Pulse and Respiratory Grouping of Sympathetic Impulses in Human Muscle Nerves

A characteristic type of spontaneous activity consisting of grouped discharges of impulses was detected when recording with a tungsten semi-microelectrode from peripheral nerve bundles innervating human skeletal muscles. An analysis of this phenomenon shows that the grouped discharges are due to volleys of impulses in efferent nerve fibres which are not skeletomotor. Some findings suggest that the efferent conduction velocity is very low (about 1 m/sec). The bursts were often “pulse-synchronous” and they tended to appear periodically during certain phases of the respiratory cycle. They were not accompanied by any significant changes in the intensity of the afferent responses to phasic or sustained muscle stretch. In some experiments, repeated noxious stimuli triggered showers of impulses in the muscle nerves, similar to the spontaneous bursts. The findings lead to the conclusion that the phenomenon is due to rhythmic outbursts of impulses in sympathetic nerve fibres.