Afferent sympathetic nerve fibers with mechanoreceptors in the right heart.

Mechanosensitivity of afferent sympathetic nerve fibers from the right heart and the pulmonary artery has been examined. Action potentials of the afferent fibers that responded to tapping the right heart and the pulmonary artery were derived from upper thoracic communicating rami of both sides of anesthetized dogs. The fibers were composed of myelinated Adelta fibers and unmyelinated fibers. The receptive fields of both groups of fibers were located widely in the right heart and the pulmonary artery. Myelinated fibers ceased to fire quickly whereas unmyelinated fibers continued to fire after withdrawl of a brief mechanical stimulus. In the myelinated group, the pressure threshold was 3-50, 6-58, and 3-10 mmHg for right ventricular, pulmonary and right atrial threshold was 15-58, 22-34, and 4-8 mmHg for right ventricular, pulmonary, and right atrial fibers, respectively. Spontaneous discharge of myelinated fibers was synchronous with each rise and/or fall in intracardiac or pulmonary pressure whereas that of unmyelinated fibers was irregular and independent. A rise in pressure produced by pulmonary embolization or occlusion caused an augumented discharge whereas a fall caused by caval vein occulsion eliminated the discharge. The results indicate the existence of both myelinated and unmyelinated fibers with mechanoreceptors in the right heart and the pulmonary artery.