Peripheral nerves consist of fascicles that contain myelinated and unmyelinated axons. Endoneurium is the small amount of matrix that is present between individual axons. The perineurium is a sheath of special, fiber-like cells that ties the axons of each fascicle together. Epineurium is the connective tissue that surrounds the entire nerve trunk and gives off vascular connective tissue septa that traverse the nerve and separate fascicles from one another. | |
Single myelinated axon | Normal nerve |
Axons thicker than one micron in the CNS and peripheral nervous system (PNS) are myelinated. Myelin is a spiral sheet of cell membrane wrapped around the axon. In the CNS, myelin is produced by oligodendroglial cells and in the PNS by Schwann cells. Each oligodendrocyte makes multiple segments of myelin that wrap around many axons. Each Schwann cell makes one segment of myelin. This is one reason why peripheral myelin regenerates more efficiently. Nodes of Ranvier are points of discontinuity between adjacent myelin sheaths in which the axon is not covered by myelin. Unmyelinated axons are covered by Schwann cell cytoplasm, but there is no spiraling of Schwann cell membrane around them. The structure of central and peripheral myelin is essentially the same. Myelin is composed of 70% lipids and 30% protein. There are some important differences in myelin proteins between CNS and PNS. These differences explain why an allergic reaction against PNS myelin does not cause central demyelination and vice versa; and why inherited metabolic disorders of myelin proteins that affect peripheral nerves do not damage central myelin. On the other hand, lipids are similar between PNS and CNS myelin. For this reason, metabolic disorders of myelin lipids, such as metachromatic leukodystrophy, affect both, the central white matter and peripheral nerves. The myelin sheath acts as an electrical insulator, preventing short-circuiting between axons. More important, it facilitates conduction. The nodes of Ranvier are the only points where the axon is uncovered by myelin and ions can be exchanged between it and the extracellular fluid. Depolarization of the axonal membrane at the nodes of Ranvier boosts the action potential that is transmitted along the axon and is the basis of saltatory (jumping) conduction. Pathological Patterns of Neuropathy
The pathology of peripheral neuropathy follows three basic patterns: Wallerian degeneration, distal axonopathy, and segmental demyelination. Wallerian degeneration. The neuronal cell body maintains the axon through the axoplasmic flow. When an axon is transected, its distal part, including the myelin sheath, undergoes a series of changes leading to its complete structural disintegration and chemical degradation.
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Acute neuropathy | Wallerian degeneration | Wallerian degeneration| Changes also occur in the neuronal body. The RER disaggregates and the neuronal body balloons. The cytoplasm becomes smooth and the nucleus is displaced toward the periphery of the cell. This process is called central chromatolysis and reflects activation of protein synthesis in order to regenerate the axon. Cytoskeletal proteins and other materials flow down the axon. The proximal stump elongates at a rate of 1 to 3 mm per day. Schwann cells distal to the transection also proliferate and make new myelin. | | |
Lipid material in acute neuropathy | Axonal sprouts | Traumatic neuroma | The degree of regeneration and recovery depends on how well the cut ends are put together and on the extent of soft tissue injury and scarring around the area of transection. If reconstruction is not good, a haphazard proliferation of collagen, Schwann cell processes, and axonal sprouts fill the gap, forming a traumatic neuroma. Wallerian degeneration was initially described in experimental axotomy. Neuropathies characterized by Wallerian degeneration include those that are caused by...