Differences in somatic and dendritic specific membrane resistivity of spinal motoneurons: an electrophysiological study of neck and shoulder motoneurons in the cat.

Abstract

1. The voltage response to a hyperpolarizing current step was used to estimate the electrotonic structure of neck and shoulder motoneurons in anesthetized cats. The coefficients (a0 and a1) and time constants (tau 0 and tau 1) of the first two exponential terms of the series of exponential terms whose sum represented the slope of the voltage response were calculated using a standardized "peeling" technique. 2. Input resistance, membrane time constant, and electrotonic length were similar to values reported for other spinal motoneurons. The voltage response of most neck and shoulder motoneurons had a slow, nonlinear component, commonly known as sag, which is also a feature of other spinal motoneurons. Estimates of motoneuron surface area were up to two times larger than the surface area of hindlimb motoneurons. 3. It was possible to estimate electrotonic length and the ratio of the dendritic conductance to the somatic conductance using Johnston's technique for only 6 of 51 motoneurons examined. The responses of these motoneurons were identical to the responses of equivalent cylinder models that had an electrotonic structure derived from the application of Johnston's technique. 4. We were unable to use Johnston's technique for the remaining motoneurons because the ratio, a1/a0, exceeded 2.0. For these motoneurons it was usually impossible to find an equivalent cylinder model whose response matched the experimental data. The failure of equivalent cylinder models to accurately predict the responses of neck and shoulder motoneurons could not be attributed to the sag process or the dendritic geometry of these cells. 5. The electrotonic structure of a group of these motoneurons was further examined using the somatic shunt model developed by Durand and Kawato. After shifting the base line of the experimental records by 1-2 mV in the depolarizing direction, it was possible to find a somatic shunt model whose response was identical to the experimentally recorded voltage response.(ABSTRACT TRUNCATED AT 400 WORDS)