Responses of vestibulospinal neurons to sinusoidal rotation of neck

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Abstract
The response characteristics of vestibular nuclear neurons to sinusoidal rotation of the cervical axis vertebra around the longitudinal axis of the animal while the head was stereotaxically clamped and maintained in horizontal position were investigated in precollicular decerebrate cats. Among the 163 vestibular nuclear neurons tested during neck rotation, 70 of 120 neurons located in the lateral vestibular nucleus (LVN) and 29 of 43 neurons located in the superior, medial and descending vestibular nuclei responded to slow neck rotation at the frequency of 0.026 Hz and at the peak amplitude of 5-10.degree.. Of 38 identified vestibulospinal neurons, 25 were affected by the neck input. The units influenced by neck rotation showed a periodic modulation of the firing rate in response to sinusoidal stimulation of neck receptors. The phase angle of the 1st harmonic of the response varied from a lead of +90.degree. to a lag of -45.degree. with respect to the side-down or side-up displacement of the neck, with the majority of the responses being related to the direction of neck orientation and not to the angular velocity of neck rotation. Responses to neck rotation were detectable at 1-3.degree. of peak displacement at the frequency of 0.026 Hz. The sensitivity of the 1st harmonic of the response, expressed in percentage change of the average discharge rate per degree of peak displacement, was only slightly influenced in the majority of tested neurons by increasing the amplitude of stimulation. The system was apparently relatively linear with respect to the amplitude of neck rotation. Moreover, both the sensitivity and the phase angle of the response were not changed in most units by varying the frequency of stimulation. A higher concentration of responsive neurons was observed in the rostroventral part than in the dorsocaudal part of the LVN. These regions project to the cervical and lumbosacral divisions of the spinal cord, respectively. The sensitivity of the responses of the cLVN neurons to the neck input was higher than that of the lLVN neurons. Units located within the ventral aspect of the LVN, namely the cLVN, were either excited or inhibited during side-down neck rotation, while units located within the dorsal aspect of the LVN, namely the lLVN, were mainly inhibited. Cerebellar activation may be responsible for inhibition of the background discharge of dorsally located LVN neurons during side-down neck rotation. The findings are discussed in relation to the results of stimulation and lesion experiments, indicating that postural changes can be elicited either during asymmetric stimulation of neck receptors or by unilateral interruption of the neck afferents.