Input-output relations of lobules I and II of the cerebellar anterior lobe vermis in connexion with neck and vestibulospinal reflexes in the cat.

In the anaesthetized cat, lobules I and II of the cerebellar anterior lobe vermis were examined to determine their role in the vestibulospinal and neck-vestibulospinal reflexes with respect to: the somatotopic representation of afferent inputs from labyrinth, neck and tail; and the inhibitory influence on vestibulospinal tract (VST) neurones receiving vestibular and neck afferent inputs. After electrical stimulation of the vestibular nerve and of neck afferents, almost identical responses via mossy fibres were evoked in the lobules, with the prominent response in lobules I and IIa of Larsell. Stimulation of the nerve supplying the dorsal region of the tail induced primarily the mossy fibre response, but also the climbing fibre response, in lobule II. The most responsive areas to tail and neck afferent stimulation did not overlap each other. In the lateral vestibular nucleus, 163 antidromically identified VST neurones were recorded extra- or intracellularly. On the basis of the response pattern to contralateral neck afferent stimulation, they were classified into 3 groups: neurones with excitation (n = 45); neurones with inhibition (n = 71); and neurones with no modulation (n = 47). Stimulation of lobules I-IIa inhibited the activities of 44 VST neurones. Out of them, 41 neurones belonged to the first group. They made up 91% of the group. Twenty-nine of these neurones, i.e. neurones receiving excitatory inputs from the neck and inhibitory inputs from the lobules, received additional excitatory input from the labyrinth. Although lobules I-IIa may be regarded as neck area in the anterior lobe vermis from the viewpoint of sensory input, they did not exert inhibitory influence only exceptionally on vestibulocollic neurones, but predominantly on VST neurones sending their axons to lower thoracic or more caudal segments in the spinal cord. It is suggested from these results that lobules I-IIa have a close relationship with the neck reflex and/or interaction of neck and vestibulospinal reflexes being concerned with the postural adjustment of a rather wide area of the body.