University:
Western Sydney University
Course:
MEDI 2003 | Clinical Sciences 2
Academic year:

2024
Views:

208

Pages:

7
Author:

Mariam A.

Vestibular pathway   Primary afferents project on to the vestibular nuclei (superior, lateral, medial and inferior divisions). Each division then sends second-order neurons throughout the brain and spinal cord, effecting the appropriate response to maintain balance. Vestibulo-ocular reflex  Allows maintenance of the same visual field despite changes from head rotations.  When the head rotates, it excites the semi-circular canals of the direction of motion (e.g. if right rotation of the head, excitation of semi-circular canals of the right vestibular system).  Signal is sent up through the vestibulocochlear nerve (CN VII) through Scarpa’s ganglion and end in the vestibular nuclei of the brainstem.  Fibres cross contralaterally to project with the abducens nucleus.  Here, they synapse with two different pathways: o Directly to the lateral rectus on the ipsilateral side of the abducens nucleus via the abducens nerve. o Projections to the contralateral oculomotor nucleus by the medial longitudinal fasciculus, which projects to the medial rectus via the oculomotor nerve.  At the same time, the semi-circular canals of the other side (e.g. left, in this scenario) are inactivated. As a result there is inhibition of vestibulocochlear nucleus on contralateral side: o Inhibition of lateral rectus on the ipsilateral side of the abducens nucleus. o Inhibition of medial rectus on the contralateral side of the abducens nucleus.  Overall, as the head rotates in one direction, muscles work together to pull the eye in opposite direction, therefore maintaining the same field of vision. Vestibulo-spinal reflex  Maintains balance, posture and stability in the environment with gravity.  Used during a fall.  Cerebellum projects into two areas: o Lateral vestibular nucleus at the mid-pons. o Medial vestibular nucleus at the rostral medulla.  Nuclei then project down their corresponding vestibulo-spinal tracts to the ventral horn of the spinal cord.  The vestibulo-spinal tract increases muscle tone in extensor muscles. o Information from medial division of vestibular nuclei acts on axial muscles. o Information from lateral division acts on peripheral muscles. Vestibulo-colic reflex  Allows control of the muscles in the head and neck.  Reflex head movement counters the movement sensed by the otoliths or semicircular canals. o A very young child will have a primitive reflex. o Moving the head will result in the limbs moving. Nystagmus  Nystagmus can be induced in a clinical setting using warm or cold water. o If used in one ear (either hot or cold), there will be a different firing rate than baseline. o The patient will experience a sense of movement, and they will have nystagmus. o If one of these organs are not working, putting in the stimulus (hot or cold water) would not produce the expected outcome, providing evidence that it is a peripheral lesion or primary afferent lesion.