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Chapter 11: Vestibular System: Pathways and Reflexes

Lincoln Gray, Ph.D., Department of Communication Sciences and Disorders, James Madison University

Reviewed and revised 07 Oct 2020

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11.1 Overview of Ascending and Descending Pathways

The first-order vestibular afferents have their cell bodies in the vestibular (Scarpa’s) ganglion, which is found at the distal end of the internal auditory meatus. Their axons travel in the vestibular portion of the VIIIth cranial nerve through the internal auditory meatus and enter the brain stem at the junction between the pons and the medulla where the IVth ventricle is the widest. Most of these afferents project to one of the four nearby vestibular nuclei in the rostral medulla and caudal pons. A few of the vestibular afferents go directly to the cerebellum through the inferior cerebellar peduncle. The cerebellum coordinates the movements that maintain balance. There are many connections between the cerebellum and the vestibular nuclei.

Figure 11.1 Summary of ascending and descending vestibular pathways. 1) Press PLAY to view the pathway. 2) Roll over each word to view the location

Figure 11.1 shows a summary of these ascending and descending vestibular pathways. Upon pressing the "play" button, the four vestibular nuclei appear in outline. Note that the medial and inferior nuclei are usually seen together in the rostral medulla. The lateral and superior nuclei are smaller and are seen in the pons. The ascending tracts are shown in blue. These arise from the superior and medial nuclei and ascend in medial longitudinal fasciculus (MLF) to the oculomotor nuclei (III, IV, VI).The lateral vestibulospinal tract is shown in green. It descends ipsilaterally to the sacral cord. The medial vestibulospinal tract is shown in red. It descends bilaterally in the MLF to thoracic levels. The cerebellar afferents are not shown in this summary, but these come from the medial and inferior vestibular nuclei.

The 1st order vestibular afferents arise in Scarpa's ganglion, which is in the distal portion of the internal auditory meatus. The axons travel in the vestibular portion of the VIIIth cranial nerve and enter the brain stem at the pontomedullary junction.

There are four 2nd order vestibular nuclei: the inferior, medial, lateral (Deiter’s) and superior vestibular nuclei. All four nuclei are found beneath the floor of the fourth ventricle in the medulla and pons, lateral to the sulcus limitans. The main projections from these nuclei are to the spinal cord (controlling head and body position), to the three, extraocular motor nuclei (III, IV, VI, controlling eye movements), to the thalamus (VPI, eventually reaching the cortex and conscious perception of movement and gravity), and to the cerebellum (coordinating postural adjustments).

The main descending tracts are the lateral vestibulospinal tract from the lateral vestibular nucleus and the medial vestibulospinal tract from the medial vestibular nucleus. The lateral vestibular tract starts in the lateral vestibular nucleus and descends the length of the spinal cord on the same side. This pathway helps us walk upright. The medial vestibular tract starts in the medial vestibular nucleus and extends bilaterally through mid-thoracic levels of the spinal cord in the MLF. This tract affects head movements and helps integrate head and eye movements. In summary, remember that the lateral vestibulo-spinal tract is ipsilateral and long; the medial vestibulo-spinal tract is bilateral but shorter.

The main ascending tracts are from the superior and medial vestibular nuclei to the extraocular muscles through the medial longitudinal fasciculus (MLF).

11.2 Vestibular Connections to the Oculomotor Nuclei

Figure 11.2 shows details of the vestibular fibers ascending in the MLF to the oculomotor nuclei. Second-order afferents arise in the medial and superior vestibular nuclei. Connections from the MLF to the abducens, trochlear and oculomotor nuclei can be seen. These connections coordinate eye movements in response to head movements. Details of this coordination for horizontal eye movements are elaborated in the section on vestibular structure and function (See Figure 10.2).

Figure 11.2 Vestibular connections to the oculomotor nuclei. 1) Press PLAY to begin the pathway. 2) Click on each layer to see it enlarge 3) Roll over a layer to identify it's name.

 

Figure 11.3 shows controversial details of the ascending vestibular pathway that gives rise to the conscious perception. Afferents from primarily the superior vestibular nucleus ascend to the thalamus (the ventral posterior intermediate nucleus) probably in the lateral lemniscus or in the reticular formation beside the MLF. The primary cortical receiving areas for the vestibular system are more diffuse than for other sensory systems, and there is some dispute about their locations. Most people believe there is a cortical vestibular area in the parietal lobe near where the intraparietal sulcus meets the postcentral gyrus. This would be close to the somatic area of the head, which makes sense for the sensation of dizziness. Others believe there is an area of vestibular cortex anterior to primary auditory cortex. (See Figure 13.3) This pathway is bilateral.

Figure 11.3 Possible pathway to vestibular cortex. 1) Press PLAY to begin the pathway. 2) Then, click on each layer to see it enlarge with the path showing OR click before playing to see the layer without the path showing. 3) Roll over a layer to identify its name.

11.3 Vestibulo-cerebellar Connections

Figure 11.4 shows details of the vestibulo-cerebellar connections. Some vestibular afferents go directly to the cerebellum through the inferior cerebellar peduncle. Most 1st order afferents synapse in the medial and inferior vestibular nuclei and then ascend to the cerebellum in the inferior cerebellar peduncle. Most of these afferents innervate the flocculonodular node. Many interconnections are found between the vestibular nuclei and the cerebellum, which coordinate the postural adjustments.

Figure 11.4 Some vestibular inputs to the cerebellum. 1) Press PLAY to begin the pathway. 2) Then, click on the layer to see it enlarge with the path showing OR click before playing to see the layer without the path showing.

11.4 Lateral and Medial Vestibulospinal Tract

Figure 11.5 shows details of the lateral vestibulospinal tract. As the name suggests, this tract arises in the lateral vestibular nucleus. The tract descends through the inferior vestibular nucleus, giving this nucleus its "salt and pepper" appearance. Below the medulla the lateral vestibulospinal tract descends in the anterior white matter down the entire length of the spinal cord. This tract modulates postural adjustments to movements. For example, the reason we stagger after spinning is due to excess activity in the lateral vestibulospinal tract.

Figure 11.5 The lateral vestibulospinal tract. 1) Press PLAY to begin the pathway. 2) Then, click on each layer to see it enlarge with the path showing OR click before playing to see the layer without the path showing. 3) Roll over a layer to identify it's name. Figure 11.6 shows details of the medial vestibulo

Figure 11.6 shows details of the medial vestibulospinal tract. As its name suggests, this tract starts in the medial vestibular nucleus. It is a bilateral tract. It descends no lower than the mid-thoracic cord. It mediates head position by controlling the muscles of the neck and shoulder

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