NEURONAL RESPONSES TO STIMULATION OF THE CERVIX, UTERUS, COLON, AND SKIN IN THE RAT SPINAL-CORD

1. Previous studies in the rat have shown that the hypogastric nerve conveys input from the cervix and uterus mainly to the T13-L3 segments of the spinal cord, whereas the pelvic nerve conveys input from the cervix and vaginal canal mainly to the L6-S2 segments. 2. To study the effects of this input, the dorsal horns of the T13-L1, L6-S2, and L4-L5 segments in 13 decerebrate, T10-spinalized, unanesthetized, and paralyzed adult female rats in estrus were searched for neurons responsive to gentle mechanical stimulation of the cervix. The 87 neurons found were then further tested for their responses to gentle mechanical stimulation of the skin and to distension of both uterine horns, distension of the colon, and shearing stimulation of the colon and vaginal canal. 3. Neurons responsive to cervix stimulation, primarily by excitation, were readily found in the ventral part of the dorsal horn in T13-L, and throughout the dorsal horn in L6-S2. Cervix-responsive neurons were less readily found throughout the dorsal horn in L4-L5, where 25% were inhibited by the stimulation. All but one neuron had cutaneous receptive fields. 4. The 30 cervix-responsive neurons in T13-L1 had large bilateral cutaneous receptive fields covering the perineum and hind-limbs. Most (76%) also responded, primarily by excitation, to uterine distension, as well as to colonic stimulation (59%). More than half were activated by both types of stimulation. 5. The 33 cervix-responsive neurons in L6-S2 had cutaneous receptive fields in the same regions as those in T13-L1, but generally smaller, particularly for neurons in the dorsal part of the dorsal horn, many of whose receptive fields were confined to the perineum. The L6-S2 neurons also exhibited less convergent input from other visceral structures, particularly the uterus. Fewer neurons (42%) responded to uterine distension, mostly by being inhibited, whereas about the same proportion (51%) responded with excitation to colonic stimulation. Only 24% responded to both uterus and colon. 6. All 24 cervix-responsive neurons in medial L4-L5 had small cutaneous receptive fields on the toes, and the neurons received less convergent input from other visceral structures (25% from the uterus, 33% from the colon, 13% from both). 7. These results indicate the presence of an extensive system of neurons throughout the caudal spinal cord of the rat, concentrated in separated thoracolumbar and lumbosacral segments, that is concerned with input from the reproductive tract. The differences in responses of neurons in T13-L1, L4-L5, and L6-S2 to reproductive and other pelvic visceral and somatic stimuli observed in the spinalized animals in this experiment only partially reflected differences in the primary afferent input to those segments. This situation points to the existence of considerable intersegmental intraspinal processing.