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

1. Pulse-synchronous multiunit muscle nerve sympathetic activity was recorded simultaneously from two nerves together with ECG in eleven healthy subjects; seven recordings were made from the two peroneal nerves during prolonged expiratory apnoeas and four from a radial and a peroneal nerve during lower body negative pressure of 10-40 mmHg. The neural 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 the peak of the burst). 2. Average baroreflex latencies were 1.3 s in the peroneal nerves and 0.9 s in the radial nerve. There were significant 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 shortened significantly when burst amplitude increased. The correlation between burst amplitude and baroreflex latency was weaker in the radial than in the peroneal nerve. The average variation of baroreflex latency 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 peroneal burst amplitudes increased, burst duration increased. This was due to increases of both the rise time and the decay time of the burst, the latter being the greater. 5. We suggest that the variation of baroreflex latency is due to changes of supraspinal synaptic delays or pathways and/or to recruitment of faster conducting sympathetic neurones when bursts become stronger.