Identification and properties of sub-retrofacial bulbospinal neurones: a descending cardiovascular pathway in the cat.

Experiments were performed on 21 chloralose-anaesthetized, paralyzed cats to identify electrophysiologically the bulbospinal vasomotor neurones of the rostral ventrolateral medulla. Neurones were recorded from a small region close to the ventral medullary surface (sub-retrofacial nucleus), where previous studies had localized points where microinjected excitant amino acids produced large pressor responses. A total of 157 neurones in this region were antidromically activated by electrical stimulation in the spinal cord, 132 from the ipsilateral dorsolateral funiculus at lower cervical level and 25 from the intermediolateral horn at T1. Of the cells tested, 33/34 were excited by iontophoresis of homocysteic acid, indicating that recordings originated from cell bodies rather than axons. The positions of 16 were marked by dye deposition, and located histologically to the sub-retrofacial nucleus. Stimulation of baroreceptors by inflating a carotid sinus blind sac to 200 mm Hg powerfully inhibited 41/48 bulbospinal neurones tested. The threshold for this effect was 80-100 mm Hg. Baroreceptor-sensitive neurones all had axonal conduction velocities between 2 and 8.8 m/s, and within this range, all but two cells were barosensitive. Barosensitive cells were unaffected by low threshold somatosensory stimulation. Baroreceptor stimulation was still able to silence these neurones' activity even when this was raised several-fold by iontophoresis of homocysteic acid. It is argued that sub-retrofacial bulbospinal neurones probably constitute a major descending vasomotor pathway, and that baroreceptors act by inhibiting them postsynaptically.