Vestibulospinal Tracts

The known output pathways from the vestibular nuclei include projections to the spinal cord, oculomotor nuclei, cerebellum and reticular formation. Vestibular activity also reaches the thalamus, superior colliculus and other higher centers, but the exact pathways are not known.

Vestibulospinal Tracts. The illustration shows the projections of vestibular neurons to the spinal cord via the lateral vestibulospinal tract (LVST) and medial vestibulospinal tract (MVST). These two tracts, which lie in the anterior and anteromedial funiculi (see Plate 2-12), act primarily on the motor apparatus that controls the proximal muscles and therefore are important in the regulation of postural equilibrium.

The LVST is uncrossed and originates primarily from the lateral vestibular nucleus. Some of its constituent fibers extend the entire length of the spinal cord, whereas others extend only part of this distance; they may branch to innervate several regions as they descend. The lateral nucleus is somatotopically organized: neurons projecting to the lower (hindlimb) levels of the spinal cord are located in the posterior and distal portion of the nucleus, and neurons ending at higher levels are situated more anteriorly and rostrally. The former region receives a heavy projection from the cerebellar vermis, whereas the latter region receives a heavy input of vestibular afferent fibers. The LVST ends in lamina VIII and parts of lamina VII; it acts on alpha and gamma neurons.

The predominant action of the LVST is to produce the contraction of extensor (antigravity) muscles and the relaxation of flexor muscles. In the case of neck, trunk, and some lower limb extensor muscles, contraction is produced in part by direct (monosynaptic) excitation of motor neurons. The excitation of other limb extensor muscles and the inhibition of flexor muscles are mediated by pathways that include spinal interneurons.

The MVST, which projects bilaterally to the cervical cord, is involved in reflex adjustments of the head and axial muscles to vestibular stimulation. It contains fibers that originate primarily in the medial vestibular nucleus and produce direct inhibition of motor neurons controlling neck and axial muscles. The tract seems to stop in the midthoracic region. The two vestibulospinal tracts are important factors in vestibular reflex reactions that are triggered by the movement of the head in space. Particularly significant in this regard is the strong vestibular action on the neck muscles, which helps to stabilize the position of the head. However, these tracts and the reticulospinal tracts (see Plate 2-11) also appear to play a much wider role in the control of the proximal musculature.

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