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and the exit of additional potassium their cell membrane, and thus the propa­
gation of action potentials along unmyeli­
repolarizes the membrane. The rapid exit
VetBooks.ir of potassium produces a small afterhyper- nated axons is as described above. Such
axons are relatively slow in conduction,
polarization (more negative than normal
resting potential) until resting conditions since each portion of the membrane must
can be re‐established (Fig. 11‐3). undergo the steps involved in depolarization
When an action potential occurs on the and repolarization as the action potential
axon of a neuron, the membrane potential makes its way along the axon.
of nearby areas is depolarized by local For faster conduction, vertebrate nervous
movement of charge. This causes the systems employ myelin, a fatty wrapping
sodium channels in the adjacent area to which is a good insulator against ionic flows.
reach their threshold voltage, and the Each cell that forms myelin (Schwann cells
events of the action potential are created in the PNS and oligodendrocytes in the
in those adjacent regions. This influences central nervous system [CNS]) covers
further regions yet again, and by this means about 1 mm along the axon. A small gap,
action potentials can be propagated along the node of Ranvier, occurs at the junction
axons (Fig. 2‐16). Propagation normally between wrappings of myelin, and it is
occurs in only one direction, primarily here that voltage‐gated sodium channels
because the sodium channels where the are preferentially concentrated. Therefore,
action potential just occurred are briefly the current created by depolarization of
insensitive or refractory to another stim­ the axonal membrane spreads from node
ulus. This refractory period is a charac­ to node, initiating the action potential only
teristic of normal sodium channels; the at these restricted sites and effectively
refractory period prevents the action skipping the intervening membrane cov­
potential from propagating back “up” the ered with myelin. Since this is analogous
axon (back toward the cell body), but it is to jumping from one node to the next, it is
brief, so that the axon is able to transmit called saltatory conduction (from Latin
action potentials at a high frequency. saltare, to jump) (Fig. 11‐4). This type of
conduction contributes to the increased
Conduction Velocity and Myelination rate of impulse conduction in myelinated
axons.
Conduction velocities of axons also
As described in Chapter 10, the axons of depend on their diameter. Large‐diameter
neurons may be either unmyelinated or axons propagate action potentials at higher
myelinated. Unmyelinated axons have velocities than do small‐diameter axons,
voltage‐gated sodium channels throughout
because large axons have less internal

Myelin Nodes of Ranvier

Action potential

Figure 11-4. Propagation of the action potential in a myelinated axon by saltatory conduction. After
initiation at the axon hillock, action potentials “leap” from node to node along the axon.