COUPLING BETWEEN VARIATIONS IN STRENGTH AND BAROREFLEX LATENCY OF SYMPATHETIC DISCHARGES IN HUMAN MUSCLE NERVES

1. Pulse-synchronous multiunit muscle nerve sympathetic activity was r ecorded simultaneously from two nerves together with ECG in eleven hea lthy subjects; seven recordings were made from the two peroneal nerves during prolonged expiratory apnoeas and four from a radial and a pero neal nerve during lower body negative pressure of 10-40 mmHg. The neur al records were displayed in mean voltage neurograms (time constant 0. 1 s) and for each mean voltage burst the following measures were taken and related to each other: amplitude, duration, rise time, decay time and baroreflex latency (from the appropriate R-wave of the ECG; to th e peak of the burst). 2. Average baroreflex latencies were 1.3 s in th e peroneal nerves and 0.9 s in the radial nerve. There were significan t positive correlations between both the amplitudes and the baroreflex latencies of corresponding bursts in peroneal-peroneal recordings and in radial-peroneal recordings. 3. In all nerves baroreflex latency sh ortened significantly when burst amplitude increased. The correlation between burst amplitude and baroreflex latency was weaker in the radia l than in the peroneal nerve. The average variation of baroreflex late ncy in peroneal-peroneal recordings was 0.20 +/- 0.02 s in both legs, and in radial-peroneal recordings the variation was 0.09 +/- 0.01 s in the radial nerve and 0.12 +/- 0.02 s in the peroneal nerve. 4. When p eroneal burst amplitudes increased, burst duration increased. This was due to increases of both the rise time and the decay time of the burs t, the latter being the greater. 5. We suggest that the variation of b aroreflex latency is due to changes of supraspinal synaptic delays or pathways and/or to recruitment of faster conducting sympathetic neuron es when bursts become stronger.