The influence of visceral mechanoreceptors on sympathetic efferent discharge in the cat.

1. Recordings have been made from eighty-three single sympathetic efferent units in the hypogastric nerve in two types of preparation. In all animals the baroreceptors were denervated to exclude changes in sympathetic discharge resulting from any variations in arterial pressure, and the spinal cord was sectioned at the 6th lumbar segment to exclude changes in efferent discharge that might have been due to pelvic nerve afferents from the bladder or other viscera. In some animals the afferent pathways were sectioned from all pelvic and lower abdominal viscera other than the bladder, so that the vesical afferent pathway was the only neural pathway that might mediate reflex events from these viscera. The hypogastric nerve afferent pathway was excited by bladder distension or by bladder contractions induced by electrical stimulation of the sacral cord.2. Approximately half the units gave an increase in spike rate during distension or during contraction of the bladder. Three-quarters of the units tested also gave an increase in spike rate during colonic distension. In no units that exhibited convergence of afferent input did colonic and vesical stimuli cause responses of opposite sign.3. Approximately 10% of units showed a reduction in discharge rate when the bladder was distended or caused to contract.4. The estimated intravesical pressure thresholds for these reflexes were in the range 8-56 mmHg which extends beyond that of the mechanoreceptors which form the afferent limb of the reflex.5. These reflex studies indicate that the hypogastric nerve afferents from the bladder can elicit sympathetic reflexes within the physiological range of intravesical pressures, and that vesico-sympathetic reflexes can be elicited in the absence of pelvic nerve afferent inputs.In 75% of sympathetic efferent units that respond to bladder distension or contraction, there is evidence for convergent inputs with similar actions from the colon.