, as the two muscles diverge laterally, they create between them a triangular space in which parts of the two  recti capitis posteriores minores muscles  are exposed. [ 1 ]


The muscle extends the head and (acting together with the  obliquus capitis inferior muscle [ 1 ] ) ipsilaterally rotates the head. [ 1 ] [ 2 ]


Its main actions are to extend and rotate the  atlanto-occipital joint .


A soft tissue connection bridging from the rectus capitis posterior major to the cervical  dura mater  was described in 2011. Various clinical manifestations may be linked to this anatomical relationship. [ 3 ]  It has also been postulated that this connection serves as a monitor of dural tension along with the  rectus capitis posterior minor  and the  obliquus capitis inferior . [ citation needed ]


This article incorporates text in the  public domain  from  page 401  of the 20th edition of   Gray's Anatomy   (1918)


The  scapulohumeral muscles  are a group of seven muscles that connect the  humerus  to the  scapula . They are amongst the muscles that act on and stabilise the  glenohumeral joint  in the  human body .


They include :


Other muscles that attach to the humerus and affect its rotation and stability :


Sebileau's muscle  is the deep muscle fibres of the  dartos tunic  which pass into the  scrotal septum . [ 1 ]  It is named after French anatomist  Pierre Sebileau  (1860–1953).


The  splenius cervicis  ( / ˈ s p l iː n i ə s   s ər ˈ v aɪ s ɪ s / ) (also known as the  splenius colli ,  /-   ˈ k ɒ l aɪ / ) is a muscle in the back of the neck. It arises by a narrow tendinous band from the  spinous processes  of the third to the sixth  thoracic vertebrae ; it is inserted, by tendinous  fasciculi , into the  posterior tubercles  of the  transverse processes  of the upper two or three  cervical vertebrae .


Its name is based on the Greek word σπληνίον,  splenion  (meaning a bandage) and the Latin word  cervix  (meaning a neck). [ 1 ]  The word  collum  also refers to the neck in Latin. [ 1 ]


The function of the splenius cervicis muscle is extension of the cervical spine, rotation to the ipsilateral side and lateral flexion to the ipsilateral side. [ 2 ]


This article incorporates text in the  public domain  from  page 397  of the 20th edition of   Gray's Anatomy   (1918)


The  splenius muscles  are:


Their origins are in the upper thoracic and lower cervical  spinous processes . Their actions are to extend and ipsilaterally rotate the head and neck.


The  subcostales  (singular: subcostalis) (Infracostales) consist of muscular and  aponeurotic   fasciculi , which are usually well-developed only in the lower part of the  thorax ; each originates from the inner surface of one  rib , and is inserted into the inner surface of the second or third rib below, near its angle.


Their fibers run in the same direction as those of the  intercostales interni .


Depresses the ribs to assist in expiration.


The  suboccipital muscles  are a group of muscles defined by their location to the  occiput . Suboccipital muscles are located below the  occipital bone . These are four paired muscles on the underside of the occipital bone; the two straight muscles ( rectus ) and the two oblique muscles ( obliquus ).


The muscles are named


They are innervated by the  suboccipital nerve .


The  superficial cervical fascia  is a thin layer of subcutaneous  connective tissue  that lies between the  dermis  of the skin and the  deep cervical fascia . [ 1 ]  It contains the  platysma , cutaneous nerves [ 1 ] [ 2 ]  from the  cervical plexus , [ 2 ]  blood vessels, and lymphatic vessels. [ 1 ] [ 2 ]  It also contains a varying amount of fat, which is its distinguishing characteristic. It is considered by some to be a part of the  panniculus adiposus , and not true  fascia . [ 1 ]


Superficial muscular aponeurotic system  (or  superficial musculoaponeurotic system [ 1 ] ) ( SMAS ) is a thin yet tough [ 2 ] : 438   unitary tissue plane of the face [ 3 ]  formed by facial fasciae, subcutis connective tissue, and facial muscles. [ 2 ] : 438   Its composition varies, containing muscle fibres in some areas, and fibrous or fibroaponeurotic tissue in others. [ 3 ]  It connects to the dermis via vertical septa. [ citation needed ]  It does not attach to bone. In most areas, a distinct plane can be defined deep to the SMAS (continuous with that formed between the platysma and underlying  investing layer of deep cervical fascia ). [ 3 ]


Superiorly, the SMAS extends to the  galea aponeurotica  of [ citation needed ]  the scalp, [ 2 ] : 438   becoming continuous with  temporoparietal fascia [ 1 ] [ 1 ]  (at the  zygomatic arch [ 1 ] ) and galea. [ citation needed ]  It becomes continuous with the  platysma muscle  inferiorly (inferior to the inferior border of the mandible), and indistinct laterally (inferior to the  zygomatic arch ). Anteromedially, it blends with the  epimysium  of some facial muscles; [ 3 ]  a link between facial muscles and the skin of the face is thereby established, enabling  facial expression . [ 2 ] : 429   Over the parotid gland, the SMAS is firmly united with the superficial layer of  parotid fascia . [ 4 ]


The SMAS is clinically important in  facial   plastic surgery  for  rhytidectomy (facelift procedure) . During this procedure, the SMAS is accessed through an arch-shaped incision anterior to the ear; a portion of the SMAS is then excised and the remaining SMAS is stretched by drawing it posterior-ward and suturing it, thus making the skin of the face which overlies the SMAS taut. [ 2 ] : 438


The  superior longitudinal muscle of tongue  or  superior lingualis  is a thin layer of oblique and longitudinal fibers immediately underlying the mucous membrane on the dorsum of the  tongue .


The superior longitudinal muscle of the tongue is one of the  intrinsic muscles  of the  tongue . [ 1 ]  It arises from the submucous fibrous layer close to the  epiglottis  and from the  median fibrous septum , and runs forward to the edges of the tongue.


The superior longitudinal muscle of the tongue is supplied by the  hypoglossal nerve  (CN XII). [ 2 ]


The superior longitudinal muscle of the tongue works with the other intrinsic muscles to move the  tongue . [ 1 ]


A considerable number of the fibers of the  thyroarytenoid muscle  are prolonged into the  aryepiglottic fold , where some of them become lost, while others continue to the margin of the epiglottis. They have received a distinctive name,  thyroepiglotticus  or  thyroepiglottic muscle , and are sometimes described as a separate muscle. This muscle's function is to widen the  laryngeal inlet .


This article incorporates text in the  public domain  from  page 1083  of the 20th edition of   Gray's Anatomy   (1918)


The  tragicus , also called the  tragus muscle  or  Valsalva muscle , [ 1 ]  is an  intrinsic muscle  of the  outer ear .


It is a short, flattened vertical band on the lateral surface of the  tragus . [ 1 ]


While the muscle modifies the auricular shape only minimally in the majority of individuals, this action could increase the opening of the  external acoustic meatus  in some. [ 2 ]


The  transverse muscle of auricle  ( transverse auricular muscle , [ 1 ]   transversus auriculae ,  transversus auricularis  or  transverse muscle of pinna [ 2 ] ) is an  intrinsic muscle  of the  outer ear .


The muscle is located on the cranial surface of the  pinna . It consists of scattered fibers, partly tendinous and partly muscular, extending from the  eminentia conchae  to the prominence corresponding with the  scapha . [ 1 ]


While the muscle modifies the auricular shape only minimally in the majority of individuals, it could help flatten the cranial profile of the auricular cartilage. [ 2 ]


The transverse muscle is developmentally derived from the  second   pharyngeal arch . [ 2 ]


The  transversus menti , or  transverse muscle of the chin , is a  facial muscle  that is often considered to be the superficial fibers of the  depressor anguli oris  muscle which cross to the other side of the face. [ 1 ]


The  transverse muscle of tongue  ( transversus linguae ) is an  intrinsic muscle  of the  tongue . [ 1 ]  It consists of fibers which arise from the  median fibrous septum . It passes laterally to insert into the submucous fibrous tissue at the sides of the  tongue . [ citation needed ]  It is innervated by the  hypoglossal nerve (cranial nerve XII) . Its contraction elongates and narrows the tongue.


The transverse muscle of the tongue is an  intrinsic muscle  of the  tongue . [ 1 ]  It consists of fibers which arise from the  median fibrous septum . It passes laterally to insert into the submucous fibrous tissue at the sides of the  tongue . [ citation needed ]


The transverse lingual muscle is innervated by the  hypoglossal nerve (CN XII) . [ 2 ]


Contraction of the transverse muscle of the tongue elongates and narrows the tongue. [ 3 ]


The  transversus thoracis muscle  ( / t r æ n z ˈ v ɜːr s ə s   θ ə ˈ r eɪ s ɪ s / ), also known as  triangularis sterni ,  lies internal to the  thoracic cage , anteriorly.  It is usually a thin plane of muscular and tendinous fibers, however on athletic individuals it can be a thick 'slab of meat', situated upon the inner surface of the front wall of the chest. It is in the same layer as the  subcostal muscles  and the  innermost intercostal  muscles.


It arises on either side from the lower third of the posterior surface of the body of the  sternum , from the posterior surface of the  xiphoid process , and from the sternal ends of the  costal cartilages  of the lower three or four true ribs.


Its fibers diverge upward and lateralward, to be inserted by slips into the lower borders and inner surfaces of the costal cartilages of the  second , third, fourth, fifth, and sixth ribs.


The lowest fibers of this muscle are horizontal in their direction, and are continuous with those of the  transversus abdominis ; the  intermediate fibers  are oblique, while the highest are almost vertical.


This muscle varies in its attachments, not only in different subjects, but on opposite sides of the same subject.


The muscle is supplied by the anterior rami of the  thoracic spinal nerves  (intercostal nerves).


It is almost completely without function, but it separates the thoracic cage from the  parietal pleura .  It depresses the ribs.


Contraction of this muscle aids in exertional expiration by decreasing the transverse diameter of the thoracic cage.


The  vertical muscle of the tongue  is an  intrinsic muscle  of the  tongue . Its fibers extend from the upper to the under surface of the tongue. It is innervated by the  hypoglossal nerve (cranial nerve XII) . Its contraction flattens, widens and elongates the tongue. [ citation needed ]


The vertical muscle of the tongue is an intrinsic muscle of the tongue. [ 1 ]  It is found only at the borders of the forepart of the  tongue . [ citation needed ]


Fibres of the vertical muscle of the tongue are arranged in an almost vertical direction, and intersect the transversely oriented fibres of the  transverse muscle of tongue . [ 2 ]  Fibers of the vertical muscle of the tongue extend from the upper to the under surface of the tongue. [ citation needed ]


The vertical of the tongue is innervated by the  hypoglossal nerve (CN XII) . [ 3 ]


Contraction of the vertical muscle of the tongue flattens, widens and elongates the tongue. [ 2 ]


This article incorporates text in the  public domain  from  page 1131  of the 20th edition of   Gray's Anatomy   (1918)


Zenker's degeneration  is a severe glassy or waxy  hyaline   degeneration  or  necrosis  of  skeletal  muscles in acute infectious diseases; a prototype of coagulative necrosis.


The condition was named by  Friedrich Albert von Zenker . It is a hyaline degeneration of skeletal muscles such as  rectus abdominis  and  diaphragm , and occurs in severe  toxaemia  as typhoid fever. It is also seen in electrical burns. [ 1 ]  Grossly the muscles appear pale and friable; microscopically, the muscle fibres are swollen, have a loss of cross striations, and show a hyaline appearance. Rupture and small hemorrhage may complicate the lesion. Coagulative necrosis occurs here.


This article about a disease of musculoskeletal and connective tissue is a  stub . You can help Wikipedia by  expanding it .


A  tendon  or  sinew  is a tough band of  dense fibrous connective tissue  that connects  muscle  to  bone . It sends the mechanical forces of muscle contraction to the skeletal system, while withstanding  tension .


Tendons, like  ligaments , are made of  collagen . The difference is that ligaments connect bone to bone, while tendons connect muscle to bone. There are about 4,000 tendons in the adult human body. [ 1 ] [ 2 ]


A tendon is made of  dense regular connective tissue , whose main cellular components are special  fibroblasts  called  tendon cells  (tenocytes). [ 3 ]  Tendon cells synthesize the tendon's  extracellular matrix , which abounds with densely-packed  collagen fibers . The collagen fibers run parallel to each other and are grouped into fascicles. Each fascicle is bound by an  endotendineum , which is a delicate loose connective tissue containing thin collagen fibrils [ 4 ] [ 5 ]  and elastic fibers. [ 6 ]  A set of fascicles is bound by an  epitenon , which is a sheath of  dense irregular connective tissue . The whole tendon is enclosed by a  fascia . The space between the fascia and the tendon tissue is filled with the  paratenon , a fatty  areolar tissue . [ 7 ]  Normal healthy tendons are anchored to bone by  Sharpey's fibres .


The dry mass of normal tendons, which is 30–45% of their total mass, is made of:


Although most of a tendon's collagen is  type I collagen , many minor collagens are present that play vital roles in tendon development and function. These include type II collagen in the  cartilaginous  zones, type III collagen in the  reticulin  fibres of the vascular walls, type IX collagen, type IV collagen in the basement membranes of the  capillaries , type V collagen in the vascular walls, and type X collagen in the mineralized fibrocartilage near the interface with the bone. [ 8 ] [ 12 ]


Collagen fibres coalesce into  macroaggregates . After secretion from the cell, cleaved by  procollagen  N- and C- proteases , the tropocollagen molecules spontaneously assemble into insoluble fibrils. A collagen molecule is about 300 nm long and 1–2 nm wide, and the diameter of the fibrils that are formed can range from 50–500 nm. In tendons, the fibrils then assemble further to form fascicles, which are about 10 mm in length with a diameter of 50–300 μm, and finally into a tendon fibre with a diameter of 100–500 μm. [ 13 ]


The collagen in tendons are held together with  proteoglycan  (a compound consisting of a protein bonded to glycosaminoglycan groups, present especially in connective tissue) components including  decorin  and, in compressed regions of tendon,  aggrecan , which are capable of binding to the collagen fibrils at specific locations. [ 14 ]  The proteoglycans are interwoven with the collagen fibrils –  their  glycosaminoglycan  (GAG) side chains have multiple interactions with the surface of the fibrils –  showing that the proteoglycans are important structurally in the interconnection of the fibrils. [ 15 ]  The major GAG components of the tendon are  dermatan sulfate  and  chondroitin sulfate , which associate with collagen and are involved in the fibril assembly process during tendon development. Dermatan sulfate is thought to be responsible for forming associations between fibrils, while chondroitin sulfate is thought to be more involved with occupying volume between the fibrils to keep them separated and help withstand deformation. [ 16 ]  The dermatan sulfate side chains of decorin aggregate in solution, and this behavior can assist with the assembly of the collagen fibrils. When decorin molecules are bound to a collagen fibril, their dermatan sulfate chains may extend and associate with other dermatan sulfate chains on decorin that is bound to separate fibrils, therefore creating interfibrillar bridges and eventually causing parallel alignment of the fibrils. [ 17 ]


The  tenocytes  produce the collagen molecules, which aggregate end-to-end and side-to-side to produce collagen fibrils. Fibril bundles are organized to form fibres with the elongated tenocytes closely packed between them. There is a three-dimensional network of cell processes associated with collagen in the tendon. The cells communicate with each other through  gap junctions , and this signalling gives them the ability to detect and respond to mechanical loading. [ 18 ]  These communications happen by two proteins essentially:  connexin 43 , present where the cells processes meet and in cell bodies  connexin 32 , present only where the processes meet. [ 19 ]


Blood vessels may be visualized within the endotendon running parallel to collagen fibres, with occasional branching transverse  anastomoses .


The internal tendon bulk is thought to contain no nerve fibres, but the epitenon and paratenon contain nerve endings, while  Golgi tendon organs  are present at the  myotendinous junction  between tendon and muscle.


Tendon length varies in all major groups and from person to person. Tendon length is, in practice, the deciding factor regarding actual and potential muscle size. For example, all other relevant biological factors being equal, a man with a shorter tendons and a longer biceps muscle will have greater potential for muscle mass than a man with a longer tendon and a shorter muscle. Successful  bodybuilders  will generally have shorter tendons. Conversely, in sports requiring athletes to excel in actions such as running or jumping, it is beneficial to have longer than average  Achilles tendon  and a shorter  calf muscle . [ 20 ]


Tendon length is determined by genetic predisposition, and has not been shown to either increase or decrease in response to environment, unlike muscles, which can be shortened by trauma, use imbalances and a lack of recovery and stretching. [ 21 ]  In addition tendons allow muscles to be at an optimal distance from the site where they actively engage in movement, passing through regions where space is premium, like the  carpal tunnel . [ 19 ]


There are about 4,000 tendons in the human body, of which 55 are listed here:


Naming convention for the table:


Traditionally, tendons have been considered to be a mechanism by which muscles connect to bone as well as muscles itself, functioning to transmit forces. This connection allows tendons to passively modulate forces during locomotion, providing additional stability with no active work. However, over the past two decades, much research has focused on the elastic properties of some tendons and their ability to function as springs. Not all tendons are required to perform the same functional role, with some predominantly positioning limbs, such as the fingers when writing (positional tendons) and others acting as springs to make locomotion more efficient (energy storing tendons). [ 22 ]  Energy storing tendons can store and recover energy at high efficiency. For example, during a human stride, the Achilles tendon stretches as the ankle joint dorsiflexes. During the last portion of the stride, as the foot plantar-flexes (pointing the toes down), the stored elastic energy is released. Furthermore, because the tendon stretches, the muscle is able to function with less or even  no change in length , allowing the muscle to generate more force.


The mechanical properties of the tendon are dependent on the collagen fiber diameter and orientation. The collagen fibrils are parallel to each other and closely packed, but show a wave-like appearance due to planar undulations, or crimps, on a scale of several micrometers. [ 23 ]  In tendons, the collagen fibres have some flexibility due to the absence of hydroxyproline and proline residues at specific locations in the amino acid sequence, which allows the formation of other conformations such as bends or internal loops in the triple helix and results in the development of crimps. [ 24 ]  The crimps in the collagen fibrils allow the tendons to have some flexibility as well as a low compressive stiffness. In addition, because the tendon is a multi-stranded structure made up of many partially independent fibrils and fascicles, it does not behave as a single rod, and this property also contributes to its flexibility. [ 25 ]


The  proteoglycan  components of tendons also are important to the mechanical properties. While the collagen fibrils allow tendons to resist tensile stress, the proteoglycans allow them to resist compressive stress. These molecules are very hydrophilic, meaning that they can absorb a large amount of water and therefore have a high swelling ratio. Since they are noncovalently bound to the fibrils, they may reversibly associate and disassociate so that the bridges between fibrils can be broken and reformed. This process may be involved in allowing the fibril to elongate and decrease in diameter under tension. [ 26 ]  However, the proteoglycans may also have a role in the tensile properties of tendon. The structure of tendon is effectively a fibre composite material, built as a series of hierarchical levels. At each level of the hierarchy, the collagen units are bound together by either collagen crosslinks, or the proteoglycans, to create a structure highly resistant to tensile load. [ 27 ]  The elongation and the strain of the collagen fibrils alone have been shown to be much lower than the total elongation and strain of the entire tendon under the same amount of stress, demonstrating that the proteoglycan-rich matrix must also undergo deformation, and stiffening of the matrix occurs at high strain rates. [ 28 ]  This deformation of the non-collagenous matrix occurs at all levels of the tendon hierarchy, and by modulating the organisation and structure of this matrix, the different mechanical properties required by different tendons can be achieved. [ 29 ]  Energy storing tendons have been shown to utilise significant amounts of sliding between fascicles to enable the high strain characteristics they require, whilst positional tendons rely more heavily on sliding between collagen fibres and fibrils. [ 30 ]  However, recent data suggests that energy storing tendons may also contain fascicles which are twisted, or helical, in nature - an arrangement that would be highly beneficial for providing the spring-like behaviour required in these tendons. [ 31 ]


Tendons are  viscoelastic  structures, which means they exhibit both elastic and viscous behaviour. When stretched, tendons exhibit typical "soft tissue" behavior. The force-extension, or stress-strain curve starts with a very low stiffness region, as the crimp structure straightens and the collagen fibres align suggesting negative Poisson's ratio in the fibres of the tendon. More recently, tests carried out in vivo (through  MRI ) and ex vivo (through mechanical testing of various cadaveric tendon tissue) have shown that healthy tendons are highly anisotropic and exhibit a negative Poisson's ratio ( auxetic ) in some planes when stretched up to 2% along their length, i.e. within their normal range of motion. [ 32 ]  After this 'toe' region, the structure becomes significantly stiffer, and has a linear stress-strain curve until it begins to fail. The mechanical properties of tendons vary widely, as they are matched to the functional requirements of the tendon. The energy storing tendons tend to be more elastic, or less stiff, so they can more easily store energy, whilst the stiffer positional tendons tend to be a little more viscoelastic, and less elastic, so they can provide finer control of movement. A typical energy storing tendon will fail at around 12–15% strain, and a stress in the region of 100–150 MPa, although some tendons are notably more extensible than this, for example the superficial digital flexor in the  horse , which stretches in excess of 20% when galloping. [ 33 ]  Positional tendons can fail at strains as low as 6–8%, but can have moduli in the region of 700–1000 MPa. [ 34 ]


Several studies have demonstrated that tendons respond to changes in mechanical loading with growth and remodeling processes, much like  bones . In particular, a study showed that disuse of the  Achilles tendon  in rats resulted in a decrease in the average thickness of the collagen fiber bundles comprising the tendon. [ 35 ]  In humans, an experiment in which people were subjected to a simulated micro-gravity environment found that tendon stiffness decreased significantly, even when subjects were required to perform restiveness exercises. [ 36 ]  These effects have implications in areas ranging from treatment of bedridden patients to the design of more effective exercises for  astronauts .


Tendons are subject to many types of injuries. There are various forms of  tendinopathies  or tendon injuries due to overuse. These types of injuries generally result in inflammation and degeneration or weakening of the tendons, which may eventually lead to  tendon rupture . [ 37 ]  Tendinopathies can be caused by a number of factors relating to the tendon extracellular matrix (ECM), and their classification has been difficult because their symptoms and histopathology often are similar.


Types of tendinopathy include: [ 38 ]


Tendinopathies may be caused by several intrinsic factors including age, body weight, and nutrition. The extrinsic factors are often related to sports and include excessive forces or loading, poor training techniques, and environmental conditions. [ 41 ]


It was believed that tendons could not undergo matrix turnover and that tenocytes were not capable of repair. However, it has since been shown that, throughout the lifetime of a person, tenocytes in the tendon actively synthesize matrix components as well as enzymes such as  matrix metalloproteinases  (MMPs) can degrade the matrix. [ 41 ]  Tendons are capable of healing and recovering from injuries in a process that is controlled by the tenocytes and their surrounding extracellular matrix.


The three main stages of tendon healing are inflammation, repair or proliferation, and remodeling, which can be further divided into consolidation and maturation. These stages can overlap with each other. In the first stage, inflammatory cells such as  neutrophils  are recruited to the injury site, along with  erythrocytes .  Monocytes  and  macrophages  are recruited within the first 24 hours, and  phagocytosis  of  necrotic  materials at the injury site occurs. After the release of  vasoactive  and  chemotactic  factors,  angiogenesis  and the  proliferation  of tenocytes are initiated. Tenocytes then move into the site and start to synthesize collagen III. [ 37 ] [ 40 ]  After a few days, the repair or proliferation stage begins. In this stage, the tenocytes are involved in the synthesis of large amounts of collagen and proteoglycans at the site of injury, and the levels of GAG and water are high. [ 42 ]  After about six weeks, the remodeling stage begins. The first part of this stage is consolidation, which lasts from about six to ten weeks after the injury. During this time, the synthesis of collagen and GAGs is decreased, and the cellularity is also decreased as the tissue becomes more fibrous as a result of increased production of collagen I and the fibrils become aligned in the direction of mechanical stress. [ 40 ]  The final maturation stage occurs after ten weeks, and during this time there is an increase in crosslinking of the collagen fibrils, which causes the tissue to become stiffer. Gradually, over about one year, the tissue will turn from fibrous to scar-like. [ 42 ]


Matrix metalloproteinases (MMPs) have a very important role in the degradation and remodeling of the ECM during the healing process after a tendon injury. Certain MMPs including MMP-1, MMP-2, MMP-8, MMP-13, and MMP-14 have collagenase activity, meaning that, unlike many other enzymes, they are capable of degrading collagen I fibrils. The degradation of the collagen fibrils by MMP-1 along with the presence of denatured collagen are factors that are believed to cause weakening of the tendon ECM and an increase in the potential for another rupture to occur. [ 43 ]  In response to repeated mechanical loading or injury,  cytokines  may be released by tenocytes and can induce the release of MMPs, causing degradation of the ECM and leading to recurring injury and chronic tendinopathies. [ 40 ]


A variety of other molecules are involved in tendon repair and regeneration. There are five growth factors that have been shown to be significantly upregulated and active during tendon healing:  insulin-like growth factor 1  (IGF-I),  platelet-derived growth factor  (PDGF),  vascular endothelial growth factor  (VEGF),  basic fibroblast growth factor  (bFGF), and  transforming growth factor beta  (TGF-β). [ 42 ]  These growth factors all have different roles during the healing process. IGF-1 increases collagen and proteoglycan production during the first stage of inflammation, and PDGF is also present during the early stages after injury and promotes the synthesis of other growth factors along with the synthesis of DNA and the proliferation of tendon cells. [ 42 ]  The three isoforms of TGF-β (TGF-β1, TGF-β2, TGF-β3) are known to play a role in wound healing and scar formation. [ 44 ]  VEGF is well known to promote angiogenesis and to induce endothelial cell proliferation and migration, and VEGF mRNA has been shown to be expressed at the site of tendon injuries along with collagen I mRNA. [ 45 ]  Bone morphogenetic proteins (BMPs) are a subgroup of TGF-β superfamily that can induce bone and cartilage formation as well as tissue differentiation, and BMP-12 specifically has been shown to influence formation and differentiation of tendon tissue and to promote fibrogenesis.


In animal models, extensive studies have been conducted to investigate the effects of mechanical strain in the form of activity level on tendon injury and healing. While stretching can disrupt healing during the initial inflammatory phase, it has been shown that controlled movement of the tendons after about one week following an acute injury can help to promote the synthesis of collagen by the tenocytes, leading to increased tensile strength and diameter of the healed tendons and fewer adhesions than tendons that are immobilized. In chronic tendon injuries, mechanical loading has also been shown to stimulate fibroblast proliferation and collagen synthesis along with collagen realignment, all of which promote repair and remodeling. [ 42 ]  To further support the theory that movement and activity assist in tendon healing, it has been shown that immobilization of the tendons after injury often has a negative effect on healing. In rabbits, collagen fascicles that are immobilized have shown decreased tensile strength, and immobilization also results in lower amounts of water, proteoglycans, and collagen crosslinks in the tendons. [ 37 ]


Several  mechanotransduction  mechanisms have been proposed as reasons for the response of tenocytes to mechanical force that enable them to alter their gene expression, protein synthesis, and cell phenotype, and eventually cause changes in tendon structure. A major factor is mechanical deformation of the  extracellular matrix , which can affect the  actin cytoskeleton  and therefore affect cell shape, motility, and function. Mechanical forces can be transmitted by focal adhesion sites,  integrins , and cell-cell junctions. Changes in the actin cytoskeleton can activate integrins, which mediate "outside-in" and "inside-out" signaling between the cell and the matrix.  G-proteins , which induce intracellular signaling cascades, may also be important, and ion channels are activated by stretching to allow ions such as calcium, sodium, or potassium to enter the cell. [ 42 ]


Sinew was widely used throughout  pre-industrial  eras as a tough, durable  fiber . Some specific uses include using sinew as  thread  for sewing, attaching feathers to arrows (see  fletch ), lashing tool blades to shafts, etc. It is also recommended in survival guides as a material from which strong cordage can be made for items like traps or living structures. Tendon must be treated in specific ways to function usefully for these purposes.  Inuit  and other  circumpolar people  utilized sinew as the only cordage for all domestic purposes due to the lack of other suitable fiber sources in their ecological habitats. The elastic properties of particular sinews were also used in  composite recurved bows  favoured by the steppe nomads of Eurasia, and Native Americans. The first stone throwing artillery also used the elastic properties of sinew.


Sinew makes for an excellent cordage material for three reasons: It is extremely strong, it contains natural glues, and it shrinks as it dries, doing away with the need for knots [ clarification needed ] .


Tendon (in particular,  beef  tendon) is used as a food in some Asian cuisines (often served at  yum cha  or  dim sum  restaurants). One popular dish is  suan bao niu jin , in which the tendon is marinated in garlic. It is also sometimes found in the Vietnamese noodle dish  phở .


In some organisms, notably  birds , [ 46 ]  and  ornithischian   dinosaurs , [ 47 ]  portions of the tendon can become ossified. In this process, osteocytes infiltrate the tendon and lay down bone as they would in sesamoid bone such as the patella. In birds, tendon ossification primarily occurs in the hindlimb, while in ornithischian dinosaurs, ossified axial muscle tendons form a latticework along the neural and haemal spines on the tail, presumably for support.


The  Achilles tendon  or  heel cord , also known as the  calcaneal tendon , is a  tendon  at the back of the lower  leg , and is the thickest in the human body. [ 1 ] [ 2 ] [ 3 ] [ 4 ] [ 5 ] [ 6 ] [ excessive citations ]  It serves to attach the  plantaris ,  gastrocnemius  (calf) and  soleus muscles  to the  calcaneus  (heel) bone. These muscles, acting via the tendon, cause  plantar flexion  of the foot at the  ankle joint , and (except the soleus)  flexion  at the  knee .


Abnormalities of the Achilles tendon include inflammation ( Achilles tendinitis ), degeneration,  rupture , and becoming embedded with cholesterol deposits ( xanthomas ).


The Achilles tendon was named in 1693 after the Greek hero  Achilles . [ 7 ]


The oldest-known written record of the tendon being named after  Achilles  is in 1693 by the Flemish/Dutch anatomist  Philip Verheyen . In his widely used text  Corporis Humani Anatomia  he described the tendon's location and said that it was commonly called "the cord of Achilles." [ 8 ] [ 9 ]  The tendon has been described as early as the time of  Hippocrates , who described it as the " tendo magnus " (Latin for "great tendon") [ dubious  –  discuss ]  and by subsequent anatomists prior to Verheyen as " chorda Hippocratis " (Latin for "Hippocrates' string"). [ 9 ]


Verheyen referred to the mythological account of Achilles being held by the heel by his mother  Thetis  when she dipped him in the  River Styx  as a baby to render his body invulnerable. As the heel by which she held him was not touched by the water, it was his one vulnerable spot (hence the expression " Achilles' heel ") and he was eventually killed by a poison dart to the heel. The name thus also refers to the particularly disabling and painful effect of an injury to this tendon. [ 9 ]  The first closed rupture was described by  Ambroise Pare  in the sixteenth century. [ 9 ]


The Achilles tendon is also known as the " tendo calcaneus " (Latin for "calcaneal tendon"). [ 9 ]  Because  eponyms  (names relating to people) have no relationship to the subject matter, most anatomical eponyms also have scientifically descriptive terms. The term calcaneal comes from the Latin  calcaneum , meaning heel.


The Achilles tendon connects muscle to bone, like other  tendons , and is located at the back of the lower leg. The Achilles tendon connects the  gastrocnemius  and  soleus  muscles to the  calcaneal tuberosity  on the  calcaneus  (heel bone). [ 10 ]  The tendon begins near the middle of the calf, and receives muscle fibers on its inner surface, particularly from the soleus muscle, almost to its lower end. Gradually thinning below, it inserts into the middle part of the back of the calcaneus bone. The tendon spreads out somewhat at its lower end so that its narrowest part is about 4 cm (1.6 in) above its insertion. [ 11 ]


The tendon is covered by the  fascia  and skin, and stands out prominently behind the bone; the gap is filled up with  areolar  and  adipose tissue . A  bursa  lies between the tendon and the upper part of the calcaneus. It is about 15 centimetres (6 in) long.


Along the side of the muscle, and superficial to it, is the  small saphenous vein . The  sural nerve  accompanies the small saphenous vein as it descends in the posterior leg, traveling inferolateral to it as it crosses the lateral border of the Achilles tendon. [ 12 ]  The tendon is the thickest tendon in the human body. [ 11 ]  It can receive a load stress 3.9 times body weight during walking and 7.7 times body weight when running. [ 13 ]


The blood supply to the Achilles tendon is poor, and mostly via a recurrent branch of the  posterior tibial artery , and some through arterial branches passing through surrounding muscles. [ 11 ]


Acting via the Achilles tendon, the gastrocnemius and soleus muscles cause  plantar flexion  of the foot at the  ankle . This action brings the sole of the foot closer to the back of the leg. The gastrocnemius also  flexes  the leg at the knee. Both muscles are innervated by the  tibial nerve . [ 14 ]  Because the fibres of the tendon spiral about 90 degrees, fibres from the gastrocnemius tend to attach to the outer part of the bone, whereas fibres from the soleus tend to attach closer to the midline. [ 11 ]