Biological Sciences 300/301, Smith College | Neurophysiology

Case 4: The case of the missing channels.

The picture of voltage-dependent channels developed for the squid axon was assumed to apply to all axons, including vertebrate myelinated axons. A family of voltage-clamp currents for the node of Ranvier in a frog axon, below, seems to bear that out; the currents look similar to currents in a squid axon.

A frog node of Ranvier under voltage clamp is held at -95 mV, and then depolarized to various potentials ranging from -60 to +60 mV in 15-mV steps. Leakage and capacity currents are subtracted by a computer. (Adapted from Hille, 1967.)

When nodes of Ranvier from a mammalian axon (rabbit nerve) were studied, the picture was quite different. Here are voltage clamp records for a rabbit myelinated axon before (A, normal) and after (B) osmotic treatment designed to loosen the myelin sheath. Which channels seem to be present in the node? Which channels are missing in A? Are they present in B? What's going on?!
(From Chiu & Ritchie, Nature 284:170, 1980.)

Additional figures of nodes stained by immunocytochemistry (*Smith campus only):

Gial cells and myelin sheath sciatic nerve ICC&EM optic nerve sciatic&optic