Vestibular analyzer

Function of vestibular system Vestibular system - organ of position and balance sense - placed in the semicurcular canals in petrous bone lying in three mutually perpendicular planes. The canals start in utricle, which is connected with sacculus. Both parts are placed in vestibulum communicating with ductus cochlearis. One outlet of each canal is transformed in ampulla, divided by the ampullary crist into two parts. Macula utriculi is in the lower part of utricle, the macula sacculi in sacculus. The crists and ampullae are covered by sensory epithelium composed of hair-cells. There are also gelatinous cupulae on ampullary crists and the statoconia membranes in maculae. Their function is to stimulate stereocilia of sensory cells. The statoconia are crystals of CaCO3 - it increases the mass of gelatinous membranes.

Function of vestibular system The semicircular canals allow analyse the rotational motion of the head. Receptors of ampullary crists react on angular acceleration. The cupulas of crists work as valves, which are deflected by streaming endolymph and stimulate the hairs of sensory cells by bending – depolarisation or hyperpolarisation takes place. The receptors of utricle and sacculus react on linear acceleration and gravitation. When changing the head position, the membrane with statoconia shifts against hairs of sensory cells - excitation arises. Important for keeping erect position - static reflexes.

The Vestibular System generates compensatory responses to head motion postural responses ocular-motor responses visceral responses To achieve this the vestibular system measures Head rotation Head acceleration Einstein’s equivalency theorem states that an accelerometer cannot distinguish between translational accelerations and tilts

SSC Coplanar Pairing Spatial arrangement of the 6 SSC cause 3 coplanar pairings R & L lateral, L anterior and R posterior; l posterior & R anterior; R & L horizontal Allows for a Push-Pull arrangement of the two sides (e.g., as head turns right, right SSC will increase firing rate & the left SSC will decrease firing rate) Advantages sensory redundancy common mode rejection/noise assist in compensation for sensor overload

Inhibitory Cutoff Depolarization of the ipsilateral hair cells occurs during angular head movements Hyperpolarization of contralateral hair cells occurs at the same time Hair cells are only able to hyperpolarize to what they were at rest = cut off of inhibitory influences from the movement going in the opposite direction even if the ipsilateral hair cells continue to spike higher firing rates

Purves 2001

Otoliths Utricle and saccule Otolith sensory structures Maculae Otolithic membrane Otoconia Movement of gel membrane & otoconia cause a shearing action to occur over the hair cells → sensitivity of otoliths

Otolith Function Respond to: Linear head motion on acceleration Static tilt Two organs respond to respective accelerations or tilts in their respective planes Saccule has vertical orientation of maculae Utricle has horizontal orientation of maculae

Bear 1996

Hair Cells 2 types: kinocilium & stereocilia Sensory structures for the peripheral end organs (maculae and ampula) Hyperpolarized or depolarized depending upon the direction of deflection of the stereocilia (movement of stereocilia towards the kinocilium causes depolarization of the hair cell) Affect the firing rate of the primary vestibular afferents to the brainstem

Bear 1996

Striola of the Macula Striola serves as a structural landmark Contains otoconia arranged in narrow trenches, dividing each otolith Orientation of the hair cells change over the course of the macula Allows otoliths to have multidirectional sensitivity

Principles of the Vestibular System Tonic firing rate Vestibular Ocular Reflex Push-pull mechanism Inhibitory cutoff Velocity storage system

Vestibular Apparatus Vestibular apparatus and cochlea form the inner ear Vestibular apparatus – provides sense of equilibrium consists of otolith organs (utricle and saccule) and semicircular canals

Vestibular Apparatus Sensory structures located within membranous labyrinth filled with endolymph and located within bony labyrinth

Vestibular Apparatus Utricle and saccule provide info about linear acceleration Semicircular canals, oriented in 3 planes, give sense of angular acceleration

Vestibular Apparatus Hair cells – receptors for equilibrium Each contains 20-50 stereocilia (hair-like extensions) 1 of these is a kinocilium---a true cilium

Vestibular Apparatus Stereocilia bend toward kinocilium – hair cell depolarizes releases NT that stimulates CN VIII When bent away from kinocilium – hair cell hyperpolarizes In this way, frequency of APs in hair cells carries information about movement

When stereocilia are bend away from kinocilium, hair cell is hyperpolarized, i.e. inhibited. It occurs because acceleratory force acts to flow of fluid in semicircular canals during circular motion of the head or whole the body. Hair cells are located along crista ampularis and protect their cilia in cupula. Hair cells are secondary sensor cells, which synapse with neurons. Axons of these nerve cells compose vestibular nerve.

Statoconia membrane in sacculus cellbio.utmb.edu/.../Ear/ organization_of_the_inner_ear.htm.

Utricle and Saccule Have a macula that contains hair cells Hair cells embedded in gelatinous otolithic membrane contains calcium carbonate crystals (otoliths) that resist change in movement

Utricle and Saccule Utricle sensitive to horizontal acceleration Hairs pushed backward during forward acceleration Saccule sensitive to vertical acceleration Hairs pushed upward when person descends

Semicircular Canals Provide information about rotational acceleration Project in 3 different planes Each contains a semicircular duct Crista ampullaris – where sensory hair cells are located 10-42

Semicircular canal function Ampula is enlargement at one end of semicircular canal. It has a small crest on top of which is a gelatinous mass known as cupula. Hair cells have two kinds of cilia – kinocilium and stereocilia. Kinocilium is large cilium located at one end of hair cell. Stereocilia are small. When stereocilia are bent towards kinocilium, hair cell is depolarized, i.e. stimulated.

Semicircular Canals Hair cell processes embedded in cupula of crista ampullaris When endolymph moves cupula moves Sensory processes bend in opposite direction of angular acceleration

From Kandel and Schwartz

Ascending Pathways Vestibular nerve Vestibular nuclei Cerebellum Oculomotor complex CN 3, 4, and 6 Along with vestibulospinal reflexes coordinate head and eye movements

Relay Centers Thalamus Vestibular Cortex Connection with vestibular cortex and reticular formation → arousal and conscious awareness of body; discrimination between self movement vs. that of the environment Vestibular Cortex Junction of parietal and insular lobe Target for afferents along with the cerebellum Both process vestibular information with somatosensory and visual input

Tonic Firing Rate Vestibular nerve and vestibular nuclei have a normal resting firing rate (70-100 cycles/sec) Baseline firing rate present without head movement Tonic firing is equal in both sides; if not, a sense of motion is felt e.g., vertigo, tilt, impulsion, spinning Excitation and inhibition of the vestibular system can then occur from stimulation of the hair cells Spontaneous recovery with light

Vestibular-Ocular Reflex (VOR) Causes eyes to move in the opposite direction to head movement Speed of the eye movement equals that of the head movement Allows objects to remain in focus during head movements

Compensatory Eye Movements VOR Optokinetic reflex Smooth pursuit reflex, saccades, vergence Neck reflexes combine to stabilize object on the same area of the retina=visual stability

Vestibular Processing Gain Keeps eye still in space while head is moving Ratio of eye movement to head movement (equals 1)

Vestibular Processing Velocity Storage Mechanism Perseveration of neural firing in the vestibular nerve by the brainstem after stimulation of SSC to increase time constant (10sec.) SSC respond by producing an exponentially decaying change in neural firing to sustained head movement Otolith & somatosensory input also drive mechanism

VOR Dysfunction Direction of gaze will shift with the head movement Cause degradation of the visual image In severe cases, visual world will move with each head movement

Oscillopsia Visual illusion of oscillating movement of stationary objects Can arise with lesions of peripheral or central vestibular systems Indicative of diminished VOR gain motion of images on fovea diminished visual acuity

Cerebellum Monitors vestibular performance Readjusts central vestibular processing of static & dynamic postural activity Modulates VOR Provides inhibitory drive of VOR (allows for VORc)

Descending Pathways Provide motor output from the vestibular system to: Extraocular muscles (part of VOR) Spinal cord & skeletal muscles (generate antigravity postural activity to cervical, trunk & lower extremity muscles) Response to changing head position with respect to gravity (righting, equilibrium responses)

Vestibulospinal Reflex (VSR) Generates compensatory body movement to maintain head and postural stability, thereby preventing falls

Neural Pathways for Equilibrium and Balance

Nystagmus and Vertigo Vestibular nystagmus – involuntary oscillations of eyes occur when spinning person stops Eyes continue to move in direction opposite to spin, then jerk rapidly back to midline Vertigo – loss of equilibrium Natural response of vestibular apparatus Pathologically, may be caused by anything that alters firing rate of CN VIII Often caused by viral infection

Otolith Ambiguity Translation Tilt Gravity Acceleration The otolith organs transduce acceleration Thus, translational accelerations and tilts of the head cause similar activity on otolith afferents. Otolith information is therefore ambiguous (Einstein’s equivalency principle) and must be resolved in order to provide proper stabilizing and orientation responses. Both Angles are equal.

Vestibular System Consists of three semi- circular canals Monitors the position of the head in space Controls balance Shares fluid with the cochlea Cochlea & Vestibular system comprise the inner ear