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FIGURE 3.3 Ultrastructure of dendritic spines and synapses in the
human brain.
A and B: Narrow spine necks (asterisks) emanate from
the main dendritic shaft (D).
The spine heads (S) contain fi lamentous
material (A, B).
Some large spines contain cisterns of a spine apparatus
(sa, B).
Dendrites and axons contain
numerous mitochondria (m).
Scale bar = 1 μm (A, B) and 0.6 μm (C).
Electron micrographs courtesy of Drs S.A.
Kirov and M.
Witcher
(Medical College of Georgia), and K.M.
Harris (University of Texas –
Austin).
CELLULAR AND MOLECULAR NEUROSCIENCE
illnesses, chronic alcoholism, schizophrenia, trisomy
21).
At the ultrastructural level (Fig.
These
fi laments most likely consist of actin and a- and b-
tubulins.
Microtubules and neurofi laments present in
dendritic shafts do not enter spines.
Those cisterns are referred to as the spine appara-
tus.
SPECIFIC EXAMPLES OF DIFFERENT NEURONAL TYPES 45
46 3.
CELLULAR COMPONENTS OF NERVOUS TISSUE
II.
3.1).
Dendrites rarely leave
the layer in which the cell body resides.
In cross-section,
the cell body is roughly triangular (Fig.
3.4) (Jones, 1975).
Dif-
ferent types of motor neurons are distinguished by
their targets.
They have a large number of
spiny dendrites that arborize locally within the ventral
horn.
Spinal motor neurons use
acetylcholine as their neurotransmitter.
These neurons are
medium-sized to large, fusiform, and frequently elon-
gated.
They have several large radiating dendrites.
Some of them contain both calbindin
and calretinin.
In the peripheral nervous system
(PNS), the Schwann cell is the major neuroglial
component.
In
invertebrates, the diameters of axons are enlarged.
In
vertebrates, the myelin sheath (Fig.
3.5) evolved to
permit rapid nerve conduction.
Adapted from Jones (1984).
NEUROGLIA 47
48 3.
CELLULAR COMPONENTS OF NERVOUS TISSUE
II.
CELLULAR AND MOLECULAR NEUROSCIENCE
conducts the action potential at speeds of 10 to 20 m/s.
As the axon mediates an escape refl ex, fi ring must be
rapid if the animal is to survive.
In the central nervous system, myelin sheaths (Fig.
3.6) are elaborated by oligodendrocytes.
The tightly compacted multilayer myelin sheath (My) surrounds
and insulates the axon (Ax).
Mit, mitochondria.
Scale bar: 75 nm.
myelin around them (Bunge, 1968).
At the end of each myelin segment, there is a bare
portion of the axon, the node of Ranvier.
Myelin seg-
ments are thus called internodes.
3.6), thus reducing the number of glial cells
required.
Brain volume is thus reserved for
further expansion of neuronal populations.
Myelin in the PNS is generated by Schwann
cells (Fig.
3.7), which are different to oligodendrocytes
in several ways.
Individual myelinating Schwann cells
form a single internode.
The biochemical composition
of PNS and CNS myelin differs, as discussed later.
Schwann cell and fi broblast-derived colla-
gens prevent normal wear-and-tear compression
damage.
II.
This protein makes up about 80% of
the protein complement of PNS myelin.
3.5).