Neural control of the heart-rate-orienting response in preweanling rats

The neural origins of heart-rate responses to auditory and painful stimuli were studied in preweanling and neonatal rats. Bradycardia and tachycardia in response to auditory and painful stimuli were found to be mediated by the 10th cranial nerve. Bilateral vagotomy abolished both responses, as did the peripherally acting cholinergic antagonist methyl scopolamine. Neither peripheral sympathectomy with guanethidine nor beta adrenergic receptor blockade with propranolol significantly altered the cardiac responses. Thus, both the tachycardia and bradycardia evoked by psychologically relevant stimuli appear to be under direct nervous system control exerted through cholinergic vagal efferents to the heart.