Monosynaptic activation of different portions of the motor neuron membrane

Abstract

Using isolated frog spinal cords, treated with pentobarbital to silence internuncial discharge, intracellular recordings from motor neurons reveal differences in dendritically initiated EPSP evoked by dorsal root stimulation and somatically initiated EPSP evoked by lateral column stimulation. Under these conditions, dorsal root EPSP never reached motor neuron threshold whereas threshold was easily reached by lateral column EPSP. EPSP's initiated by dorsal root volleys were slower in their time course and smaller in their amplitude than those initiated by lateral column volleys. EPSP's initiated by lateral column volleys reduced the amplitude of antidromic spikes, while those produced by dorsal root stimulation did not. Lateral column induced EPSP was also capable of reducing the amplitude of orthodromic spikes. Some observations on duration of transmitter action are reported. It is concluded that dendritic excitation following a dorsal root volley influences the level of polarization of the cell body by electrotonic propagation of the resulting EPSP.