Conduction Velocity of the Action Potential
Main Factors:
Cell diameter: The greater the cell
diameter, the greater the conduction velocity.
Myelin: The greater the myelination, the
greater the conduction velocity
- Large myelinated fibers = fast conduction
- Small unmyelinated fibers = slow conduction
Myelin sheaths are produced by glia:
oligodendrocytes in the CNS and Schwann cells in the PNS. They wrap many layers
of lipid membranes that act as excellent insulators.
In myelinated axons, voltage-gated sodium
channels are restricted to gaps in the myelin sheath called nodes of Ranvier, as well as the contact between the extracellular fluid and the axonal membrane. As a
result, action potentials are not generated in the regions between the nodes;
instead, the current generated by an action potential at a node travels within
the axon all the way to the next node. This mechanism for propagating action
potentials is called saltatory conduction. The action potential appears to be jumping from node to node
along the axon.
The major selective advantage of
myelination is its space efficiency. Recall that both cell diameter and
myelination can increase the conduction velocity of an action potential. However, a myelinated axon 20
μm in diameter has a conduction speed
faster than an unmyelinated axon of diameter 40 times greater. Hence, more
myelinated axons can be packed into the space occupied by just one giant axon.
 |
| Propagation of action potentials in a myelinated axon |
Reference:
Campbell, et al. Biology: A Global Approach. 11th ed., Pearson, 2017.
Robert B. Dunn. 2002. USMLE Step 1: Physiology Notes.
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