Physiology of balance

PHYSIOLOGY OF BALANCE BY DR MARJAN

1. PHYSIOLOGY OF BALANCE DR.MARJAN

2. Main functions of vestibular system is to generate information to CNS for To ensure gaze stabilisation To enable balanced locomotion and body position To provide general orientation of the body with respect to gravity To readjust autonomic functions after body reorientation

3. For this we need a proper functioning - visual, propioceptive system along with vestibular system

4. VESTIBULAR SYSTEM PERIPHERAL- which is made up of membranous labyrinth[( horizonta l(lateral), superior( anterior) & posterior SCC ,utricle and saccule ] and vestibular nerve . CENTRAL- Consists of vestibular nuclei and their projections to thalamus, cerebellum, cortex, descending spinal cord extraocular nuclei (nuclei and fibre tracts in the CNS to integrate vestibular impulses with other systems to maintain body balance

5. Every motion in space can be broken down into 6 degrees of freeom 3 rotational ( roll,pitch and yaw) & 3 translational ( left–right, up– down,front –back). The semicircular canals (SCCs) measure predominantly rotations whereas the maculae of the utricle and saccule detect mainly translations.

6. PERIPHERAL RECEPTORS They are of two types: 1. Cristae They are located in the ampullated ends of the three semicircular ducts. These receptors respond to angular acceleration . 2. Maculae They are located in otolith organs (i.e. utricle and saccule). Macula of the utricle- in the floor in a horizontal plane. Macula of the saccule-in the posterior wall in a vertical plane. They sense position of head in response to gravity and linear acceleration.

7. Cristae It is a crest like mound of connective tissues on which lie the sensory epithelial cells. The cilia of the sensory hair cells project into the cupula , (gelatinous mass extending from the surface to the ceiling of the ampulla and forms a water tight partition) It get displaced to one or the other side like a swing door, with movements of endolymph . The gelatinous mass of cupula consists of polysaccharide and contains canals into which project the cilia of sensory cells.

8. Hair cells are of two types Type I cells- flask-shaped with a single large cup-like nerve terminal surrounding the base. Type II cells - cylindrical with multiple nerve terminals at the base. From the upper surface of each cell, project a number of cilia, 20–300, the stereocilia The kinocilium is thickest &largest among them and is located on the edge of the cell. Sensory cells are surrounded by supporting cells which show microvilli on their upper ends

9. MACULA A macula consists mainly of two parts: ( i) a sensory neuroepithelium , made up of type I and type II cells. (ii) an otolithic membrane , which is made up of a gelatinous mass and on the top, the crystals of calcium carbonate called otoliths or otoconia The cilia of hair cells project into the gelatinous layer. The linear, gravitational and head tilt movements cause displacement of otolithic membrane and thus stimulate the hair cells

10. Potassium flows into cell Calcium flows into cell Voltage shifts to a less negative value More neurotransmitter is released

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12. OTOLITH ORGANS (UTRICLE & SACCULE) The utricle lies in the posterior part of bony vestibule. It receives the five openings of the three semicircular ducts. The saccule also lies anterior to the utricle and opposite the stapes footplate. Connected to utricle - utriculosaccular duct. Utricle lies relatively horizontal & Saccule –vertical

13. Otolith organs-detects gravitational acceleration continuously Also sense linear accelerations caused by translational movements of the head as well as static tilts of the head Macules are the receptors

14. SEMICIRCULAR CANALS 3 in number These canals lie at right angles to each other Respond to angular acceleration and deceleration. The canal which lies at right angles to the axis of rotation is stimulated the most. Thus horizontal canal will respond maximum to rotation on the vertical axis. Due to this arrangement of the three canals in three different planes, any change in position of head can be detected.

15. SUPERIOR/ANTERIOR SCC- Head nodes up and down as in YES motion(pitch) POSTERIOR SCC- When head tilt towards shoulder(roll) LATERAL SCC- When head shakes side to side in ‘NO’ motion(yaw)

16. Vestibulo -ocular reflex It is a reflex , where activation of vestibular system of the ear causes eye movement, which is to stabilize image on the centre of the retina The left and right SCCs are oriented in the head such that any movement always induces an antagonistic response in both canals

17. The left and right canals as parallel systems, i.e. the right anterior (RA) canal is parallel with the left posterior (LP) - RALP plane. Right posterior (RP) canal the left anterior (LA) constitutes the LARP plane. Both horizontal canals are also parallel with each other in the lateral plane The hair cells of both right and left canals as a mirror image, the deflection on the ‘leading’ side induces a movement of the stereocilia towards the kinocilium , whereas on the opposite ‘following’ ear the movement of the stereocilia is away from the kinocilium . V;

18. The simplified principle of VOR generation – PUSH-PULL PRINCIPLE “ (yaw-plane rotation - horizontal SCC) is as follows: 1. During head rest, hair cells in both SCCs have a resting discharge rate of 90 spikes per second. 2. Head rotation is to the right. 3. Endolymph fluid lags behind due to inertia. 4. The cupula bends to the left in each canal.

19. 5. In the (leading) right SCC the stereocilia bend towards the kinocilium .- discharge rate increases 6. In the (following) left SCC the stereocilia bend away from the kinocilium - discharge rate decreases

20. The vestibular nuclei interpret the difference in discharge rate between left and right SCCs as movement to the right, and trigger the oculomotor nuclei to drive the eyes to the left to maintain gaze stabilization

21. Three arc Neuron Representation of VOR

22. Thus, nystagmus is horizontal-horizontal canal , vertical -posterior canal Rotatory -superior canal

23. NYSTAGMUS The eye response to a head rotation – combination of a slow phase/drift(until the eye reaches the edge of the outer canthus ) and a fast phase to reset the eye in its initial position- is called nystagmus The direction of the nystagmus is defined by the fast reset phase, since that is easiest identified by the clinician. The slow phase, however, represents the actual vestibular output and is quantified. It is quantified by measurement of the slope of the upward trace, which indicates the speed of the eye movement (degrees/s).

24. In electronystagmography or video- nystagmography , An upward excursion-represents eye deviation to the right. If the slope of the sawtooth is upward to the right, i.e. a positive slope, this corresponds to a slow drift of the eye to the right, followed by a quick leftward reset saccade, as represented by a steep downward trace. This is defined as a left nystagmus .

25. The quick component of nystagmus is always opposite to the direction of flow of endolymph. Thus, if a person is rotated to the right for sometime and then abruptly stopped, the endolymph continues to move to the right due to inertia,the nystagmus will be directed to the left Remember nystagmus is in the direction opposite to the direction of flow of endolymph.

26. Head Impulse Test/ Thrust test It is a simple, reliable, bedside test to detect unilateral loss of semicircular canal function clinically This test can distinguish between vestibular neuritis and cerebellar infarction

27. when the eyes rotate together with the head and are no longer focused on the target, the SCC on the side towards which the head was turned is not functioning properly

28. VESTIBULAR NERVE Vestibular or Scarpa’s ganglion is situated in the lateral part of the internal acoustic meatus. It contains bipolar cells. The distal processes of bipolar cells innervate the sensory epithelium of the labyrinth while its central processes aggregate to form the vestibular nerve .

29. Vestibular Portion of C.N. VIII superior division: utricle, anterior part of saccule, and horizontal & anterior canals inferior division: posterior part of saccule& posterior canal

30. CENTRAL VESTIBULAR CONNECTIONS The fibres of vestibular nerve end in vestibular nuclei and some go to the cerebellum directly . Vestibular nuclei are four in number, the superior, medial, lateral and descending. Afferents to these nuclei come from: 1. Peripheral vestibular receptors (SCC, utricle and saccule) 2 . Cerebellum 3. Reticular formation 4. Spinal cord 5. Contralateral vestibular nuclei

31. Efferents from vestibular nuclei go to : 1. Nuclei of CN III, IV, VI via medial longitudinal bundle ( Vestibulo-Oculomotor Pathways ) It is the pathway for vestibulo -ocular reflexes and this explains the genesis of nystagmus . 2 . Motor part of spinal cord ( vestibulospinal fibres ). This coordinates the movements of head, neck and body in the maintenance of balance . Lateral V-S-throughout spinal cord Medial V-S-cervical & thoracic Reticulospinal tract-via brainstem reticular formation

32. 3. Cerebellum ( vestibulocerebellar fibres ). It helps to coordinate input information to maintain the body balance. 4. Autonomic nervous system . This explains nausea, vomiting, palpitation, sweating and pallor seen in vestibular disorders (e.g. Ménière’s disease). 5. Vestibular nuclei of the opposite side . 6. Cerebral cortex (temporal lobe). This is responsible for subjective awareness of motion

33. In the brainstem Vestibular inputs undergo integration Integrated signal is combined with original (velocity driven) signal Processing to reset spatial map for eye musculature

34. *VESTIBULOSPINAL REFLEX The lateral vestibulospinal tract . Projects to cervical, thoracic, and lumbar segments via the ventral funiculus Entirely ipsilateral Originates in the lateral vestibular nucleus,predominantly an otolith signal Allows the legs to adjust for head movements. Provides excitatory tone to extensor muscles Decerebrate rigidity is the loss of inhibition from cerebral cortex and cerebellum on the LVST, and exaggerates the effect of the tonic signal in the LVST.

35. VESTIBULOCOLLIC REFLEX The Medial Vestibulospinal Tract Originates in the medial vestibular nucleus, predominantly a canal signal Projects to cervical segments via the medial longitudinal fasciculus Predominantly ipsilateral . Keeps the head still in space –  mediating the vestibulo-collic reflex

36. MAINTENANCE OF BODY EQUILIBRIUM In static neutral position, each side contributes equal sensory information, i.e. push and pull system of one side is equal to that of the other side. If one side pulls more than the other, balance of the body is disturbed . During movement, i.e. turning or tilt, there is a temporary change in the push and pull system, are corrected by appropriate reflexes and motor outputs to the eyes ( vestibulo -ocular reflex), neck ( vestibulocervical reflex ), and trunk and limbs ( vestibulospinal reflex)

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