Cholinesterase-positive and catecholamine-containing nerves in the guinea-pig pericardium

The functions of the pericardium are traditionally defined in terms of lubrication and support. However, its complement of autonomic nerve fibers, shown by physiologc experimnts to be sensory, suggests that the pericardium may also serve as a mechanoreceptor site. In this study acetycholinesterase-positive, and catecholamine-containing elements of the pericardial plexus of the guinea-pig were visualized, using thiocholine and fluorescence techniques, respectively. A cholinesterase-positive nerve net, containing the preterminal segments of unmyelinated fibers and simple nerve endings, extended over the entire parietal pericardium. Some of the cholinesterase-positive endings, apparently separate from effector structures were assumed to be sensory. A few heavily myelinated fibers were contributed by the phrenic nerve. The parietal pericardium covering the atria contained many adrenergic nerves and endings. Not all pericardial adrenergic endings were linked to effector structures, such as blood vessels. Some fibers ended freely. It is suggested that liberation of catecholamines from “unattached terminals” may serve to lower the threshold, and prolong the adaptation of adjacent cholinesterase-positive mechanoreceptor terminals.