ACTION POTENTIAL PROPAGATION

A. Unmyelinated Axon Action Potential Conduction

1. Region undergoing an AP acts to depolarize surrounding regions

2. Adjacent regions reach threshold first, generating APs in the adjacent regions

3. New site of activity (adjacent region) depolarizes the excitable membrane adjacent to it

4. Sequence continues until AP has propagated along the whole axon

Question: When the AP reaches the ends of the axon, will it turn around and continue in the opposite direction from which it came?

B. Myelinated Axon Action Potential Conduction (Saltatory Conduction)

1. Functional anatomy: myelinated segments separated by nodes (Nodes of Ranvier)

Note that the density of Na channels at the nodes is higher than the density on the remainder of the axon, so the nodes have a lower threshold for excitation

2. Consequences of myelinization

a. myelin prevents current flow through the underlying axon membrane; thus current is confined to the nodes

b. AP propagation jumps from node to node ("saltatory" conduction)

c. conduction velocity is increased (compared to an unmyelinated axon of the same diameter)

Note: myelinated axons are white; cell bodies, dendrites, and unmylinated axons are gray

1. Determining Factors

a. axon diameter: radius increase => velocity increase

the larger the fiber diameter, the lower the core electrical resistance

b. myelination

increases conduction velocity, since membrane capacitance (or charge storage) is decreased and distance of passive spread is increased

2. Velocity values

a. unmyelinated axons

relatively slow, order of 1 meter/sec, because small and unmyelinated

b. myelinated axons

relatively fast, range of 10-120 meters/sec, because larger (up to 20 um) and myelinated

Note: 1 meter/sec = 3.6 kilometer/hr = 2.24 miles/hr