Urinary bladder and hindlimb stimuli inhibit T1-T6 spinal and spinoreticular cells

Upper thoracic spinal neurons are primarily excited by cardiopulmonary spinal afferent input but are excited and inhibited by splanchnic afferent input. These data suggest that the greater the number of segments between a spinal neuron and spinal afferent input the greater the probability that the afferent input will inhibit the spinal neuron. Based on this idea we hypothesized that visceral (urinary bladder) and somatic (hindlimb) afferent input would inhibit upper thoracic spinal neurons. To test this hypothesis the activities of 69 spinal and 27 spinoreticular tract neurons in 45 alpha-chloralose-anesthetized cats were studied. Only neurons excited by both visceral and somatic thoracic afferent input were studied. Urinary bladder distension (UBD) inhibited 48 (50%), excited 6 (6%), and did not affect 41 (43%) of these neurons. Also, UBD inhibited the excitatory responses of these cells to noxious visceral and somatic stimuli. Hindlimb pinch also inhibited greater than 50% of the neurons. These data indicate that visceral and somatic afferent input to the lumbosacral spinal cord inhibits the activity of upper thoracic neurons. This inhibitory effect may play a role in localization of sensory and motor responses to noxious stimuli.