Abstract
The medullary projections of afferent neurons with cell bodies in the petrosal ganglion have been investigated using an antidromic mapping technique. Of the ninety-three units studied, fifty-eight were shown to have patterns of discharge indicating that they were baroreceptors and thirty-five showed responses to stimuli indicating that they were arterial chemoreceptors. Twelve baroreceptor and thirteen chemoreceptor 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 fibres with myelinated (six units) and non-myelinated (six units) axons showed similar patterns of central projection. All could be activated from the ipsilateral nucleus of the tractus solitarius (n.t.s.), most often from its lateral divisions rostral to the obex. The dorsolateral and dorsomedial portions of the n.t.s. were most often innervated, with the commissural subnucleus receiving an innervation in seven of the twelve neurones studied. Stimulation of the ventrolateral subnucleus was effective in activating two afferent fibres whilst stimulation of the ventral subnucleus was effective in only one case. All chemoreceptor afferent fibres had calculated conduction velocities less than 4 m/s and all were activated from the dorsomedial and medial subnuclei of the ipsilateral n.t.s. In twelve of the thirteen neurones investigated in detail there was evidence of an innervation of the commissural nucleus both at the level of the obex and behind it. In three cases this extended into the contralateral portion of the commissural nucleus. In four 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.