56514243 physiology-of-hearing-balance

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56514243 physiology-of-hearing-balance

  1. 1. Physiology of Hearing & Balance Dr. Archana Sudhir
  2. 2. The Nature of Sound  Sound is any audible vibration of molecules  Vibrating object pushes air molecules into zones of compression separated by zones of rarefaction
  3. 3. Properties of Sound  Frequency – the number of waves that pass a given point in a given time  Pitch – perception of different frequencies (we hear from 20–20,000 Hz)  Intensity – The power transmitted by a wave through an unit area.  Loudness – The perception of intensity.
  4. 4. Main Components of the Hearing Mechanism Divided into 4 parts (by function):  Outer Ear  Middle Ear  Inner Ear  Central Auditory Nervous System
  5. 5. Functions of the Outer Ear  Gathers sound waves  Increases Pressure in a frequency sensitive way.  Aids in localization
  6. 6. Functions of the Middle Ear  Couple sound energy to the cochlea  Impedance matching  Protects Cochlea  Preferential application of sound to one window.
  7. 7. Impedance Transformer  Large area of TM in comparison to small area of foot plate (pressure increases inversely to the ratio of these areas)  Ossicular lever ratio (Malleus is 1.3 times longer than incus)  Buckling action of TM  Ligaments suspending ossicles.
  8. 8. Impedance Efficiency  Middle ear converts low pressure high displacement movements of the ear drum into high pressure low displacement movements needed for the cochlear fluid movement.  50% of sound energy from TM gets transmitted and absorbed in the cochlea.  Without middle ear only 1% of sound energy will be absorbed by the cochlea.
  9. 9. Role of Middle Ear Muscles  Tensor tympani attaches to the neck of malleus. It pulls the drum medially.  Stapedius muscle attaches to the posterior aspect of neck of stapes.  Contraction of these muscles increase the stiffness of ossicular chain thus blunting low frequencies.  Stapedius contracts in response to loud sounds and acts as an in built ear plug.
  10. 10. Bone Conduction  Bone vibration conducted through ext canal  Skull vibration – ossicles lag behind.  Differential distortion of bony cochlea  Direct vibration of osseous spiral lamina  Skull vibration via CSF to endolymph
  11. 11. Structures of the Inner Ear  Bony Labyrinth  Bony Cochlea  Vestibule  Semi Circular Canals  Membranous Labyrinth  Cochlea Duct  Utricle & Saccule  Semi Circular Canals
  12. 12. Organ of Corti  16,000 hair cells have 30-100 stereocilia(microvilli )  Microvilli make contact with tectorial membrane (gelatinous membrane that overlaps the spiral organ of Corti)  Basal sides of inner hair cells synapse with 1st order sensory neurons whose cell body is in spiral ganglion
  13. 13. Movement of pressure waves through the cochlea
  14. 14. MOVEMENTS OF THE BASILAR MEMBRANE AND THE DEFLECTION OF THE STEREOCILIA.
  15. 15. Potassium Gates of Cochlear Hair Cells  Stereocilia bathed in high K+ concentration creating electrochemical gradient from tip to base  Stereocilia of OHCs have tip embedded in tectorial membrane which is anchored  Movement of basilar membrane bends stereocilia  Bending pulls on tip links and opens ion channels  K+ flows in -- depolarizing it & causing release of neurotransmitter stimulating sensory dendrites at its base
  16. 16. Theories Of Hearing  Place theory of Helm holtz  Telephone theory of Rutherford  Volley theory of Wever  Traveling wave theory of Bekesy
  17. 17. CENTRAL AUDITORY PATHWAYS
  18. 18. Auditory Cortex
  19. 19. APPLIED PHYSIOLOGY  EAC BLOCK - 30db HL  TM PERFORATION - 26db HL  TM PERFORATION WITH OSSICULAR INTERRUPTION - 26.5 +7.3+ 5=38.3dbHL  TOTAL LOSS OF TM WITH OSSICULAR INTERRUPTION - 26.5 +7.3+ 16.2=50dbHL  OSSICULAR INTERRUPTION WITH INTACT TM - 38+15=54dbHL  OSSICULAR INTERRUPTION WITH INTACT TM WITH CLOSED OVAL WINDOW - 60dbHL
  20. 20. Vestibular Apparatus  Vestibule  Utricle  Saccule  Semicircular canals - lateral, superior, posterior  Vestibular nerve
  21. 21. Equilibrium  Static equilibrium is perception of head orientation  perceived by macula  Dynamic equilibrium is perception of motion or acceleration  linear acceleration perceived by macula  angular acceleration perceived by crista
  22. 22. The Saccule and Utricle  Saccule & utricle chambers containing macula  patch of hair cells with their stereocilia & one kinocilium buried in a gelatinous otolithic membrane weighted with granules called otoliths  otoliths add to the density & inertia and enhance the sense of gravity and motion Otoliths
  23. 23. Macula of Saccule and Utricle  With the head erect, stimulation is minimal, but when the head is tilted, weight of membrane bends the stereocilia (static equilibrium)  Linear acceleration is detected since heavy otolith lags behind (one type of dynamic equilibrium)
  24. 24. Crista Ampullaris of Semicircular Ducts  Crista ampullaris consists of hair cells buried in a mound of gelatinous membrane  Orientation of ducts causes different ducts to be stimulated by rotation in different planes
  25. 25. Crista Ampullaris & Head Rotation  As head turns, the endolymph lags behind pushing the cupula and stimulating its hair cells
  26. 26. Equilibrium Projection Pathways  Unmyelinated plexus at the base of sensory epithelium gives rise to primary vestibular neuron  Central processes of primary vestibular neurons synapses with vestibular nucleus of pons, cerebellum
  27. 27. Vestibular Nuclei Cristae of SCC & Cerebellum Superior vestibular nuclei Bechterew Medial Longitudinal Fasciculus Cerebellum & Utricular Macula Lateral vestibular nuclei Dieter Vestibulo Spinal Tract, Reticulo Spinal Tract Cristae Cerebellum Medial vestibular nuclei Schwalbe Medial Longitudinal Fasciculus Utricular & Sacular Maculae Descending vestibular nuclei Cerebellum & Reticular Formation
  28. 28. Ascending Vestibular Projections Lateral & Superior vestibular nuclei Thalamus Sensori Motor Cortex Visual Projections Proprioceptive Projections
  29. 29. Vestibular Reflexes  Vestibulo-spinal  Helps maintain center of gravity  Vestibulo-ocular  Helps maintain stability of visual field  Vestibulo-collic:  Helps to maintain stability of the head during movement of the torso.
  30. 30. Vestibulo Ocular Reflexes
  31. 31. CLINICAL RELEVENCE GIDDINESS 1. NON CORRECTABLE VISUAL IMPAIRMENT. 2. NEUROPATHY. 3. VESTIBULAR DYSFUNCTION. 4. CERVICAL SPONDYLOSIS. 5. ORTHOPAEDIC DISTURBANCES. 6. CARDIAC DISORDERS. 7. NEUROLOGICAL DEFICITS.
  32. 32. ASSESMENT  HISTORY  IDENTIFICATION OF PRESENCE/ ABSENCE OF VESTIBULAR COMPONENT. 1. VESTIBULO-SPINAL FUNCTION. 2. VESTIBULO – OCULAR FUNCTION.
  33. 33. VESTIBULO-SPINAL FUNCTION  ROMBERGS TEST  UNTERBERGERS TEST
  34. 34. VESTIBULO-OCULAR FUNCTION  NYSTAGMUS  INVOLUNTARY DEVIATION OF EYES AWAY FROM DIRECTION OF GAZE FOLLOWED BY A RETURN OF THE EYES TO THEIR ORIGINAL POSITION.  3 TYPES 1. CENTRAL 2. OCULAR 3. VESTIBULAR
  35. 35. VESTIBULAR NYSTAGMUS  RHYTHMIC  FAST AND SLOW PHASES  NAMED AFTER FAST PHASE.  3 TYPES 1. SPONTANEOUS 2. POSITIONAL 3. INDUCED.
  36. 36. VESTIBULAR NYSTAGMUS  SPONTANEOUS NYSTAGMUS  GRADE 1.  GRADE 2.  GRADE 3.  POSITIONAL NYSTAGMUS  HALLPIKE MANOEUVRE
  37. 37. INDUCED NYSTAGMUS  ROTATIONAL TESTS  Nystagmus Induced by accelerating and decelerating rotating chair, tests both labyrinths simultaneously  CALORIC TESTS  COWS- cold water opposite side, warm water same side, direction of nystagmus  Extent of caloric response indicates function of labyrinth
  38. 38. Electronystagmograghy  Positive potential between the cornea and retina recorded as eyes move from straight ahead gaze  Test includes different head positions, eyes open, closed and caloric tests

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