The Inhibition of Sympathetic Preganglionic Neurons by Somatic Afferents

The inhibitory effect of sciatic and ulnar nerve afferent stimulation on the firing frequency of sympathetic preganglionic neurons was studied in anesthetized or unanesthetized decerebrate cats, with intact spinal cords or with spinal cords sectioned at C2. The spontaneous firing and the firing evoked by antidromic stimulation, by iontophoretic glutamate, and by mechanical injury could be depressed, in the preparations both with intact and sectioned spinal cord. The depression was not preceded by excitation. The minimum stimulus strength required for the inhibition was, on average, 15 times the nerve threshold. The inhibition elicited by single shocks lasted several hundred milliseconds and was longer in the intact than in the spinal preparation. The results show that the neural pathway used by high threshold somatic afferents for inhibition of the sympathetic preganglionic neurons is complete within the spinal cord and suggest that the inhibition is probably acting on the preganglionic neuron membrane.