Cerebellar Control of the Vestibulospinal Reflex Arc

Abstract

Stimulation of macular labyrinthine receptors produced by tilting of the animal elicits strong influences on extensor moto-nurons. These responses are mediated by the three-neuronal pathway rpresented by primary vestibular afferents, ipsilateral lateral vestibular nucleus ot Deitere (LVN) and corresponding extensor moto neurons. This vestibulospinal reflex arc acts as a closed loop system for the control of head position through the activity of heck and limb musculature. Any change in head position is in fact detected by the vestibular organ, Whose signals in turn evoke contraction in neck and body musculature to prevent alteration in head position and resyore head and body to their normal attitude. In this way the vestibulospinal reflex arc represents a typical example of the feedback control, where the output is returned to the input. It was generally assumed that this closed loop control mechanism did not require a control from the cerebellum, since neither primary nor secondary vestibular fibere were seen to project to the vermal cortex of the cerebellar anterior lobe, which exerts a powerful inhibitory influence on the LVN. Recent experiments have shown that neurons of the ascending spinoreticulocerebellar pathway, which project to the vermal cortex of the anterior lobe, respond to macular labyrinthine stimulation. This effect is due to vestibulospinal volleys originating from the LVN, which act monosynaptically on neurons of the cervical spinoreticular pathway. In this way the reflex discharge of LVN neurons, induced by macular stimulation, may be modified by the inhibitory influence that the cerebellar Purkinje neurons exert upon them. The cerebellar cortex of the anterior lobe thus represents a side path to the vestibulospinal reflex arc, which. contributes to the positional changes induced by the macular input. In add1tion to signals originating from macular receptors, the spino reticulocerebellar pathway also transmits to the cerebellum somato-ensory information from the periphery, including the nuchal and the forelimb afferents. It appears, therefore, that the cerebellum integrates not only the command signals originating from macular recepora, but also the somatosensory input originating from neck and forelimb receptors, as a result of the actual displacement of the head occurring during the positional reflex. The resulting activity of the cerebellar Purkinje (P) neurons will then produce appropriate adjustments in the reflex response of the vestibulospinal neurons to macular labyrinthine stimulation, thus leading to an exact compensation for alteration in head position.