Biology · Ch 19 — Neural Control and Coordination
Conduction of the Nerve Impulse
Conduction of the Nerve Impulse
Generating an action potential at one point of the axon is only the first step; the impulse must then travel (be conducted) along the full length of the axon until it reaches the axon terminals. How this happens, and how fast, depends heavily on whether the fibre is myelinated.
In an unmyelinated fibre, the entire length of the axonal membrane is exposed and carries voltage-gated ion channels. When an action potential occurs at one point, the local depolarization spreads passively to the immediately adjacent patch of membrane, bringing it to threshold and triggering a fresh, full-sized action potential there; this new action potential then triggers the next patch, and so on — the impulse is regenerated continuously, point by point, all along the membrane. Because each of these regenerative steps takes a small but finite amount of time, and the fibre also loses signal strength gradually along its length between regenerations, conduction along an unmyelinated fibre is comparatively slow.
In a myelinated fibre, most of the axon's length is wrapped in the electrically insulating myelin sheath, and only the nodes of Ranvier — the short gaps between successive segments of myelin — have the membrane exposed and rich in voltage-gated channels. An action potential occurring at one node cannot regenerate along the insulated internodal segment; instead, the depolarizing current spreads passively (and very rapidly, since there is no ion movement needed under the myelin) along the inside of the axon until it reaches the next node, where it is strong enough to trigger a fresh action potential. The impulse therefore appears to 'jump' from node to node rather than travelling continuously — this is called saltatory conduction (from the Latin saltare, to leap). Because full regeneration of the impulse only needs to happen at the widely spaced nodes rather than at every point along the membrane, saltatory conduction is both considerably faster and more energy-efficient (since fewer ions need to be pumped ba …
Worked out. A schematic of a myelinated axon showing the impulse (marked by a series of arrows) jumping from one node of Ranvier to the next rather than travelling continuously along the membrane, because voltage-gated ion channels are concentrated only at the exposed nodes and the myelin-covered internodal segments are electrically insulated, which makes conduction markedly faster and more energy …