Nerves
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Introduction
Nerves are bundles of axons outside the central nervous system.
Cranial nerves leave the brain to innervate primarily the head.
Spinal nerves leave the spinal cord to innervate the rest of the body.
In typical human nerves, unmyelinated axons outnumber myelinated axons by 2:1.
Nerve Structure
Fascicles, surrounded by perinurium, contain axons. Fascicles anastamose with each other, and axons can move between them.
Myelin
The myelin sheath serves to increase action potential propogation in the nervous system. Myelin is wrapped around most neuronal axons by Schwann cells in the PNS and oligodendrocytes in the CNS, accounting for up to 40% of fibre diameter. Large fibres may have as much as 300 layers, while small fibres have as few as 2 or 3. As layers are wrapped, cytoplasm is squeezed out, leaving membrane layers in close contact with each other. This increases resistance and decreases capacitance of myelin, making it ideal for preventing ion flow across membranes.
Myelin function
Myelin increases neuronal communication, increasing axonal transmission speeds by 10x. Saltatory conduction is the leaping of action potentials under the myelin sheath between Nodes of Ranvier. Sodium channels are clustered at the N of R, leading to ease of reaching threshold. In this way, the excitable membrane is spread across the neuron and leads to signal being propogated almost simultaneously.
Potassium channels are present under the myelin.
Myelin also is involved in axonal integrity, increasing diameter and affecting ion channel localization to nodes of Ranvier. It also increases the number of neurofilaments, major constituents of the axonal cytoskeleton.
Myelin Composition
Myelin contains various proteins, including the P0 protein, myelin basic protein (MBP), P2 protein, and PMP-22 in the PNS and proteolipid protein (PLP) and MBP in the CNS.
MBP is a highly charged extrinsic membrane protein which binds negatively charged lipids.
E-cadherin and beta-catenin are involved in the warpping of myelin layers.
Demyelination in Disease
Many neurologic diseases are caused by demyelination. Depending on the degree of myelin loss, decreased conduction velocity, failure to transmit high-frequency trains of action potentials, or total conduction block can occur. Internodal time can increase from 20 μsec to 500 μsec. Positive conduction abnormalities can also result, with ectopic impulse generation, increased mechanosensitivity, or fibre cross talk occurring.
Conduction through demyelinated axons is very sensitive to temparature. Increased temperature can produce increased conduction block. Lower temperatures slow gating kinetics of Na+ channels, increasing action potential duration and slowing conduction velocity. These broader spikes make it more likely that threshold will be reached and conduction will continue
- Mulitple Sclerosis is the most common demyelinating disease, and is characterized by chronic autoimmune episodes of neurologic deficits.
- Guillain-Barre Syndrome is an acute post-viral autoimmune disease where myelin is attacked.
Nerve Course
In thoracic nerves, subdivision happens in an orderly fashion. In cervical and lumbosacral areas, however, nerve plexuses form, with complex paths ensuing.
Resources and References
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