PHYSIOLOGY OF HEARING AND EQUILIBRIUM DR MUBEENA SOUND

PHYSIOLOGY OF HEARING AND EQUILIBRIUM DR MUBEENA

SOUND • A form of energy • propagates in the form of waves • The speed of sound depend on the medium through which the wave pass – air - 343 m/s – water - 1482 m/sec • Audible frequencies t for humans 20 to 20, 000 cycles per second (cps, Hz). 2

EAR AS A TRANSDUCER SOUNDENERGY MECHANICAL ENERGY ELECTRICAL ENERGY 3

Values of hearing: ü 15 -25 d. B ü 35 d. B ü 40 -60 d. B ü 65— 70 d. B ü 130 d. B ü 140 -180 d. B —Whisper —Background noise ( home ) –- normal speaking voice —painful noise —jet air craft engine noise

FUNCTION OF EXTERNAL EAR 5

Functions of EXTERNAL EAR Sound collection Increasing pressure on the tympanic membrane in a frequency sensitive way Sound localization 6

FUNCTION OF MIDDLE EAR

FUNCTIONS OF MIDDLE EAR IMPEDENCE MATCHING ATTENUATION PHASE DIFFERENCIAL EFFECT 8

IMPEDANCE MATCHING. • A person in water can not hear sound produced out of it. • As 99. 9% sound get reflected from surface of water due to Impedance.

Impedence mismatch IF THERE WAS NO MIDDLE EAR SYSTEM , 99% OF SOUND WAVES WOULD HAVE REFLECTED BACK FROM OVAL WINDOW MIDDLE EAR BY ITS IMPEDENCE MATCHING PROPERTY ALLOWS 60% OF SOUND ENERGY TO DISSIPATE IN INNER EAR 10

“Impedance Matching” by the middle ear System (a) Area of tympanic membrane relative to oval window (b) The lever action of middle ear ossicles (c) The shape of tympanic membrane 11

(a) HYDRAULIC ACTION OF TYMPANIC MEMBRANE • Total effective area of tympanic membrane 55 mm 2 • Area of stapes footplate is 3. 2 mm 2 • Effective areal ratio is 14: 1 • Thus by focusing sound pressure from large area of tympanic membrane to small area of oval window the effectiveness of energy transfer between air to fluid of cochlea is increased 12

LEVER ACTION OF OSSICLES. – Handle of Malleus 1. 3 times longer than Long process of Incus, providing Mechanical Leverage Advantage. – So Ossicles increases force of movement by 1. 3 times.

CURVED MEMBRANE EFFECT – Movement of Tympanic membrane more at Periphery than at Center where Malleus is attached. – So provide some leverage.

IMPEDANCE MATCHING • So all these together Increase sound pressure 22 folds

PHASE DIFFERENTIAL BETWEEN OVAL AND ROUND WINDOW. • Sound don’t reach both windows simultaneously. • When oval window receive compression, round window receive rarefaction. • If sound reaches simultaneously no movement of Perilymph & no hearing.

ATTENUATION REFLEX. • Stimulus - loud sound. • Latent period – 4080 ms. (sudden loud sound; bomb explosion – Deafness) • Reflex Activity – contraction of 2 muscles – Tensor tympani – Stapedius.

ATTENUATION REFLEX. • Tensor Tympani – pull Malleus inwards • Stapedius – pulls stapes outwards. • Both makes Ossicular system rigid & no vibrations. • Sound intensity Decreased by 30 -40 db.

ADVANTAGES OF ATTENUATION REFLEX. • Prevents Damage to cochlea from loud sound. • Attenuates & Mask all low frequency environmental sounds & allow person to concentrate on sounds above 1000 Hz. • Reduces sound produced during vocalization & chewing.

NATURAL RESONANCE OF EXTERNAL EAR AND MIDDLE EAR. • Natural Resonance – allow some frequency to pass more easily to inner ear. – External auditory canal – 3000 Hz – Tympanic membrane – 8001600 Hz. – Middle ear – 800 Hz. – Ossicular chain – 500 -2000 Hz.

INNER EAR FUNCTION OF INNER EAR 21

COCHLEA • A TRANSDUCER that translates sound energy into a form suitable for stimulating the dendrites of auditory nerve. 22

TRANSDUCTION OF SOUND WAVES • Transduction of sound from Mechanical to Electrical occur in ORGAN OF CORTI in inner ear. – Vibration of Basilar membrane. – Stimulation of hair cells – Membrane potential change in hair cells – Neural transmission of signals.

VIBRATION OF BASILAR MEMBRANE. • Sound waves from middle ear pass to inner ear through Oval window by in & out movement of stapes. • Wave spread along Scala Vestibuli to Scala tympani as a travelling wave. • As it passes it Vibrate basilar membrane.

STIMULATION OF HAIR CELLS • As organ of Corti Moves up, tectorial membrane slide foreward moving stereocilia Away from limbus. • As organ of Corti Moves Down, tectorial membrane slide backward moving stereocilia towards limbus.

STIMULATION OF HAIR CELLS • Bending of stereocilia stimulate hair cells • Depolarization – as stereocilia bend away from limbus. • Hyperpolarization – as stereocilia bends towards limbus.

ELECTRICAL POTENTIAL OF COCHLEA AND CN VIII 27

Endocochlear potential • An electrical potential of about +80 millivolts exists all the time between endolymph and perilymph, with positivity inside the scala media and negativity outside. • This is called the endocochlear potential, and it is generated by continual secretion of positive potassium ions into the scala media by the stria vascularis 28

Cochlear microphonic • When basilar membrane move in response to sound stimulus electrical resistance at the tip of hair cells change allowing flow of K+ through hair cells and produce voltage fluctuations called cochlear micro phonic. • This is AC potential 29

Summating potential • Produced by hair cells • DC potential superimposed on VIII nerve action potential 30

COCHLEAR MICROPHONIC POTENTIAL • Similar to generator potential as – No latency or refractory period. – Do not obey all or none law. – Not propogated • Base of cochlea respond to high frequency, apex respond to low frequency of sound.

Compound action potential • All or none response of auditory nerve fibres 32

Central auditory pathway • nerve fibers from the spiral ganglion of Corti enter the dorsal and ventral cochlear nuclei • second-order neurons pass mainly to the opposite side of the brain stem to terminate in the superior olivary nucleus • the superior olivary nucleus, the auditory pathway passes upward through the lateral lemniscus. 33

AUDITORY CORTEX

Monday, February 22, 2021

PHYSIOLOGY OF EQUILIBRIUM

Centre of gravity To balance the centre of gravity must be above the support point.

Systems regulating body balance • Humans use three systems: 1 - visual CNS 1 - Cerebral cortex 2 - Brainstem 3 - Cerebellum Muscle commands 1 - 2 - Vestibular 2 - 3 - Proprioceptive

the vestibular labyrinth Anterior Lat Po era ior r ste l

Utricle responds to horizontal acceleration (going in a car, “yes”) and the saccule to vertical acceleration (going in a lift, tilting)

The Utricle and Saccule • Present in the vestibule of the labyrinth • Utricle is vertically oriented • Saccule is horizontally oriented • Sensory hair cells are embedded in the maculae of the utricle and saccule • Hair cells are covered by a membrane called otolithic membrane

Maculae

The Semicircular Canals 1. Fluid filled 2. Each canal has a dilated end = Ampulla 3. The ampulla houses the sensory hair cells which are covered by a gelatinous material a. Ampulla b. Cristae = hair cells c. Cupulae = gelatinous material

Crista ampullaris

Anatomy: Maculae of Utricle or Saccule Physiology: Linear acceleration of head Otolithic membrane Hair cell

Angular acceleration • The three canals lie at right angles to each other but the one 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 and so on.

• Stimulation of semicircular canals produces nystagmus and the direction of nystagmus is determined by the plane of the canal being stimulated. • Horizontal horizontal canal • rotatory superior canal • vertical posterior canal.

• The stimulus to semicircular canal is flow of endolymph which displaces the cupula. The flow may be towards the cupula (ampullopetal) or away from it (ampullofugal). • 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 (i. e. ampullopetal for left canal), the nystagmus will be horizontal and directed to the left • Nystagmus is in the direction opposite to the direction of flow of endolymph.

Balance and Orientation Pathways

VERTIGO AND DIZZINESS • Disorientation in space causes vertigo or dizziness and can arise from disorders of any of the three systems: vestibular, visual or somatosensory. • Normally, the impulses reaching the brain from the three systems are equal and opposite. If any component on one side is inhibited or stimulated, the information reaching the cortex is mismatched, resulting in disorientation and vertigo.

• The vestibular inhibition on one side (e. g. acute vestibular failure, labyrinthectomy, Me nie re’s disease, VIIIth nerve section) causes vertigo. • Dizziness can similarly result from the ocular causes, e. g. high errors of refraction or acute extraocular muscle paralysis with diplopia.

MOTION SICKNESS • Due to mismatch of information reaching the vestibular nuclei and cerebellum from the visual, labyrinthine and somatosensory systems. • It can be controlled by the usual labyrinthine sedatives.

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