VESTIBULAR PHYSIOLOGY BY DR .SHLOKA.pptx

vestibular physiology

1. VESTIBULAR PHYSIOLOGY DR SHLOKA PATEL ENT PG-1

2. The vestibular system is located in the inner ear It responds to movement and gravity, contributing to the development of balance equilibrium postural control muscle tone maintaining a stable visual field while you are moving bilateral coordination.

3. FUNCTION Vestibular system collects information about position and motion of head. Together with visual and proprioceptive signals the brain uses this information to coordinate the eyes ,head and body during movement and to give conscious perception of orientation and motion Any disturbance in this integrative process can lead to dizziness

4. Anatomical organization of Vestibular system PERIPHERAL CENTRAL PERIPHERAL CENTRAL Horizontal scc anterior scc Posterior scc Utricle and saccule Lateral nuclei Medial nuclei Superior nuclei Inferior vestibular nuclei and projections of all these nuclei to thalamus, cortex, cerebellum, descending spinal cord and extra ocular motor nuclei

5. How it works? Detection of body motion as detected by head motion Detection of Motion of the head in space relative to gravity Maintenance of fovea in object to visual fixation Maintenance of balance Activity of autonomous nervous system

6. Each membranous semicircular canal is filled with endolymph a potassium rich extracellular fluid and is bathed in perilymph which has the approximate composition of CSF. The end of the canal opens into the vestibule Near one end of each membranous canal is a widening known as ampulla, which contains the crista ampullaris and cupula The crista ampullaris is a saddle shaped gelatinous structure along one wall of memebranous canal that contains the sensory hair cells of the vestibular system.

7. Basic structure of Macula Sensory Epithelium -sensory cells -supporting cell basement membrane: nerve fibres pass through Otoconial Membrane -otolith/ otoconia: barrel shaped, contains Ca2+ -gelatinous layer -honeycomb mesh: in which the cilia protrudes

8. Basic structure of CRISTA Sensory Epithelium -Basement membrane -sensory cells -supporting cells The gelatinous layer overlying the sensory epithelium is called as CUPULA

9. The cupula act as a membranous diaphragm, stretching from the crista to the opposite walls of the canal Because the membranous labyrinth is tethered to the skull, the crista and cupula accelerates with the head as it rotates, but the endolymph’s inertia causes it to lag behind. The endolymph accumulates with higher pressure on one side of the cupula than the other, indenting it and bending the hair cells within the crista. When the head ceases to accelerate, the cupula and crista gradually return to their resting positions

10. ANATOMICAL ORIENTATION OF SEMICIRCULAR CANALS Horizontal scc is around 30 degree to the horizontal axis of the cranium Vertical scc i.e. both posterior as well as superior are also arranged in a direction that right Sup.scc is always parallel to opposite left post. Scc and left sup.scc is almost parallel to right post scc And in the same side these 3 SCC are at right angles to one another

11. Each horizontal canal is maximally excited by rotation towards the side of the canal and inhibited by the opposite direction. This results in an excitatory slow phase movement towards the side opposite the canal and resetting saccade towards the canal Superior canal is excited by rotation downwards and to the side, in the plane of the canal, this results in a vertical- tortional nystagmus with the slow phase of vertical component upward and resetting saccade downward The posterior canal is excited by an upward rotation and to the side, in the same plane of canal so that slow phase is downward and resetting phase upwards

12. ANGULAR ACCELERATION The semicircular canals or ducts are oriented approx. at right angle to each other each corresponding roughly to one of the 3 planes of motion i.e. turning left and right, nodding up and down, and tilting to a side they contain a fluid called endolymph. When the head turns , the ducts that are located on the same plane of motion rotate but the fluid lags behind because of inertia ,this causes the fluid to briefly move in the opposite direction as the head, and either push or pull on the cupula ,bending the cilia on the hair cells and thus activating them to send nerve impulses to brain. The direction of the bend determine if the signals generated are excitatory or inhibitory, Because two sides of the head are mirror images, a head turn generates excitatory signals on one side( ampullo -petal movement) and inhibitory signals ( ampullo -fugal movement) on the other Angular accelerations are best sensed by the SCCs

13. LINEAR ACCELERATION The 2 otolithic organs are two patches of hair cells oriented nearly perpendicular to each other. The saccule being vertical and the utricle being horizontal, the cilia of these cells are embedded in a gel like layer sprinkled with calcium carbonate crystals called OTOCONIA commonly known as ear rocks. The crystal add weight to the layer pulling it down with gravity ,When the head is in upright position the gelatinous layer bears down evenly on the layers of the utricle ,the cilia remains straight and no signals are generated. On the vertical saccule, however the heavy gel is pulled down by gravity at one end bending the cilia generating nerve impulse. The reverse is true when the head is horizontal. Horizontal linear acceleration is best sensed by utricular maculae. i.e the utricle senses lateral tilt and translation of head whereas saccule measures front to back tilt and translation as well as motion aligned with pull of gravity

14. Orientation of saccule and utricle Maculae of the saccule in vertically oriented Maculae of the utricle is in horizontal axis Maculae of saccule and utricle are oriented at right angle to each other

15. VESTIBULAR HAIR CELLS The primary sensory cell is the hair cells Crista comes in the ampullated end of the semicircular canal Maculae is present in the saccule and utricle There are around in total of 23,000 hair cells in total of 3 semicircular canals crista. 45,000 to 60,000 hair cells in total maculae of the otolith organs Spatial orientation of these hair cells with the sensory epithelium is imp. Saccule and utricle are right angle to one another

16. ARRANGEMENT OF HAIR CELLS in otolithic organs Striola – linear structure which separates the maculae into two parts in both saccule and utricle There are two types of hair cells type 1 and type 2 In each hair cell there is 1 kinocilium and more of stereo cilium In saccule kinocilium is arranged away from the striola In utricle kinocilium are arranged towards the striola If the kinocilium are beaten away from the striola of saccule maculae they get depolarised In the utricle the kinocilium moving towards the striola will get depolarised

17. Both hair cells located in sensory epithelium of semicircular canals and otolith organs are together responsible for coding head acceleration into a modulation in the discharge rate of vestibular afferent fibres Each hair cells contain a bundle of 50-100 stero cillia and one long kinocilium at the end of each bundle. The location of which is relative to stereocillia gives each hair cells an intrinsic polarity In horizontal canal the kinocillium of every hair cell is located on side of cilliary bundle facing the utricle whereas this arrangement is reversed in superior and posterior canals Displacement of cilliary bundle opens the potassium channels which in turn depolarises the hair cells from RMP of -50 mV This depolarisation leads to calcium influx at the basal end of hair cells and increased flow of neurotransmiter into the synapse whereas displacement in the opposite direction hyperpolarises the cell and reduces the neurotransmiter release

18. There are 3 planes of head motion Two is in the vertical axis i.e. yaw (NO NO)& pitch(flexion and extension) Another can be in the horizontal axis i.e lateral head tilt called ROLL

19. ACTION POTENTIONAL The movement of endolymph along with the cupula and the otolith organ will causes a force and when the maximum force is parallel to the surface of the sensory epithelium ,the stereo cilia will move towards the kinocilium and causes depolarization and this causes opening of the iodine channels of the hair cells and the influx of potassium and calcium ions in hair cells This causes a production of a neural excitation potential so the mechanical energy is transduced into a ionic flow and there will be neural action potential these are sensed by the afferent fiber attached to the hair cells and thus the nerve endings will catch the action potential.

20. BOUTON – only afferents have innervations onto Type 2 hair cells in peripheral zone of cristae. These are regularly discharging and have low rotational sensitivity. Dimorphic afferents have calyx endings on Type 1 hair cells and bouton endings terminating at type 2hair cell Those dimorphic endings terminating in peripheral zone are regularly firing or discharging whereas the central ones are irregularly but with higher rotational sensitivity. This function of different hair cells is for coding type of head movement

21. The afferent nerve fibres in superior vestibular nerve extend from sensory structure in the superior, horizontal canal and the utricle to the vestibular nuclei in the brainstem The inferior vestibular nerve leads from posterior semicircular canal and saccule All most all vestibular nerve afferents have a spontaneous or resting state with some fibres firing at 100 spikes per second . This enables each afferent to respond both excitatory and inhibitory stimuli

22. According to ewald and his experiment the excitatory stimuli leads to longer amplitude responses than inhibitory stimuli. This is at least in part attributable to resting discharge rate of vestibular nerve afferents and central vestibular neurals These neurons can be excited up to a firing rate of at least 350 spikes per second but can be inhibited to only 0 spike per second thus there is 3-4 fold higher range of excitation in comparison to inhibition The first observation,head eye movements takes place always in direction of canal being stimulated & flow of endolymph termed as EWALD’S FIRST LAW Ewald second law is “excitatory responses are larger than inhibitory one These relationship provide a basis for understanding many of the symptoms and signs that occur in injury to the labyrinth

23. Central processes involved in control of vestibular reflexes The central part of the vestibular system consist of the vestibular nuclei which integrates information from vision, proprioreception via spinal and cervical afferents and vestibular signals These nuclei send projects from occulomotor system where they control the eye movements. The muscles of the neck and spine where they steady the head, thalamus and cortex Central vestibular circuit also include parts of cerebellum which is invovled in recalibrating the system whenever needed the projections to occulomotor nuclei reflexly maintain a steady image on the retina while the head is turning, tilted or moving linearly in space. Quantification of these reflexes is the most common way of evaluating these reflexes

24. PRIMARY VESTIBULAR REFLEXES Vestibulo ocular reflex Vestibulospinal reflex Vestibulo colic reflex

25. VESTIBULO OCULAR REFLEX a.k.a Oculocephalic reflex Reflex eye movement that stabilizes images on the retina during head movement by producing an eye movement in direction opposite to head movement, thus preserving the image on the centre of the visual field E.g when the head moves to the right, the eyes moves to the left and vice versa

26. The VESTIBULOOCULAR REFLEX that controls eye muscles to keep visual object in focus while the head is moving it does so by moving the eye in the opposite direction as the head

27. AVOR - Angular vestibulo -ocular reflex is a three neuron arc consisting of vestibular afferent neuron, a vestibular interneuron and an occulomotor neuron innervating the extra ocular muscles. For example turning the head to the left excites the left horizontal SCC and the eyes tract to the right in response this action leaves angle of the target relative to the eye unchanged and the image steady to the retina. Similar relationship is applied to posterior and superior canal The AVOR is the fastest and one of the most exquisitely accurate reflexes in the whole body. Its latency= 7 ms leads to production of eye movement that typically has less than 5% errors with respect to rapid eye movement. This so far is important as even a small time delay might lead to instability of visual image on the retina. The visual system is unable to provide effective feedback to stabil z e gaze during rapid head movement because it has a latency of 50ms or more Deficits of AVOR therefore lead to symptom of OSCILLOPSIA (the apparent motion of objects that are known to be stationary)

28. LVOR (Linear vestibulo - occular reflex) is responsible for compensation of linear accelerations for example tilting head side to side evokes ocular counter rolling which is tortional movement of eyeball about line of sight that partially compensates for the effect of rotation The motion in which the head is moved laterally however requires the eyes to track with horizontal rotation. The otolith organs cannot distinguish between tilt and the translational movement so the system uses other information from SCC and other sensory inputs to decide how to move the eye

29. Velocity storage is the name given to the process that maintains the sense of rotation even after the rotation has stopped Signals from SCC decay away slowly with the time constant of approximately 5 seconds however the time constant of AVOR typically lies between 12-20 seconds, this difference between the two values is due to velocity storage mechanism hence in patients with peripheral vestibular dysfunction the velocity storage mechanism may cease to function. This causes rotation-induced nystagmus, and the conscious perception of rotation, to decay in shorter period of time than normal

30. NEURAL INTEGRATOR - the neural integration circuit may be located in the medial vestibular and hypoglossi nuclei. It is the process that provide the signal to hold the eyes away from the primary position facing straight ahead. when looking away from primary position the extra ocular muscle require a burst of activity to move the eye from eccentric position then a sustained level of discharge that signal the muscle to hold the eye in eccentric position hence the patient with vestibular loss this process may become dysfunctions and the eye may drift inappropriately in the original position

31. Vestibulo spinal(VSR) & vestibulo-collic reflex(VCR) -Function of both the reflex is maintenance of proper orientation in space & motion -Extensor muscles of neck, trunk, arms&limbs are the effector organs for For VCR &VSR - similar to VOR the same push-pull mechanism are used for controlling the balance between extensor &flexor muscles. - these reflexes are mediated through projection of the vestibular nuclei on to medial &lateral vestibulospinal tract. - thus these pathways project to lower limb &neck muscles to maintain upright position &balanced locomotion

32. VESTIBULAR PATHWAY Balance ,orientation of space is the networking b/w the information from vestibule , vision , proprioception and superficial sensation Afferent fibers ( primary vestibular neurons) from both type 1 and type 2 hair cells will go to the SCARPA’S GANGLION which is situated in the internal auditory canal. SCARPA’S GANGLION has a inferior portion and a superior portion so the primary vestibular neurons from superior scc , lateral scc , utricle, and small portion of saccule will go to the superior portion. Major portion of the saccule and as well as the posterior scc will go to the inferior portion of the scarpa’s ganglion. From scarpa’s ganglion fibers will go to the vestibular nuclei which is situated in the floor of the fourth ventricle at the ponto medullary junction. Some efferent fibers go to the cochlea as well we call them BUNDLE OF OORT.

33. Vestibular nucleus has got 4 divisions SUPERIOR vestibular nucleus also known as bechterew . Afferent fibers from the cristae and the cerebellum will synapse with it LATERAL vestibular nucleus also known as dieter. Afferent fibers from utricular maculae, cerebellum , Spinal and commissural fiber synapses here MEDIAL vestibular nucleus also known as schwalbe .Afferent fibers are mainly from cristae, utricular maculae, cerebellum, reticular formation. DESCENDING vestibular nucleus ,Afferent fibers from maculae of utricle and saccule From vestibular nucleus the efferent fibers will go to the opposite vestibular nucleus to the same nucleus of cranial nerve 3,4,6 and also to the opposite side Then they will go to the VESTIBULOSPINAL TRACT THEN TO CEREBRAL CORTEX THALAMUS RETICULAR FORMATION AUTONOMIC NERVOUS SYSTEM

34. By all these mechanism i.e VOR VSR VCR The body get information about linear as well as angular acceleration and movement of head in relation to gravity.

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