Viregeneration Of Nerve Cells

A. CNS. Effective regeneration does not occur in the CNS. For example, there is no regeneration of the optic nerve, which is a tract of the diencephalon. There are no basement membranes or endoneural investments surrounding the axons of the CNS.

B. PNS. Regeneration does occur in the PNS. The proximal tip of a severed axon grows into the endoneural tube, which consists of Schwann cell basement membrane and endoneurium. The axon sprout grows at the rate of 3 mm/day.

VII. GLIAL CELLS are the nonneural cells of the nervous system. A. Macroglia consist of astrocytes and oligodendrocytes.

1. Astrocytes perform the following functions:

a. They project foot processes that envelop the basement membrane of capillaries, neurons, and synapses.

b. They form the external and internal glial-Iimiting membranes of the CNS.

C. They play a role in the metabolism of certain neurotransmitters [e.g., 7-

aminobutyric acid (GABA), serotonin, glutamate].

d. They buffer the potassium concentration of the extracellular space.

B. They form glial scars in damaged areas of the brain (i.e., astrogliosis).

f. They contain glial fibrillary acidic protein (GFAP), which is a marker for astrocytes.

g. They contain glutamine synthetase, another biochemical marker for astrocytes.

h. May be identified with monoclonal antibodies (e.g., A2B5).

2. Oligodendrocytes are the myelin-forming cells of the CNS. One oligodendrocyte can myelinate as many as 30 axons.

B. Microglia arise from monocytes and function as the scavenger cells (phagocytes) of the CNS.

C. Ependymal cells are ciliated cells that line the central canal and ventricles of the brain. They also line the luminal surface of the choroid plexus. These cells produce cerebrospinal fluid (CSF).

D. Tanycytes are modified ependymal cells that contact capillaries and neurons. They mediate cellular transport between the ventricles and the neuropil. They project to hypothalamic nuclei that regulate the release of gonadotropic hormone from the adenohypophysis.

E. Schwann cells are derived from the neural crest. They are the myelin-forming cells of the PNS. One Schwann cell can myelinate only one internode. Schwann cells invest all myelinated and unmyelinated axons of the PNS and are separated from each other by the nodes of Ranvier.

VIII. THE BLOOD-BRAIN BARRIER consists of the tight junctions of nonfenestrated endothelial cells; some authorities include the astrocytic foot processes. Infarction of brain tissue destroys the tight junctions of endothelial cells and results in vasogenic edema, which is an infiltrate of plasma into the extracellular space.

IX. THE BLOOD-CSF BARRIER consists of the tight junctions between the cuboidal epithelial cells of the choroid plexus. The barrier is permeable to some circulating peptides (e.g., insulin) and plasma proteins (e.g., prealbumin).

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