As we have seen, action potentials can move down an axon without diminution at speeds up to 1 meter per second. But even such fast speeds are insufficient to permit the complex movements typical of animals. In humans, for instance, the cell bodies of motor neurons innervating leg muscles are located in the spinal cord, and the axons are about a meter in length. The coordinated muscle contractions required for walking, running, and similar movements would be impossible if it took one second for an action potential to move from the spinal cord down the axon of a motor neuron to a leg muscle. The presence of a myelin sheath around an axon increases the velocity of impulse conduction to 10-100 meters window Cytosol window Cytosol per second. As a result, in a typical human motor neuron, an action potential can travel the length of a 1-meter-long axon and stimulate a muscle to contract within 0.01 seconds.
In nonmyelinated neurons, the conduction velocity of an action potential is roughly proportional to the diameter of the axon, because a thicker axon will have a greater number of ions that can diffuse. The human brain is packed with relatively small, myelinated neurons. If the neurons in the
▲ FIGURE 7-39 Formation and structure of a myelin sheath in the peripheral nervous system. Myelinated axons are surrounded by an insulating layer of compressed membranes.
(a) Electron micrograph of a cross section through an axon (Ax) surrounded by a myelin sheath (MS) and Schwann cell (SN).
(b) At higher magnification this specialized spiral membrane appears as a series of layers, or lamellae, of phospholipid bilayers. In this image, the termini of both the outer and innermost wraps are evident (arrows). (c) As a Schwann cell repeatedly wraps around an axon, all the spaces between its plasma membranes, both cytosolic and exoplasmic, are reduced. Eventually all the cytosol is forced out and a structure of compact stacked plasma membranes, the myelin sheath, is formed. (d) The two most abundant membrane proteins, P0 and PMP22, in peripheral myelin are expressed only by Schwann cells. The exoplasmic human brain were not myelinated, their axonal diameters would have to increase about 10,000-fold to achieve the same conduction velocities as myelinated neurons. Thus vertebrate brains, with their densely packed neurons, never could have evolved without myelin.
The myelin sheath is a stack of specialized plasma membrane sheets produced by a glial cell that wraps itself around the axon (Figure 7-39). In the peripheral nervous system, domain of a P0 protein, which has an immunoglobulin fold, associates with similar domains emanating from P0 proteins in the opposite membrane surface, thereby "zippering" together the exoplasmic membrane surfaces in close apposition. These interactions are stabilized by binding of a tryptophan residue on the tip of the exoplasmic domain to lipids in the opposite membrane. Close apposition of the cytosolic faces of the membrane may result from binding of the cytosolic tail of each P0 protein to phospholipids in the opposite membrane. PMP22 may also contribute to membrane compaction. Myelin basic protein (MBP), a cytosolic protein, remains between the closely apposed membranes as the cytosol is squeezed out. [Parts (a) and (b) courtesy Grahame Kidd, Lerner Research Institute. Part (d) adapted from L. Shapiro et al., 1996, Neuron 17:435, and E. J. Arroyo and S. S. Scherer, 2000, Histochem. Cell Biol. 113:1.]
these glial cells are called Schwann cells. In the central nervous system, they are called oligodendrocytes. In both vertebrates and some invertebrates, glial cells accompany axons along their length, but specialization of these glial cells to form myelin occurs predominantly in vertebrates. Vertebrate glial cells that will later form myelin have on their surface a myelin-associated glycoprotein and other proteins that bind to adjacent axons and trigger the formation of myelin.
Figure 7-39d illustrates the formation and basic structure of a myelin sheath, which contains both membrane and cy-tosolic components. A myelin membrane, like all biomembranes, has a basic phospholipid bilayer structure, but it contains far fewer types of proteins than found in most other membranes. Two proteins predominate in the myelin membrane around peripheral axons: P0, which causes adjacent plasma membranes to stack tightly together, and PMP22. Gene knockout studies in mice have recently identified PMP22 as essential for myelination. In the central nervous system, a different membrane protein and a proteolipid together function similarly to P0. The major cytosolic protein in all myelin sheaths is myelin basic protein (MBP). Mice that contain the shiverer mutation in the MBP gene exhibit severe neurological problems, evidence for the importance of myelination in the normal functioning of the nervous system.
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