Vestibulospinal tract

sources from Haines Fundamental neuroscience 3rd edition

The vestibulospinal system comprises medial and lateral vestibulospinal tracts . The medial vestibulospinal tract is made up of axons that originate in the medial and inferior vestibular nuclei and descend bilaterally into the spinal cord as part of the medial longitudinal fasciculus. The lateral vestibulospinal tract is formed by axons that originate in cells of the lateral vestibular nucleus and descend ipsilaterally through the anterior portion of the brainstem to course in the anterior funiculus of the spinal cord.

The medial vestibulospinal tract projects only as far as cervical or upper thoracic spinal cord levels and influences motor neurons controlling neck musculature. The lateral vestibulospinal tract, in contrast, extends throughout the length of the cord. Cells in rostral portions of the lateral vestibular nucleus project to the cervical cord, cells in the middle portion project to the thoracic cord, and cells in the caudal part terminate in lumbosacral levels. The fibers of this tract terminate in the medial portions of laminae VII and VIII and excite motor neurons that innervate paravertebral extensors and proximal limb extensors . These muscles function to counteract the force of gravity and, therefore, are commonly called antigravity muscles. Through their effects on these extensor muscles, lateral vestibulospinal fibers function in the control of posture and balance. Evidence from experimental studies suggests that some vestibulospinal axons synapse directly on alpha motor neurons but that most exert their influence through spinal interneurons.

Activity in the lateral vestibulospinal tract is driven primarily by three ipsilateral inputs-two excitatory and one inhibitory . The two sources of excitatory input are the vestibular sensory apparatus and the cerebellar nuclei, mainly the fastigial nucleus. The inhibitory input consists of Purkinje cell axons from the cerebellar cortex.

The lateral vestibulospinal tract is the path by which input from the vestibular sensory apparatus is used to coordinate orientation of the head and body in space. Maintenance of body and limb posture is also influenced by extensive cerebellovestibular projections, which can be either excitatory or inhibitory. The cerebral cortex essentially has no direct projections to the vestibular nuclei; consequently, the vestibulospinal tract is not directly influenced by cortical mechanisms.