1. Simultaneous recordings of multi-unit muscle nerve sympathetic activity and arterial blood pressure were made in 29 subjects, 17 healthy and 12 hypertensive. The neural activity, quantified by counting the number of pulse-synchronous sympathetic bursts in the mean voltage neurogram (burst incidence), was plotted against blood pressure. The effect of spontaneous temporary blood pressure fluctuations was studied by correlating different pressure parameters of individual heart beats to the occurrence of a sympathetic burst.
2. Between subjects there were marked differences in burst incidence but no correlation was found to interindividual differences in blood pressure level.
3. When for each heart beat the occurrence of a burst was correlated to different pressure parameters there was a close negative correlation to diastolic, a low correlation to systolic, and an intermediate negative correlation to mean blood pressure.
4. In a given subject, when comparing heart beats with the same diastolic pressure, the occurrence and the amplitudes of the sympathetic bursts were higher during falling than during rising pressure. This directional dependence of the muscle—nerve sympathetic activity was slightly more pronounced in the hypertensive group, but this was considered secondary to the hypertension.
5. The findings of an intimate correlation with dynamic variations in blood pressure and the absence of correlation to the static blood pressure suggest that the sympathetic outflow to skeletal muscle is of importance for buffering acute blood pressure changes but has little influence on the long-term blood pressure.