MAKING PERIPHERAL NERVES AND CENTRAL TRACTS
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| SHEATH AND SATELLITE CELL FORMATION |
Oligodendrocytes
in
the CNS and Schwann cells in the PNS form myelin sheaths by similar processes.
In an action similar to the continuous wrapping of a bolt of cloth, the
oligodendroglial cell membrane becomes wrapped around the axon many times. As
the wrapping occurs, the oligodendroglial cytoplasm
retracts or is extruded so that the two layers of the cell’s plasma membrane,
which originally were separated by cytoplasm, come together and fuse. Except
for small islands of cytoplasm, which may be trapped between the fused
membranes, the fusion is complete. The cell membrane of the myelinating
oligodendrocyte, like cell membranes elsewhere, is composed of alternate
layers of lipid and protein molecules. Thus myelin is made up of numerous fused
layers of lipoprotein membrane.
Myelination
is closely associated with the development of the functional capacity of
neurons. Unmyelinated neurons have a low conduction velocity and show fatigue
earlier, whereas myelinated neurons fire rapidly and have a long period of
activity before fatigue occurs. Neurons that ultimately are capable of rapid transmission
of impulses become fully functional at about the time their axons become
completely insulated with a myelin sheath. In general, the motor neurons of
cranial nerves become myelinated before their sensory counterparts. The sensory
neurons of the trigeminal nerve and the cochlear division of the
vestibulocochlear nerve begin to acquire myelin only in the fifth and sixth
months of development. The optic nerve neurons begin to be sheathed at birth,
and myelination is completed by the end of the second week after birth.
As
development continues, the nerve fibers (axons) of both the CNS and the PNS
eventually become sheathed or
encapsulated. In the PNS, neurons become completely encapsulated by parts of
other cells, except at their terminal endings and at the nodes of Ranvier. The
Schwann cell ensheathes both the myelinated and unmyelinated axons of somatic
motor neurons and pre- ganglionic autonomic motor neurons as they pass out of
the CNS. These cells, derived from both the neural crest and the wall of the neural
tube, also ensheath both the central and peripheral processes of the somatic
and visceral sensory neurons, as well as the axons of post-ganglionic
autonomic (sympathetic and parasympathetic) motor neurons.
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| DEVELOPMENT OF MYELINATION AND AXON ENSHEATHMENT |
Another
type of cell, which is derived from both the neural crest and the wall of the
neural tube and which participates in covering the neurons of the PNS, is the satellite
cell. Satellite cells completely encapsulate the cell
bodies of sensory neurons in the sensory ganglia of both the cranial and spinal
nerves, and also the post-ganglionic neurons of the sympathetic and
parasympathetic ganglia. Finally, there is a specialized glial cell that shares
properties of Schwann and satellite cells but is only found apposed to the
unmyelinated axons of the olfactory nerve; these axons originate from receptor
neurons in the nose that are continually replaced throughout life and thus must
regrow into the olfactory bulb in the CNS and make new synapses. These olfactory
ensheathing cells are apparently specialized to support the constant regrowth
of the axons and the establishment of new connections in the CNS. Accordingly,
there is great interest in these cells as a substrate for improve axon growth
in other regions of the CNS after injury.
