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lateral vestibulospinal tract to motor neurons innervation extensor muscles

FIGURE 5 Central vestibular pathways. (A) Position of the vestibular nuclei in a dorsal view of the brain with the cerebral cortex removed and the cerebellum rendered transparent. (B) Major connections of the vestibular nuclei. (Left) Superior and medial vestibular nuclei (light blue). The main input is from the semicircular canals.

to the unopposed excitatory drive of the vestibular nucleus. Under the constant stimulation from motor neurons, muscles of the legs and arms remain rigidly contracted, unable to relax or participate in normal movements.

Medial and superior vestibular nuclei receive information primarily from the semicircular canals. Fibers from the medial nucleus send their second-order fibers in the medial vestibulospinal tract to motor neurons located in upper levels of the spinal cord. This pathway elicits reflex movements in neck muscles in order to stabilize head position while walking and, along with other inputs, to coordinate head movements with eye movements. Additional output fibers from the medial nucleus join processes from the superior nucleus in forming a major pathway that travels within the medial longitudinal fasciculus (MLF), one of the most prominent fiber bundles within the brain stem.

As described in Chapter 51, fibers within the MLF project to the three pairs of motor nuclei that control eye movement, enabling the eyes to move appropriately as the head moves. When the head is turned to one side, eyes reflexly rotate in the opposite direction. Without vestibulo-ocular reflexes being mediated through the MLF, visual images are not stabilized on the retina during body movement and visual performance is greatly impaired. Fibers in the MLF are highly myelinated, which is one reason why they serve as a prominent landmark within the less myelinated regions of the brain stem. Myelination increases conduction velocity of the fibers, a much-desired attribute for a reflex pathway such as the MLF, which requires a rapid response time. Unfortunately, the heavy myelination makes it a target for demyelinating diseases such as multiple sclerosis and accounts for some of the visual effects (e.g., diplopia and blurred vision) that are often associated with this disease.

The inferior vestibular nucleus receives information from all vestibular organs as well as from the cerebellum. It provides second-order fibers for: (1) the vestibulospinal pathway, which helps maintain posture, and (2) the vestibuloreticular pathway, which terminates in various areas of the reticular formation of the brain stem and helps activate protective reflexes. The latter

Both superior and medial nuclei interact to coordinate eye movements with rotational head movements by way of the ascending medial longitudinal fasciculus. The medial nuclei also coordinate neck movements by way of the descending medial vestibulospinal tract. (Right) The lateral and inferior vestibular nuclei (dark blue) receive information from the utricle and saccule as well as the semicircular canals. They send information down the lateral vestibulospinal tract to axial motor neurons controlling posture and balance. The lateral tract also sends commands to the reticular formation for protective reflexes such as the gag reflex. The nuclei also communicate with the cerebellum (not shown.)

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