Tone of sympathetic nerves and regulation of heart activity

Tone of sympathetic nerves to the heart was studied in rats and guinea pigs. Experiments with pharmacological blockade of the sympathetic nervous system and vagotomy confirmed the general notion on the absence of tonic effects of sympathetic nerves on the heart. Reduction of the heart rate reported previously probably attests to various experimental designs (type and depth of anesthesia, possible hypothermia, duration of observations, and pharmacological preparations). As differentiated from the vascular tone, the heart rate under rest conditions depends on the vagal tone and circulating humoral substances.