Stabilization of the discharge rate of sympathetic preganglionic neurons.

A characteristic feature of the sympathetic preganglionic neuron (SPN) is the low rate of firing during both tonic and evoked activity. Firing rates between 1 and 2 Hz are typical of tonic activity, and the rates increase only slightly during sustained reflex activation. The low mean firing rate of the SPN may result from mechanisms which depress the excitability of the neuron and /or from a very low synaptic efficacy of its excitatory inputs. In recent years depressant mechanisms, other than baroreceptor inhibition, have been identified which may be involved in the control of SPN firing rate. Some of these mechanisms are spinal. This paper reviews data on 3 depressant mechanisms, namely post-impulse depression, recurrent inhibition and inhibition by myelinated spinal afferents, which are operating within the spinal cord.