Further Studies on the Functional Properties of Spinal Axons in Vivo

Mammalian spinal tracts in situ demonstrate a phase of marked hyperexcitability during hypoxia or on the application of an excess of potassium or citrate ion. This is in keeping with the fact that they also show post-spike supernormality as well as hyperexcitability under cathodal polarization (17). Behavior of this kind indicates that central axons carry a well developed L fraction of membrane properties. The rhythmic state in central axons in situ, unlike peripheral nerve or spinal root, is not induced by the action of excess potassium ion. This appears to be related to the absence of a positive after-potential in dorsal columns (17). However, sodium citrate can elicit autonomous firing in central axons. When synchronized by an applied stimulus the resulting periodic oscillations have a fundamental frequency (340 to 400 C.P.S.) which is significantly greater than that of peripheral nerve.