The nerve cell membrane is polarized at rest, with positive charges lined up along the outside of the membrane and negative charges along the inside. During the action potential, this polarity is abolished and for a brief period is actually reversed (Figure 1). Positive charges from the membrane ahead of and behind the action potential flow into the area of negativity represented by the action potential (“current sink”). By drawing off positive charges, this flow decreases the polarity of the membrane ahead of the action potential. Such electrotonic depolarization initiates a local response, and when the firing level is reached, a propagated response occurs that in turn electrotonically depolarizes the mem brane in front of it.
Fig1. Local current flow (movement of positive charges) around an impulse in an axon. Top: Unmyelinated axon. Bottom: Myelinated axon. Positive charges from the membrane ahead of and behind the action potential flow into the area of negativity represented by the action potential (“current sink”). In myelinated axons, depolarization appears to “jump” from one node of Ranvier to the next (saltatory conduction).
The spatial distribution of ion channels along the axon plays a key role in the initiation and regulation of the action potential. Voltage-gated Na+ channels are highly concentrated in the nodes of Ranvier and the initial segment in myelinated neurons. The number of Na+ channels per square micrometer of membrane in myelinated mammalian neurons has been estimated to be 50–75 in the cell body, 350–500 in the initial segment, less than 25 on the surface of the myelin, 2000–12,000 at the nodes of Ranvier, and 20–75 at the axon terminals. Along the axons of unmyelinated neurons, the number is about 110. In many myelinated neurons, the Na+ channels are flanked by K+ channels that are involved in repolarization.
Conduction in myelinated axons depends on a similar pattern of circular current flow as described above. However, myelin is an effective insulator, and current flow through it is negligible. Instead, depolarization in myelinated axons travels from one node of Ranvier to the next, with the current sink at the active node serving to electrotonically depolarize the node ahead of the action potential to the firing level (Figure 4–9). This “jumping” of depolarization from node to node is called saltatory conduction. It is a rapid process that allows myelinated axons to conduct up to 50 times faster than the fastest unmyelinated fibers.
