Responses of lateral reticular neurons to sinusoidal stimulation of labyrinth receptors in decerebrate cat.

1. The electrical activity of 106 individual neurons located in the precerebellar lateral reticular nucleus (NRL) and the surrounding medullary reticular formation (RF) has been recorded in precollicular decerebrate cats during sinusoidal tilt around the longitudinal axis of the whole animal leading to stimulation of labyrinth receptors. 2. Among these lateral reticular neurons tested, 48 of 712 (67.6%) NRL neurons and 11 of 35 (31.4%) RF neurons responded to slow rotation of the animal at the standard frequency of 0.026 Hz and at the peak amplitude of displacement of 5-10 degrees. 3. All the responsive units showed a periodic modulation of firing rate during the sinusoidal stimulus. In particular, 35 of 57 units (i.e., 61.4%) were excited during side-up and depressed during side-down tilt of the whole animal; on the other hand, 14 of 57 units (i.e., 24.6%) showed the opposite behavior. In both instances, the peak of the responses occurred with an average phase lead of about 16 degrees with respect to the extreme side-up or side-down position of the animal. The remaining eight units (i.e., 14%) showed a phase shift of the peak of their response of about 90 degrees with respect to the animal position. 4. The sensitivity of the responses, expressed in percentage change of the average firing rate per degree of displacement, did not change by increasing the peak amplitude of tilt from 5 to 15 degrees at the frequency of 0.026 Hz. This finding indicates that the system was relatively linear with respect to the amplitude of stimulation. The sensitivity of the units, however, slightly increased but the phase angle of the responses did not change by increasing the frequency of tilting from 0.015 to 0.15 Hz at the peak amplitude of 5 or 10 degrees. These findings indicate that the responses depended on stimulation of macular labyrinth receptors. 5. Most of the lateral reticular units affected by tilt received also a bilateral convergent input from the hindlimbs. 6. These observations are related to the results of previous studies in which the responses of macular afferents, vestibular nuclei neurons, and corticocerebellar Purkinje (P) cells to sinusoidal tilt of the whole animal have been investigated. A possible role of lateral reticular neurons in the labyrinth control of posture in decerebrate cat is also discussed.