When an axon in a peripheral nerve is cut, the distal portion of the axon that was severed from the cell body degenerates and is phagocytosed by Schwann cells. The Schwann cells, surrounded by the basement membrane, then form a regeneration tube (fig. 7.9) as the part of the axon that is connected to the cell body begins to grow and exhibit amoeboid movement. The Schwann cells of the regeneration tube are believed to secrete chemicals that attract the growing axon tip, and the regeneration tube helps to guide the regenerating axon to its proper destination. Even a severed major nerve may be surgically reconnected—and the function of the nerve largely reestablished—if the surgery is performed before tissue death occurs.
Fox: Human Physiology, Eighth Edition
7. The Nervous System: Neurons and Synapses
© The McGraw-H Companies, 2003
■ Figure 7.9 The process of peripheral neuron regeneration. (a) If a neuron is severed through a myelinated axon, the proximal portion may survive, but (b) the distal portion will degenerate through phagocytosis. The myelin sheath provides a pathway (c) and (d) for the regeneration of an axon, and (e) innervation is restored.
Motor neuron cell body
Site of injury m f
Distal portion of nerve fiber degenerates and is phagocytosed
Proximal end of injured nerve fiber regenerating into tube of Schwann cells
Skeletal muscle fiber
Former connection reestablished
Multiple sclerosis (MS) is a neurological disease usually diagnosed in people between the ages of 20 and 40. It is a chronic, degenerating, remitting, and relapsing disease that progressively destroys the myelin sheaths of neurons in multiple areas of the CNS. Initially, lesions form on the myelin sheaths and soon develop into hardened scleroses, or scars (from the Greek word sklerosis, meaning "hardened"). Destruction of the myelin sheaths prohibits the normal conduction of impulses, resulting in a progressive loss of functions. Because myelin degeneration is widespread and affects different areas of the nervous system in different people, MS has a wider variety of symptoms than any other neurological disease. Although the causes of MS are not fully known, there is evidence that the disease involves a genetic susceptibility combined with an immune attack on the oligo-dendrocytes and myelin, perhaps triggered by viruses. Inflammation and demyelination then occur, leading to the symptoms of MS.
Injury in the CNS stimulates growth of axon collaterals, but central axons have a much more limited ability to regenerate than peripheral axons. This may be due in part to the absence of a continuous neurilemma (as is present in the PNS), which precludes the formation of a regeneration tube, and to inhibitory molecules produced by oligodendrocytes and astrocytes in the injured CNS. In addition to the limited ability of CNS neurons to regenerate, injury to the spinal cord has recently been shown to actually evoke apoptosis (cell suicide—chapter 3) in neurons that were not directly damaged by the injury.
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