Some properties of preganglionic neurons in upper thoracic spinal cord of the cat.

Abstract

Intracellular recordings were made from sympathetic preganglionic neurons located in the upper thoracic spinal cord of anesthetized cats. The cells were identified by their antidromic responses to stimulation of the white rami of T2 and T3. The action potentials recorded from them had prolonged durations between 4-5 ms and were followed by pronounced afterhyperpolarizations. The passive electrical properties of the cells, determined by passing current pulses through the recording microelectrode, were more similar to those of autonomic ganglion cells than to those of somatic motoneurons. If reliance can be placed on these measurements, the values obtained would be consistent with the relative dimensions of these neurons. In most neurons, natural activity took the form of ongoing subthreshold synaptic potentials; action potentials were intermittently initiated from this activity and no evidence of pacemaker-type potentials intrinsic to the neurons was obtained. When the pressure in an ipsilateral isolated carotid sinus was decreased, the rate of generation of action potentials was increased in 4 of 7 neurons tested. During increases in arterial blood pressure or imposed pulsation of ipsilateral arterial baroreceptor perfusion pressure, the rate of discharge of action potentials was decreased. This decreased rate of discharge was associated with membrane hyperpolarization and the occurrence of discrete inhibitory synaptic potentials. The discharge of sympathetic preganglionic neurons depends on the integrative response of the neuronal membrane to both excitatory and inhibitory synaptic inputs.