Physiology of Equilibrium

Physiology of equilibrium, Semicircular canal, Otolith organ, Ewald's law, VOR, Alexander's law, Vestibulo ocular reflex

1. PHYSIOLOGY OF EQUILIBRIUM PRESENTER – DR. SHILPA M J

2. EQUILIBRIUM

3. THE INNER EAR

4. Equilibrium is a balanced state of the position of the body in space Equilibrium depends on inputs from: 1. Vestibular receptors - monitor gravity, linear acceleration, and rotation. 2. Visual Cues 3. Proprioceptor impulses from muscle & joint sensation 4. Cutaneous receptor impulses Cerebellum co-ordinates all the inputs 18-Jun-19 5

5. Vestibular system PERIPHERAL Measures accelerations of the head Rotational by the 3 semicircular canals Linear by the utricle & saccule CENTRAL 4 vestibular nuclei Give projections to Thalamus Cortex Cerebellum Descending spinal cord Extra ocular motor nuclei

6. Abnormal Perception Unusual combination of sensory inputs - Pattern not recognised False perception of movement of self or others Percieved as Vertigo Physiological Vertigo Visual stimuli - wearing of new spectacles & unaccustomed heights Disease –neurological , visual , vestibular

7. Motion 6 degrees of freedom 3 Rotational– SCC Yaw (horizontal) pitch (flexion & extension) roll (lateral head tilt) 3 Translational– Utricle & Saccule Left - Right To & Fro Up-Down

8. Vestibular Apparatus SCC: Angular rotation Horizontal canal – Vertical axis Vertical canals – Horizontal axis Utricle & Saccule ( Otolith ): Linear Rotation Linear movement in relation to force of gravity .

9. In the SCC Membranous labyrinth is tethered to the skull Crista & cupula rotate with the head Endolymph lags behind due to inertia Accumulated endolymph indents one side of cupula Bends the hair cells 18-Jun-19 11

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11. In the Otolith organs Hair cells – arranged along the central striola Utricle = kinocilia oriented towards the striola Saccule = kinocilia oriented away from the striola

12. Maculae Utricle = lie horizontally against the ceiling Saccule = hangs sagittaly on the wall Gelatinous matrix into which hair cells project Contain Otoconia Otoconia – increase the density of the maculae When the head moves – maculae lag behind – hair cells bend

13. Vestibular receptor cells Receptor cells or hair cells – neuroepithelial type. About 23000 in cristae & 45000-60000 in two maculae. Two types type 1 – flask shaped type 2 - cylindrical 50 -100 stereocilia - rigid,nonmotile . Single Kinocilium –motile. Spatial organization of these by glycocalyx & extracellular filaments. 18-Jun-19 15

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16. On stimulation of hair cell

17. Neurotransmitters involved Afferent - GABA - glutamate. Efferent - acetylcholine. Others – enkephalin - calcitonin gene related peptide - substance –p 18-Jun-19 19

18. Each vestibular nerve – 25,000 bipolar neurons with cells bodies in Scarpa’s ganglion Superior Vestibular N Afferents Horizontal SCC Superior SCC Utricle Inferior Vestibular N Afferents Posterior SCC Saccule

19. 3 groups of Vestibular nerve Afferents Bouton only Terminates onto type II HC’s in peripheral zone of cristae Regularly discharging with low rotational sensitivity Calyx only Type I HC’s in central zone Irregularly discharging Rotational sensitivity proportional to frequency Dimorphic Calyx endings on type I HC’s Bouton endings on type II HC’s

20. RALP – LARP planes of SCC 18-Jun-19 22

21. Semicircular canals Detect only changes of rotation Anatomical design is such that of a full circular tube, filled with viscous fluid. Movement of the endolymph is usually along the direction of the cylindrical canalicular cavity The cupula in the ampulla is comparable to a flexible membrane with a spring constant which defines its elastic property.

22. On rotation, 3 forces act Inertial force – proprortional to the mass of the endolymph and cupula Elastic restoring force – positions cupula back to the centre after stimulation Viscous forces – act on the fluid when it slides past the internal walls of the tube These 3 forces add up to give Head Angular Acceleration ( Pendulum Model)

23. Mechanics of Semicircular canal response M – Mass of Endolymph B – Endolymph Viscosity K – Cupular spring constant Alpha – Angular displacement Omega – Angular acceleration Theta - Angle Displacement of Cupula - depends on Inertial force, Elastic Force, Viscous force.

24. Cupular deflection Final signal fed to the brain due to hair cell depolarisation or hyperpolarisation Behaviour of cupular endolymph system can be described by two quantities Gain – Expresses amount of output per input Phase – Describes time difference between output & input Optimal situation is characterised by gain of 1 & a phase of 0 degree present between 0.1 & 5 Hz enabling gaze stabilisation for these frequencies

25. When sudden angular velocity is applied to head cupula reaches 67 percent of its ultimate deflection after 3 milliseconds. SCC limits the deflection of the cupula preventing it from large excursions that may damage sensory epithelium. For even a high head rotation of about 500 degrees/sec, cupular deflection is only 1.5 degree

26. EWALD’S LAW 1 st law Fenestration of a SCC followed by mechanical stimulation of membranous canal leads to eye & head movements in the plane of that canal 2 nd law Excitatory stimuli lead to larger amplitude responses than Inhibitory stimuli Vestibular nuclei afferents & central vestibular neurons can be excited ≥ 350 spikes/sec and inhibited only upto 0 spikes/sec

27. Vestibular responses Vestibulo -ocular reflexes . Vestibulo -spinal reflexes . Vestibulo-collic reflexes . 18-Jun-19 29

28. Angular Vestibulo Ocular Reflex 3 neuron arc Vestibular afferent arc Vestibular interneuron Occulomotor neuron Brain interprets stimulation of SCC as motion of head – eyes move reflexively in an equal but opposite amount to compensate

29. Fastest & most accurate reflex in body Latency of about 7 milliseconds Eye movements occur with < 5% error during rapid head movements Deficit in VOR = OSCILLOPSIA Apparent motion of objects that are known to be stationary during rapid head movements Seen in Semicircular canal dehiscence syndrome Vertigo & oscillopsia induced by loud noises/ stimuli that change middle ear or intracranial pressure Ewald’s 1 st law applicable

30. Linear VOR Otolith organs cannot distinguish between tilting of the head & linear movements Tiliting of the head causes Ocular Counter Rolling torsional movement of the eye about the line of sight that partially compensates for the effect of rotation

31. Canal movements and Eye Movements Vertical SCC and Saccule – Vertical eye movements Horizontal SCC and Utricle – Horizontal eye movements Vertical SCC and Utricle – Torsional Eye movements Canal Stimulation Contracted Eye muscle Relaxed Eye muscle Lateral Ipsi - MR CL - LR Ipsi – LR CL - MR Anterior Ipsi – SR CL – IO Ipsi – IR CL – SO Posterior Ipsi – SO CL – IR Ipsi – IO CL - SR

32. When the head is tilted Sideways – posterior and anterior canals are stimulated, hence torsional nystagmus is seen. Forwards – left and right anterior canals are stimulated causing upward eye movement. Backwards – left and right posterior canals are stimulated causing downward eye movement.

33. Push – Pull Principle of VOR Leading side = side of head tilt Utriculopetal deflection is seen – causes excitation of hair cells Following ear = opposite side Utriculofugal deflection is seen – causes inhibition of hair cells = decreased neuronal activity Both are required for optimal VOR

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35. When Head rotates to Left Endolymph inertia makes apparent movement to right Endolymph move towards left ampula , away from right ampula Left LSCC is stimulated & Right LSCC is inhibited Left 3 rd & 4 th & Right 6 th cranial nerves are stimulated Both eyes slowly move to right – Vestibular Component Cortical corrective component – Both eye are suddenly jerked to left – Left beating Nystagmus

36. Velocity storage mechanism Process that maintains the sense of rotation even after the rotation has stopped Cause is difference in Time constant of SCC = 5 secs VOR = 10 – 12 secs Time constant = response to decay to 37% of its initial value Peripheral vestibular dysfunction VSM ceases to function Rotation induced nystagmus Conscious perception of rotation

37. Gaze holding Generated by the Neural integrator located at M edulla Medial vestibular nucleus When looking away from the midline, extraocular muscles require Burst of activity to move eyes to the eccentric position Sustained level of discharge for the muscles to hold the eyes in that position

38. ALEXANDER’S LAW Eye movements in pt. with Unilateral vestibular loss Central process contributes to eye motion and nystagmus by allowing the eye to drift to center, regardless of its position. The interaction of this motion & the motion of the eye caused by the imbalance in vestibular activity between the two labyrinths cause nystagmus to be more pronounced when looking away from the lesion.

39. CENTRAL PROJECTIONS OF PERIPHERAL VESTIBULAR SYSTEM Lateral SCC input to medial vestibular nuclei triggers two types of neurons – Type 1 & type 2 neurons Some type 1 neurons are excitatory & some are inhibitory Primary vestibular afferents synapse with inhibitory as well as excitatory type 1 neurons

40. Code for excitation & inhibition of vestibular nuclei

41. Commissural pathway In the VN, there are type 2 (secondary) neurons which behave in opposite manner to type 1 neurons - inhibitory Activating these neurons silence neighbouring type 1 neurons VOR is enhanced in positive feedback loop by commissural pathway. They prove to be crucial in case of unilateral lesions.

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44. 18-Jun-19 47 Class of eye movement Main function Vestibular Gaze stabilisation (0.5-5 hz ) Visual fixation Holds image of stationary object on fovea Optokinetc During sustained head movements Nystagmus Resets eyes during sustained head movement and directs gaze towards oncoming visual scene Saccades Bring object of interest on fovea Vergence Moves eye in opposite direction to place target simultaneously on both foveas

45. Optokinetic & smooth pursuit Visually mediated reflexes Ability of the brain to determine speed of image drift on retina Smooth pursuit system – in a tennis match – holds the image of a slow moving target on the fovea Optokinetic system – on a merry go round – holds the image of the surrounding world steady during sustained head movement Vestibulo-collic reflexes Stabilizes head during oscillations.

46. Vestibulospinal tract Helps in maintenance of posture both static & dynamic. Otolith input with proprioceptive & visual cues Reflexes- Positional Acceleration Righting

47. SUMMARY OF PATHWAYS AND FUNCTION PATHWAY FUNCTION VESTIBULOOCULAR REFLEX GAZE STABILISATION VESTIBULOSPINAL REFLEX & VESTIBULOCOLLIC REFLEX ENSURE UPRIGHT POSITION OF BODY AND TRUNK AND HEAD STABILISATION IN SPACE VESTIBULAR CORTEX ORIENTATION,PERCEPTION OF SELF POSITION,NAVIGATION AUTONOMIC FUNCTION ALTER BODY ORIENTATIONS

48. Vestibular pathway 18-Jun-19 51

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