Effects of Autonomic Blockade on Heart Rate Responses to Reaction Time and Sustained Handgrip Tasks

This study used both beta‐adrenergic and cholinergic pharmacological blockade to investigate differences in autonomic mediation of heart rate responses during reaction time and sustained handgrip tasks. Forty‐five healthy young men received a venous infusion of saline and then performed each task for the first time. Then, each subject received a second infusion consisting of either saline, propranolol hydrochloride, or atropine sulfate, and repeated each task. For each infusion, the 15 subjects in each group were unaware of which of the three substances they were receiving. The results indicate that heart rate increases during the reaction time task were mediated by increased beta‐adrenergic activity, whereas heart rate increases during handgrip were mediated by a combination of increased beta‐adrenergic activity and vagal withdrawal. These results support the hypothesis that neural mechanisms mediating cardiac adjustments during some types of psychological stress differ qualitatively from mechanisms mediating similar‐appearing adjustments during some types of physical exertion.