Convergence patterns of the posterior semicircular canal and utricular inputs in single vestibular neurons in cats

The convergence of the posterior semicircular canal (PC) and utricular (UT) inputs in single vestibular nuclei neurons was studied intracellularly in decerebrate cats. A total of 160 vestibular neurons were orthodromically ac tivated by selective stimulation of the PC and the UT nerve and classified according to whether or not they were antidromically activated from the spi nal cord and oculomotor nuclei into vestibulospinal (VS), vestibulooculospi nal (VOS), vestibuloocular (VO), and unidentified vestibular neurons. Fifty -three (33%) of 160 vestibular neurons received convergent inputs from both the PC and UT nerves. Seventy-nine (49%) vestibular neurons responded to P C inputs alone, and 28 (18%) neurons received inputs only from the UT nerve . Of 53 convergent neurons, 8 (15%) were monosynaptically excited from both nerves. Thirty-five (66%) received monosynaptic excitatory inputs from the PC nerve and polysynaptic excitatory or inhibitory inputs from the UT nerv e, or vice versa. Approximately one-third of VS and VOS neurons received co nvergent inputs. A majority of the VS neurons descended to the spinal cord through the lateral vestibulospinal tract, while almost all the VOS neurons descended to the spinal cord through the medial vestibulospinal tract. The convergent neurons were found in all vestibular nuclei but more in the lat eral nucleus and descending nucleus. The VS neurons were more numerous than VO neurons or VOS neurons.