The central projections of carotid baroreceptors and chemoreceptors in the cat: a neurophysiological study.

Abstract

The medullary projections of afferent neurons with cell bodies in the petrosal ganglion were investigated using an antidromic mapping technique. Of the 93 units studied, 58 had discharge patterns indicating that they were baroreceptors and 35 showed responses indicating they were arterial chemoreceptors. Baroreceptor (12) and chemoreceptor (13) afferents had sufficiently stable unitary discharges to permit a detailed estimation of some of their central projections using stimulation through monopolar tungsten micro-electrodes to evoke antidromic spikes. In order to estimate their pattern of projection, depth-threshold contours for each penetration through the dorsomedial medulla and the values of antidromic latency were considered. Baroreceptor afferent fibers with myelinated (6 units) and non-myelinated (6 units) axons showed similar patterns of central projection. All could be activated from the ipsilateral nucleus of the tractus solitarius (NTS), most often from its lateral divisions rostral to the obex. The dorsolateral and dorsomedial portions of the NTS were most often innervated, with the commissural subnucleus receiving an innervation in 7 of the 12 neurons studied. Stimulation of the ventrolateral subnucleus was effective in activating 2 afferent fibers while stimulation of the ventral subnucleus was effective in only 1 case. All chemoreceptor afferent fibers had calculated conduction velocities < 4 m/s and all were activated from the dorsomedial and medial subnuclei of the ipsilateral NTS. In 12 of the 13 neurons investigated in detail there was evidence of an innervation of the commissural nucleus both at the level of the obex and behind it. In 3 cases this extended into the contralateral portion of the commissural nucleus. In 4 cases a sparse innervation of the lateral subnucleus, comprising its dorsolateral aspects, was seen. The potential significance of these distinctive patterns of projection of arterial baroreceptors and chemoreceptors is discussed in relation to cardiovascular and respiratory control.