Accommodative reactions of neuronal elements in the spinal cord.

Abstract

The minimal gradient requirements (expressed as Hill''s "time constant of accommodation, [lambda] ~) was determined for neural elements of the spinal cords of cats anesthetized with Nembutal. This was done by applying linearly rising currents at various rates of rise through intracellular electrodes. Rectangular pulses of long duration were used for determinations of rheobase. Dorsal column fibers showed [lambda] values ranging from 5 to 70 msec. Ventral column fibers gave [lambda] values of 5-10 msec. Interneurons had [lambda] values ranging from 5-50 msec. Motoneuron [lambda] values varied between 5 and 1,700 msec. In 19% of the 80 motoneurons tested [lambda] could not be measured because it exceeded 2,000 msec. Motoneurons with short [lambda] values (less than 200 msec.) possessed some characteristics of "phasic" motoneurons while those with [lambda] s of over 500 msec, resembled to their characteristics "tonic" motoneurons. In all elements excitation occurred when rising currents approximated rheobasic strength. Stimulation of the reticular formation increased values, with or without changing the rheobase. Peripheral afferent nerve stimulations had effects similar to those produced by reticular formation stimulation. Hyperpolarization of the cell membrane by applied current flow reduced accommodative power. Minimal gradient requirement was reduced much more, percentagewise, than rheobase was increased. Subliminal depolarization had the opposite effects.