Vestibular System √

UTRICLE

most horizontal motion

SACCULE ( HEAD TILT)

mostly vertical motion

Caloric Testing

- Caloric testing can be used to test the function of the brainstem in an unconscious patient. - figures show eye movements resulting from cold or warm water irrigation in one ear for (1) a normal subject, and in three different conditions in an unconscious patient: (2) with the brainstem intact; (3) with a lesion of the medial longitudinal fasciculus (MLF; note that irrigation in this case results in lateral movement of the eye only on the less active side); and (4) with a low (D)brainstem lesion

ADAPTATION

- Despite its great sensitivity, the hair cell can adapt quickly and continuously to static displacements of the hair bundle caused by large movements. - Such adjustments are useful in otholit organs , since the help maintain sensitivity to small linear and angular accelerations of the head, despite constant input from gravitational forces that much greater. - Adaption occurs in both directions in which the hair bundle displacement generates a receptor potential, at different rates for each direction. - When hair bundle is pushed toward kinocilium, tension is initially increased in the gating spring - During adaptation, tension decreases back to the resting level, perhaps because one end of the gating spring repositions itself along the shank of the stereocilium - When hair bundle is displaced in opposite direction, away from kinocilium, tension in spring initially decreases; adaptation then involves an increase in spring tension - theory is that a calciumreg-ulated motor such as a myosin ATPase climbs along actin filaments in the stereocilium and actively resets the tension in the transduction spring.

Vestibulo-ocular reflex

- Normally, when head is not being rotated, output of the nerves from the right and left sides are equal-->no eye movements - When head is rotated in horizontal plane, vestibular afferent fibers on side toward turning motion increase their firing rate, while afferents on the opposite side decrease their firing rate (Figures A and B). -net difference in firing rates then leads to slow movements of eyes counter to turning motion. -->this reflex response generates the slow component of a normal eye movement pattern called physiological nystagmus, which means "nodding" or oscillatory movements of the eyes (Figure B1).

Central pathways

- Projections of vestibular nucleus to nuclei of cranial nerves III (oculomotor) and VI (abducens). - connections to oculomotor nucleus and to the contralateral abducens nucleus are excitatory (red), whereas the connections to ipsilateral abducens nucleus are inhibitory (black). -There are connections from oculomotor nucleus to medial rectus of left eye and from the adbucens nucleus to lateral rectus of right eye. - This circuit moves the eyes to the right, that is, in the direction away from the left horizontal canal, when the head rotates to the left. Turning to the right, which causes increased activity in the right horizontal canal, has the opposite effect on eye movements. - The projections from the right vestibular nucleus are omitted for clarity.

VESTIBULAR HAIR CELLS

- Same build and functionality as inner ear hair cells, expect that they have a kinocilium, which is the same reference point as the highest cilia in the inner ear hair cells. - movement of the stereocilia toward kinocilium opens mechanically gated transduction channels located at tips of the stereocilia--> depolarization - Movement of stereocilia in direction away from kinocilium closes the channels, hyperpolarizing hair cell and thus reducing vestibular nerve activity - It also keeps the biphasic nature (transduction channels are open in the absence of stimulation) - In the ampulla located at the base of each semicircular canal, the hair bundles are oriented in the same direction. - In the sacculus and utricle, the striola (special area) divides the hair cells into populations with opposing hair bundle polarities.

vestibulo oular reflex with water

- individual is placed onr back and head is elevated to about 30° above horizontal, horizontal semicircular canals lie in almost vertical orientation. - Irrigating one ear with cold water will lead to spontaneous eye movements because convection cur rents in the canal mimic rotatory head movements away from the irrigated ear - In normal individuals, these eye movements consist of slow movement toward irrigated ear and a fast movement away from it.

ANATOMY OF THE LABYRINTH

- labyrinth is buried deep within temporal bone and consists of two otolith organs (utricle, saccule) and three semicircular canals. - otolith organs respond primarily to linear accelerations and static head position relative to the gravitational axis. - semicircular canals respond to rotational accelerations. - vestibular hair cells are located in the utricle and saccule and in three juglike swellings called ampullae, located at the base of the semicircular canals next to the utricle. - Within each ampulla, the vestibular hair cells extend their hair bundles into the endolymph of the membranous labyrinth.

Electrical Tuning

- other tuning mechanisms are especially important in otolith organs, where, unlike the cochlea, there are no obvious macromechanical resonances to selectively filter and/or enhance biologi-cally relevant movements. - such mechanism is an electrical resonance dis-played by hair cells in response to depoarization: The membrane potential of a hair cell undergoes damped sinusoidal oscillations at a specific frequency in response to the injection of depolarizing current pulses

sustained response of a vestibular nerve fiber inner- vating the utricle

- responses were recorded from axons in a monkey seated in chair that could be tilted for several seconds to produce steady force. - Prior to tilt, the axon has a high firing rate, which increases or decreases depending on the direction of the tilt. - Notice also response remains at a high level as long as tilting force remains constant; thus, such neurons faithfully encode the static force being applied to the head (Figure 13.6A). When head is returned to original position,--> firing level of the neurons returns to baseline value. - Conversely, when tilt is in opposite direction, neurons respond by decreasing their firing rate below the resting level (Figure 13.6B) and remain depressed as long as static force continues.

Semicircular canals

- sense heads rotation rom self-induced movements or from angular acceleration - ampulla which houses the sensory epithelium, - cristacontains the hair cells. - As head rotates, cupula (gelatinous mass, that bridges the width of the ampulla, forming a fluid barrier through which endolymph cannot circulate) bends in opposite direction of the rotation. - cupula is distorted by movements of the endolymphatic fluid - Pairs of canals act in concert--> work in concert with partner located on other side of the head that has its hair cells aligned oppositely. - 3 pairs : Left horizontal with right horizontal canal, Left anterior with right posterior canal, Right anterior with left posterior canal

How Otolith Neurons Sense Linear Forces

- structure enables to sense both static displacements, by tilting the head relative to the gravitaional axis, and transient displacements by translational movements of the head - hair bundle displacement will occur transiently in response to linear accelerations and tonically in response to tilting of the head - properties of hair cells are reflected in the responses of the vestibular nerve fibers that innervate the otolith organ - nerve fibers have steady and relatively high firing rate when the head is upright - change in firing rate in response to a given movement can be either sustained or transient, thereby signaling either absolute head position or linear acceleration

The Otolith Organs

- tilting or translational movements are detected by 2 otolith organs: the saccule and the utricle - contain sensory epithelium--> maucla - macula consists of hair cells and associated supporting cells - overlying layer over haircells is the --> otolithic membrane, in which are embedded crystals of calcium carbonate called otoconia - oconia make otolithic membrane considerably heavier than structures and fluids surrounding it--> when head tilts, gravity causes membrane to shift relative to sensory epithelium --> displacement of the hair bundles generates a receptor potential in the hair cells

Semicircular canals: vestibular nerve response

- vestibular fibers that innervate semicircular canals exhibit high level of spontaneous activity - transmit information by either increasing or decreasing their firing rate - experiment: initial period of acceleration, then periord of several seconds at constant velocity, and finally a period of sudden deceleration to a stop - maximum firing rates observed correspond to period of acceleration; maximum inhibition corresponds to period of deceleration - during constant-velocity phase, response adapts so that firing rate subsides to resting level; after movement stops, neuronal activity decreases transiently before returning to resting level.

ELECTRICAL RESONANCE

Electrical resonance achieved through potassium influx and efflux → cancel each other out.

Nomenclature

Roll: angular acceleration Pitch: angular acceleration Yaw: angular acceleration Translation: linear acceleration

THE LABYRINTH

The labyrinth uses the same type of specialized cell as the cochlea to transduce physical motion into neural impulses. The motions transduced in this system arise from head movements, inertial effects due to gravity and ground-borne vibrations.

Function

The main functions of the vestibular system are balance, spatial orientation and the stabilization of vision.