The vestibular system

Vestibular hair cell function

Like auditory inner hair cells, the cilia of hair cells in the vestibular system are bathed in endolymph, depolarized by cilia movement toward the kinocilium, and hyper polarized by movement away from the kinocilium. They also use TRPA1 channels opened by tip links

Post rotary nystagmus

Post rotary nystagmus is when the head stops, because the endolymph keeps moving

Vestibular neuroanatomy- cortex

Area 2v also receives visual and proprioceptive inputs. Electrical stimulation of 2v evokes sensations of spinning or dizziness. Area 3a also receives somatosensory inputs and may be involved in head movements. These cortical areas are probably responsible for our perception of movement

Semicircular canals with constant velocity angular rotation

1) The endolymph lagging behind the bony canal bends the cupula to cause the sense of rotation. 2) After continued rotation at a constant velocity, the endolymph moves at the same speed at the canal and the cupula does not bend. There is no sense of rotation. 3) When the head stops rotating, the endolymph continues to move which bends the cupula in the opposite direction to create the sensation of rotating in the opposite direction. Begin rotating at a constant velocity, endolymph lags behind bony canal, endolymph catches up to bony canal, endolymph continues to move after canal stops

Conjugate horizontal gaze

Conjugate horizontal eye movements in which both eyes move in the same direction and speed occur because the input to the abducens nucleus acts on abducens motoneurnons, that activate the ipsilateral lateral rectus and abducens internuclear interneurons that activate the contralateral medial rectus motoneurons. Abducens internuclear interneurons, Abi axons, ascend in the medial longitudinal fasiculus, MLF. If the left abducens nucleus receives excitation, then the right abducens nucleus receives inhibition

3 axes of eye rotation

Elevation: rotate superior part of eye upward. Adduction: nasal rotation. Intorsion: rotate superior part of eye nasally. Depression: Rotate superior part of eye downward. Abduction: temporal rotation. Extorsion: rotate superior part of eye temporally

The vestibulo-ocular reflex (VOR) prevents retinal slip

Eye movements serve one of two goals. They reflect the line of sight or they eliminate retinal slip. Retinal slip is image motion on the retina during head motion. The job of the VOR is to percent retinal slip

Semicircular canals

Inside each bony semicircular canal is an endolymph filled membranous labyrinth with an expanded region, the ampulla. Within the ampulla is a gelatinous mass, the cupula fixed to the canal. Embedded in the cupula are the stereo cilia and kinocilium of vestibular hair cells. Law of inertia is the quality in matter that lets it stay still if still and keep moving if it is moving. The semicircular canals work because of endolymph inertia

Retinal slip

Image motion on the retina during head torsion. The job of the VOR is to prevent retinal slip

Semicircular canal organization and hair cells

In each semicircular canal ampulla, all hair cells have the same orientation. Deflection of the hair cell bundles by movement of the cupula produces depolarization or hyper polarization in all of the hair cells in the ampulla. Hair cell orientation determines the activation direction for vestibular end organs

Extraoculur muscles work in a push pull fashion

Medial rectus, adduction, and lateral rectus, abduction. Inferior oblique, extorsion, superior oblique, intorsion. Superior rectus, election, inferior rectus, depression

How the otoliths sense linear acceleration

Otolith hair cell cilia are bathed in endolymph, but the hair cell tips are in contact with a calcium carbonate mass, the otoconia. When the head undergoes linear acceleration, inertia of the otoconia causes it to lag behind the head and bend the stereocilia

Semicircular canal organization and hair cells (part 3)

Rotating your head downward, forward, excites the anterior canals and inhibits the posterior canals. Rotating your head up, backward, excited the posterior canals and inhibits the anterior canals. By looking at the direction of cupula bending that excites hair cells in each canal, it is clear that semicircular canals on the opposite side of the head form opposing pairs. Left and right horizontal canals. Left anterior and right posterior canal. Left posterior and right anterior canals. With the head stationary, semicircular canal seventh nerve afferents have a spontaneous discharge of about 90 spikes per second. Angular head acceleration produces excitation of one seventh nerve and inhibition of the contralateral seventh nerve in the oposing semicircular canal. The vestibular neurons in your brain look at the difference in activity between the left and right vestibular nerves to determine the direction of rotation

Vestibular neuroanatomy- eye movements

Semicircular canal afferents project to neurons in the ipsilateral vestibular nucleus, superior and medial vestibular nuclei. These vestibular neurons project to extrraocular motoneuron nuclei, abducens, trochlear, and oculomotor that produce eye movements

Otoliths respond to linear acceleration

Seventh nerve afferents from the otoliths respond to static head tilt continuously because gravity provides a constant acceleration. The otoliths also sense changes in linear acceleration. An otolith seventh nerve afferent can respond to a .0005 g acceleration. This is equivalent to taking 40 seconds for an elevator to go between floor.

Vestibulo ocular reflex prevents retinal slip by rotating the eyes opposite to the direction of head rotation

The VOR rotates the eyes at the same velocity but in the opposite direction of the head rotation. This pattern keeps gaze, where they eye is looking in space, stable in spite of the head rotation. Angular acceleration of the semicircular canals is the only stimulus that activates the VOR. Vision does not activated the VOR

Vestibulo-ocular reflex and nystagmus

The VOR rotates the eyes the same velocity but in the opposite direction of the head rotation. Nystagmus is alternating fast phase and slow phase of eye movements. The slow phase for vestibular nystagmus is the VOR rotating the eyes opposite to the head rotation and the fast phase, an oppositely directed saccadic eye movement resets the eye. With vestibular nystagmus, the fast phase is in the direction of head rotation and the slow phase is the VOR opposite the direction of head rotation. Constant velocity head rotation in the dark initially produces nystagmus that decays with continued rotation

Semicircular canals are the eye movement coordinate system

The medial and lateral recti rotate the eye in a plane parallel to the horizontal canals. The superior and inferior recti rotate in the eye in a plane parallel to the ipsilateral anterior canal. The superior oblique and inferior oblique rotate the eye in a plane parallel to the ipsilateral posterior canal. There is no need for a coordinate system transformation for the VOR

Semicircular canals and constant velocity rotation

The three semicircular canals each respond to angular acceleration in the plane of the canal. In response to constant angular velocity, the canals only respond to the initial step in angular velocity from 0 to the new velocity and when the velocity returns to 0

Vestibular system organization- semicircular canals

The three semicircular canals each respond to angular acceleration in the plane of the canal. They are insensitive to angular acceleration perpendicular to the plane of the canal. The three semicircular canals are perpendicular to one another. This organization forms a coordinate system that is important for eye movements. Canals on the opposite side of the head form opposing pairs: Left and right horizontal canals. Left anterior and right posterior canals. Left posterior and right anterior canals.

Vestibular system organization- otoliths

The two otoliths sense linear acceleration. The utricle responds best to linear acceleration of the head moving forwards and backwards and side to side. The saccule responds best to up and down linear acceleration of the head, i.e gravity.

Vestibular neuroanatomy-posture

The vestibular nucleus contains 4 nuclei. 1) Superior vestibular nucleus, SVN. 2) Lateral vestibular nucleus, LVN. 3) Medial vestibular nucleus, MVN. 4) Inferior vestibular nucleus, IVN. Otolith afferents project to neurons in the ipsilateral vestibular nucleus, lateral and inferior vestibular nuclei. These vestibular neurons project to the spinal cord. The ventromedial pathways from motor cortex include the vestibulospinal tract

Two types of semicircular canal afferents

Type 1 afferents have an irregular pattern of discharge in which the time between action potentials is variable. Type 1 nerve afferents envelope type 1 hair cells, a calyx synapse. Type 1 afferents act as event detectors for high frequency had rotations. Type 2 afferents exhibit a regular discharge pattern in which there is little variability in the time between action potentials. Type 2 afferents form typical synapses, a bouton, on type 2 hair cells. Type 2 afferents provide detailed information about head rotation. Like other sensory systems, the vestibular system has parallel information pathways going into the brain

Vestibular eye movements

Vestibular eye movements eliminate retinal slip by moving the eyes in the opposite direction, but at the same soeed at the head rotation. The VOR only occurs because of bending of the cupula in the semicircular canals. Vision does NOT play a tole in causing the VOR. The semicircular canals rely on endolymph inertia to respond to angular acceleration. Nystagmus is alternating fast and slow eye movements. Constant velocity head rotation in the dark produces vestibular nystagmus that decays as the endolymph cache sup with the bony semicircular canals and oppositely directed post rotary vestibular nystagmus occurs when head rotation stops, because the endolymph keeps flowing

The VOR is a three neuron reflex

Vestibular neurons receive an excitatory input from the semicircular canals. The second order excitatory vestibular neurons excited the contralateral abducens motoneurons and abducens internuclear interneurons. The second order inhibitory vestibular neurons inhibit the ipsilateral abducens motoneurons and abducens internuclear interneurons

Semicircular canal organization and hair cells (part 2)

When rotating the head to the left, endolymph flow bends the cilia in the left horizontal semicircular canal toward the kinocilium, depolarization. The same leftward head turn, bends the cilia in the right horizontal semicircular canal away from the kinocilium. This, the left and right horizontal semicircular canals are antagonistic

Semicircular canal

With semicircular canal angular acceleration, inertia of the endolymph causes it to lag behind the bony canal bending the cupula and the embedded hair cell cilia. We sense angular head movements because of bending of the cupula. The semicircular canals work because of endolymph inertia

Vestibulo-ocular reflex and nystagmus (part 2)

With vestibular nystagmus, the fast phase is in the direction of hear rotation and the slow phase is the VOR, opposite the direction of head rotation. Constant velocity head rotation in the dark initially produces nystagmus that decays with continued rotation. When the subject stops rotating, the eyes develop nystagmus in the opposite direction, post rotary nystagmus. With post rotary nystagmus, the fast phase is opposite the previous direction of head rotation and the slow phase is opposite in the previous VOR direction