Physiological and ultrastructural observations on regenerated carotid sinus nerves after removal of the carotid bodies in cats

Abstract

Carotid bodies were excised bilaterally in cats and the cut carotid sinus nerves allowed to regenerate freely into the region of the carotid bifurcation for 31 to 315 days. Electrophysiological recordings from regenerated nerves showed spontaneous activity and mechanoreceptive units in some nerves. None of the nerves were excited by physiological or chemical stimuli known to excite chemoreceptors, but electrical stimulation of the regenerated axons evoked reflex ventilatory responses. Serial section light-microscopic analysis of the regenerated nerves revealed bundles of axons branching from the carotid sinus nerve near the site of transection and coursing into the area of the carotid bifurcation. Electron-microscopic examination of the most distal region of the regenerated nerve which could be identified showed axonal bundles comprised of myelinated and unmyelinated fibers characteristic of the mature sinus nerve; axonal sprouts were rarely encountered. Carotid body type 1 or type II cells were never observed. Infrequently, neuronal specializations containing mitochondria and synaptic vesicles were seen in connective tissue in the distal regions of the regenerated nerve. The vascularity of the regenerated nerve was at least comparable to that of the normal carotid sinus nerve. The results indicate that chemoreceptivity is not a property of carotid sinus nerve sensory axons when they are allowed to regenerate freely into the area of the carotid bifurcation after removing the carotid body. Consequently, these observations do not provide support for theories which consider chemoreception to be an intrinsic property of carotid sinus neurons. It is concluded that some extrinsic factor normally provided by the carotid body is essential to the transduction process which initiates depolarization of the sensory axons terminating within this organ.