Responses of vestibular nucleus neurons to tilt following chronic bilateral removal of vestibular inputs

Recordings were made from the vestibular nuclei of decerebrate cats that ha d undergone a combined bilateral labyrinthectomy and vestibular neurectomy 49-103 days previously and allowed to recover. Responses of neurons were re corded to tilts in multiple vertical planes at frequencies ranging from 0.0 5 to 1 Hz and amplitudes up to 15 degrees. Many spontaneously active neuron s were present in the vestibular nuclei; the mean firing rate of these cell s was 43+/-5 (SEM) spikes/s. The spontaneous firing of the neurons was irre gular: the coefficient of variation was 0.86+/-0.14. The firing of 27% of t he neurons was modulated by tilt. The plane of tilt that elicited the maxim al response was typically within 25 degrees of pitch. The response gain was approximately 1 spike/s/degrees across stimulus frequencies. The response phase was near stimulus position at low frequencies, and lagged position sl ightly at higher frequencies (average of 35+/-9 degrees at 0.5 Hz). The sou rce of the inputs eliciting modulation of vestibular nucleus activity durin g tilt in animals lacking vestibular inputs is unknown, but could include r eceptors in the trunk or limbs. These findings show that activation of vest ibular nucleus neurons during vertical rotations is not exclusively the res ult of labyrinthine inputs, and suggest that limb and trunk inputs may play an important role in graviception and modulating vestibular-elicited refle xes.