Electrophysiological study of sympathoexcitatory structures of the bulbar ventrolateral surface as related to vasomotor regulation.

The responses in T3-4, T10-11 and L2-3 white rami to stimulation of different zones of the bulbar ventrolateral surface were maximal when the region of about 4 mm laterally to the midline was stimulated. A weak surface stimulation of all these zones elicited only a long latency response consisting of three waves. A short latency response appeared when supramaximal stimuli were applied only to the intermediate zone--the region up to 6 mm rostrally to the hypoglossal nerve root level (zone S and caudal part of zone M). The data presented show that long and short latency responses are conducted from the intermediate zone to the spinal cord via dorsolateral funiculus fibres with a conduction velocity of about 5.6 +/- 0.6 m/s. In addition, a special descending sympathoexcitatory pathway oriented to T2 preganglionic neurons with a conduction velocity of about 12.3 +/- 3.2 m/s was demonstrated. Antidromic discharges of the output sympathoexcitatory neurons elicited by dorsolateral funiculus stimulation were found in the intermediate zone only at a depth of about 400-2000 microM. Stimuli applied to different regions of the ipsilateral bulbar ventrolateral surface activate at least two groups of surface fibres (conduction velocities 6.7-8.0 and 2-3.2 m/s) which, in turn, activate the output neurons with a rather constant delay of about 20 ms equal to a difference between the latencies of long and short latency white rami responses. The mechanism of delay formation seems to be concentrated in the intermediate zone and formed probably by a chain of interneurons. A possible scheme of neuronal organization of the bulbar ventrolateral sympathoexcitatory structures is presented and discussed. The descending tonic activation of spinal vasomotor neurons is formed by spontaneous discharges of antidromically identified output neurons with a mean firing rate of about 14.4 imp./s. Some neurons are reflexly activated within the time limits of the late somatosympathetic reflex response. Coagulation of the intermediate zone resulted in a profound fall of blood pressure, disappearance of pressor and late somatosympathetic reflexes, whereas the spinobulbospinal somatosomatic reflex remains unchanged. The baroreceptor inhibition is partly realized through the elements of sympathoexcitatory intermediate zone because the preferable inhibition of the long latency white rami response was demonstrated in the middle of R-R interval and during a sharp increase in the arterial pressure induced by vasoconstrictor drugs. Thus, the structures of the intermediate zone seem to play a key role in supporting of blood pressure level and organization of pressure reflexes.(ABSTRACT TRUNCATED AT 400 WORDS)