Unlike the well-defined sensory and motor nuclei of the cranial nerves, the reticular formation comprises clusters of neurons scattered among a welter of interdigitating axon bundles it is therefore difficult to subdivide anatomically. The reticular formation is a complicated network of circuits located in the core of the brainstem and extending from the rostral midbrain to the caudal medulla ( Figure 17.3). The projections from the vestibular nuclei that control axial muscles and those that influence proximal limb muscles originate from different cells and take different routes (called the medial and lateral vestibulospinal tracts). Many of the cells in the vestibular nuclei that receive this information are upper motor neurons with descending axons that terminate in the medial region of the spinal cord gray matter, although some extend more laterally to contact the neurons that control the proximal muscles of the limbs. As described in Chapter 14, the vestibular nuclei are the major destination of the axons that form the vestibular division of the eighth cranial nerve as such, they receive sensory information from the semicircular canals and the otolith organs that specifies the position and angular acceleration of the head.
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