The effect of somatosensory stimulation on second-order and efferent vestibular neurons in the decerebrate decerebellate guinea-pig.

The extracellularly recorded activity of medial vestibular nucleus neurons and efferent vestibular neurons was analysed in the decerebrate decerebellate guinea-pig. Neurons were identified by means of electrical stimulation of the anterior semicircular canal. Thirty-six neurons were monosynaptically activated during semicircular canal stimulation. These cells were regarded as second-order vestibular neurons. Thirty neurons were antidromically activated and therefore identified as efferent vestibular neurons. Both types of neurons investigated had spontaneous impulse activity. All neurons responded to sinusoidal roll tilt. All the second-order vestibular neurons were excited during ipsilateral tilt and inhibited by contralateral tilt. Eighteen efferent vestibular neurons also showed this pattern, while the remaining 12 were excited by contralateral tilt and inhibited by ipsilateral tilt. Some neurons responded to passive forelimb extension or pressure of the forelimb plantar surface; none of the neurons responded to passive forelimb flexion or light plantar touch. Eleven second-order neurons (30%) were excited by somatosensory stimuli, seven (20%) were inhibited and 18 (50%) showed no response. Twenty efferent neurons (67%) were excited by somatosensory stimuli, none were inhibited and 10 (33%) showed no response. The responses of vestibular neurons to somatosensory stimulation are discussed with respect to their importance in vestibulospinal control during locomotion.