Heart rate changes reflecting modifications of efferent cardiac sympathetic outflow by cutaneous and muscle afferent volleys

In cats anesthetized with chloralose and urethane graded electrical stimulation of cutaneous and muscle nerves of the fore- and hindlimb was performed and the resulting changes in heart rate and blood pressure were recorded. The vagus nerves were cut and the right carotid artery was tied off. The left carotid sinus was intact. Repetitive stimulation of cutaneous group II afferents did not change the heart rate nor the blood pressure. Cutaneous group III afferent activity led to an increased heart rate in about 70% of all trials. Very consistent increases in heart rate were always seen when repetitive group IV cutaneous volleys were elicited. Volleys in group I and II muscle afferents were ineffective. With group III muscle volleys bradycardiac (in about 40% of all trials) or tachycardiac (in about 30%) responses were observed. The nature of the response depended on the experimental situation but was difficult to predict or to modify. Stimulation of group IV muscle afferents always induced definite increases in heart rate. The latency for the onset of the heart rate changes was of the order of 3--10 s, and the time of peak during prolonged stimulation was about 20--30 s. There was a marked early and a small late response adaptation during prolonged repetitive stimulation, but the control heart rate was only reached some 40--65 s after the end of the stimulation. For group IV volleys the minimal effective frequency of the repetitive stimulation was between 0.25 and 1.0 Hz, the optimal one between 6 and 15 Hz. At higher stimulus frequencies a decline of the responses was observed. At the optimal frequency about 5 volleys were sufficient to induce a just detectable heart rate change. Mainly from the results obtained with group III fiber stimulation it is concluded that the intensity and direction of somatically induced heart rate changes are not strictly related to the afferent fiber groups (i.e. fiber threshold and diameter), as suggested by the concept of "pressor" and "depressor" nerve fibers, but to the modality of the receptive units which the fibers subserve.