THE DISCHARGE BEHAVIOR OF SINGLE VASOCONSTRICTOR MOTONEURONS IN HUMANMUSCLE NERVES

1. The discharge behaviour of fourteen single sympathetic vasoconstric tor efferents was studied using a tungsten microelectrode inserted per cutaneously into a motor fascicle of the radial or peroneal nerve in e ight awake supine subjects. Units were classified as vasoconstrictor b ecause their firing properties correlated appropriately to changes in cardiac interval and arterial pressure. 2. On average, individual vaso constrictor units discharged in only 21% of heart beats, with an overa ll mean frequency of 0.47 Hz. Usually only one spike was generated per cardiac cycle. Calculated from cardiac cycles in which a unit fired f rom two to seven spikes, the mean within-burst firing rate was 18.8 +/ - 2.5 Hz (mean +/- S.E.M.); but instantaneous frequencies above 50 Hz were occasionally observed. 3. Measured from a defined R-wave of the E CG, the spike onset latency varied over 358 +/- 33 ms, suggesting cons iderable variation of synaptic delays in the baroreflex are. This late ncy had a relatively uniform temporal relationship with the burst onse t or peak latency, compatible with a fixed recruitment order of indivi dual sympathetic neurones. 4. In view of the low average firing rate o f individual units we suggest that the variable instantaneous firing r ates may optimize the contractile responses of vascular smooth muscle.