Sympathetic fibers in the aortic nerve of the cat

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Summary In 27 cats the presence of sympathetic fibers in the left aortic nerve was studied. Electrical activity with a cardiac rhythm was recorded in the central cut end of the left aortic nerve in neck and thorax. This activity increased with intravenous norepinephrine or intra-aortic saline and decreased with the occlusion of the homolateral carotid artery. Irregularly occurring spontaneous activity was also detected. The former was thought to originate in the common carotid baroreceptor areas while the nonrhythmic discharges may be of central origin. Electrical stimulation of the preganglionic trunk of the stellate ganglion gave a potential complex recorded in the thoracic part of the aortic nerve. This complex was decreased in amplitude with high frequency stimulations and abolished with ganglion-blocking agents but reappeared with postganglionic stimulation. The conduction velocities of the stimulated fibers, measured in two experiments, were 1.3 and 2 m/sec. With preganglionic stimulation of the cervical sympathetic trunk, a low threshold response, conducted at 15–42 m/sec was recorded in the thoracic aortic nerve. This response was not modified by electrical stimulation at high frequencies, ganglion blocking agents or cutting of the sympathetic trunk cranial to the stimulating electrodes. Records taken from the peripheral part of the left aortic nerve at variable times after its severance in the neck showed that baroreceptor activity persists in some units; unaltered myelinated and unmyelinated fibers were observed with both optic and electron microscopy. Myelinated fibers probably joined the sympathetic trunk in the neck before the point of section as suggested by the responses obtained by preganglionic stimulation of the sympathetic trunk and may be responsible for the recorded baroreceptor activity. Unmyelinated fibers probably correspond to the postganglionic sympathetic fibers recorded in the aortic nerve with stellate ganglion stimulation and may represent a sympathetic supply to the baro- and chemoreceptor aortic areas.