5.9 Conductionvelocitiesofunmyelinated(blue) andmyelinated(red) felineaxonsasfunctionsofaxondiameter.Solid lines representmeasureddata.Dotted lines representextrapolationsthatshowtheadvantageofmyelinationoversimplyincreasingaxondiameterasamechanismforincreasedconductionvelocity.(FromSchmidt-Nielsen
K.
Animal Physiology: Adaptation and Environment. 5thed.Cambridge,UK:CambridgeUniversityPress;1997.) flow down the axon decreases more than it does to current flow across the membrane. This increases the length constant (see
Fig. 5.3 ), which means that a greater amount of the current entering at one site is delivered to neighboring regions of the axon, which brings those regions to threshold more quickly, and thus the action potential is conducted faster along fibers with large diameters.
However, increasing the diameter also increases the surface area of the plasma membrane over which inner negative and outer positive charges are held to each other. Discharging this increased capacitance tends to slow conduction and mitigate the increase in conduction velocity gained by increasing diameter.
In vertebrates, many nerve fibers are coated with myelin, and such fibers are said to be myelinated. Myelin consists of the plasma membranes of Schwann cells (in the peripheral nervous system) or oligodendroglia (in the central nervous system [CNS]), which wrap around and insulate the nerve fiber (
Fig. 5.10A and B ). The myelin sheath consists of several to more than 100 layers of glial cell plasma membrane. Gaps about 1-2 µm wide, known as nodes of Ranvier, separate the contribution of one Schwann cell (or oligodendrocyte) from that of another. For all but the axons of smallest diameter, a myelinated axon has much greater conduction velocity than does an unmyelinated fiber of the same caliber because the myelin sheath increases the effective membrane resistance of the axon, decreases the capacitance of the axon membrane, and limits the generation of action potentials to the nodes of Ranvier. In short, myelination greatly alters the electrical properties of the axon.
Because the many wrappings of membrane around the axon increase the effective membrane resistance rm/ra and the length constant are much greater. The increased membrane resistance means that less current is lost through the membrane per length of axon, and thus the amplitude of a conducted signal decreases less with distance along the axon and needs to be regenerated (by opening of Na+ channels) less often.
In addition, the thicker myelin-wrapped membrane results in a much larger separation of charges across it than exists across the bare membrane of an axon, so that the charges across it are much less tightly bound to each other. This is analogous to when the plates of a capacitor are moved apart and reduce its capacitance. Because the effect of membrane capacitance is to slow the rate at which the membrane potential can be changed, the reduced capacitance of myelinated axons means that the depolarization occurs more rapidly. For all these reasons, conduction velocity is greatly increased by myelination, and the current generated at one node of Ranvier is conducted at great speed to the next (see
Fig. 5.10
In myelinated axons, the Na+ channels that bring about generation of an action potential are highly concentrated