Vestibular+System

=**Vestibular System**=


 * 1. Describe the major pathways of the vestibular system.**

__Primary Pathways__ Primary axons from the vestibular labyrinth can project to the ipsilateral vestibular nuclei (using glutamate neurotransmitter): medial vestibular nucleus (MVN), lateral vestibular nucleus (LVN), superior vestibular nucleus (SuVN), or Inferior/Spinal vestibular nucleus (IVN/SpVN).

Alternatively, the fibers from the vestibular labyrinth can project directly to the ipsilateral cerebellum (nodulus, flocculus, and uvula) through the juxtarestiform body located just medial to the restiform body nithe inferior cerebellular peduncle.

The cochlear nucleus is located lateral to the inferior cerebellar pudencle; the vestibular nucleus is located medial to the inferior cerebellar pudencle.

__Secondary Pathways__ Secondary axons from the vestibular nuclei can project to the cerebellum (nodulus, flocculus, and uvula, ipsilaterally) through the juxtarestiform body.

Secondary axons may also project bilaterally to nuclei controlling the extraoccular muscles (CN III oculomotor, CN IV trochlear, and CN VI abducens) through the medial longitudinal fasciculus (MLF).

Lastly, secondary fibers can project to the spinal cord ventral horn. Fibers from the LVN can descend the lateral vestibulospinal tract ipsilaterally to facilitate motoneurons for extensor muscles in both arms and legs. Fibers from the MVN can descend the medial (anterior) vestibulospinal tract bilaterally to cervical levels to affect neck movements.

__Other Pathways__ The vestibular nuclei also receive connections from the cerebellar cortex (purkinje cells) of vermis and flocculus (using GABA neurotransmitter), cerebellar fastigial nucleus (the most medial of deep nuclei), and the nucleus dorsalis (Clarke’s nucleus) of the spinal cord (which fibers travel with the dorsal spinocerebellar tract).

Axons from the vestibular nuclei also project to the thalamus, where they next go to the vestibular cerebral cortex (somewhere in parietal, temporal, and insular locations). They can also project to the reticular formation which is involved in vestibulo-autonomic reflexes. Some fibers go to the contralateral vestibular nuclei and some double back to vestibular receptor cells, following the same route to the inner ear as the olivocochlear bundle and using the same ACh neurotransmitter.

In summary, vestibular system sends information to regions controlling motor activity without going through the cerebral cortex (i.e., this allows reflex control of motor activity coordinated with head movement).


 * 2. Describe the fluid spaces of the vestibular labyrinth and their relationship to those of the cochlea.**

The fluid spaces of the vestibular labyrinth are similar to those in the cochlea in that there are concentric channels extending throughout the labyrinth with a perilymphatic duct outside the channel containing perilymph and an endolymphatic duct inside the channel containing endolymph. As with the cochlea, the perilymph is high in Na+ and low in K+ while the endolymph is high in K+ and low in Na+ concentrations.


 * 3. Describe how linear accelerations are converted to electrical activity in the otolith organs.**

The cilia are organized in both directions in each macula. In the utricle, the macula are organized predominately in the horizontal plane, while, in the saccule, the macula are organized predominately in the vertical plane. Otoliths are supported on a gelatinuous matrix that the cilia hair cells are embedded in. The hair cells synapse on to vestibular nerve fibers.

Linear acceleration leads to distortion of the gelatinous material in which the cilia are embedded, leading to bending of the cilia and depolarization in some hair cells and hyperpolarization in others.

Depolarization leads to release of excitatory neurotransmitters onto the vestibular nerve fibers and increase action potential firing. Hyperpolarization leads to decreased excitatory neurotransmitter release and decreased action potential firing. All directions of excitation are represented in each macula.


 * 4. Describe how angular accelerations are converted to electrical activity in the semicircular canals.**

In the semicircular canals, hair cells in each crista are organized in the same orientation as the cilia. The cupula blocks the endolymphatic duct in the ampulla and the cilia of the hair cells are embedded in the cupula. The hair cells synapse with the vestibular nerve fibers.

When the head turns, the inertia of the endolymph makes it push against the cupula which turns with the canal, leading to distortion of the cupula and bending of the cilia. This eads to depolarization or hyperpolarization of the hair cells, depending on the direction the cilia are bent.

Depolarization leads to release of excitatory neurotransmitters onto the vestibular nerve fibers and increase action potential firing. Hyperpolarization leads to decreased excitatory neurotransmitter release and decreased action potential firing. All directions of excitation are represented in each macula.

**5. Describe the reflexes whereby body and eye movements are coordinated with head movements.**

__Phasic Posture Reflexes__ Excitation of extensor muscles oppose the direction of acceleration, allowing posture to be maintained.

__Righting Reflex__ In general, vestibular, visual, and somatosensory systems function as a team for the maintenance of balance and orientation relative to gravity.

__Vestibulo-ocular Reflexes__ These reflexes allow an individual to maintain the direction of gaze during movement. Eye movements follow nystagmus pattern with slow movement in one direction when tracking one target, followed by rapid movement (saccade) in another direction to refocus on a new target when the original target can no longer be tracked. The direction of nystagmus is defined by the saccade (the direction of rapid movement when the eyes refocus to look at something else).

__Vestibulo-autonomic Reflexes__ Reflexive preparation of the circulatory system for movements.


 * X. Compare the similarities and differences between vestibular and auditory nerve fibers.**

All vestibular nerve fibers are myelinated. In the auditory system, type II fibers are unmyelinated.

Hair cells in the vestibular system have kinocilium that are unique to the vestibular system and are the only “true” cilium (the rest are all stereocilia). Kinocilium develop in the auditory system but are lost during development by adulthood.

Vestibular nerve fibers terminate on type I hair cells with chalice terminals that are unique to the vestibular system.