Vestibulospinal reflexes and the reticular formation.

While both vestibulospinal and reticulospinal tracts contribute to vestibulospinal reflexes, their respective roles are not fully understood. Previous evidence suggests that reticulospinal fibers make an important contribution to the horizontal vestibulocollic reflex (VCR) of the decerebrate cat. Recent work addresses their contribution to the vertical VCR. On the basis of study of reflex and vestibulocollic neuron dynamics, it appears that processing which is necessary to produce some of the spatial properties of the vertical VCR takes place outside the vestibular nuclei. Recording from pontomedullary reticulospinal neurons receiving vestibular input and projecting to different levels of the spinal cord reveals that almost no cells receive only vertical canal input, while approximately half receive otolith input. As is the case for vestibulocollic neurons, these reticulospinal neurons also lack the properties required to produce all of the VCR's spatial properties. Two conclusions are that in response to stimuli in vertical planes pontomedullary reticulospinal fibers are best suited to contribute to otolith reflexes, and that spatial properties of the VCR depend in part on convergence of inputs within the neck itself.