Brain stem mechanisms in the control of arterial pressure.

Electrical stimulation of a circumscribed region within the rostral part of the ventrolateral medulla elicits a large increase in arterial pressure accompanied by intense regional vasoconstriction. Microinjection of glutamate has established that the vasomotor effects are due to excitation of cell bodies rather than axons of passage within the ventrolateral region. Bilateral localized destruction of the same region results in a profound decrease in resting arterial pressure as well as virtual abolition of reflexes controlling sympathetic vasomotor activity. Neuroanatomical studies using the method of retrograde transport of horseradish peroxidase (HRP) have shown that this region contains a dense group of neurons projecting to the thoracolumbar spinal cord and receives major monosynaptic inputs from the nucleus tractus solitarius and parabrachial nucleus. There is some evidence that the spinally-projecting ventrolateral cells contain epinephrine. The anatomical and physiological evidence taken together indicate that the bulbospinal pathway originating from the ventrolateral cells plays a major role in the tonic and phasic regulation of arterial pressure.