Physiology of Hearing

PHYSIOLOGY OF HEARING

The human ear : A) External earEar pinna Collect & direct sound to external auditoy meatus Sound localization 30 % of hearing power  External auditory meatus Counduct & concentrate sound to tympanic membrane Resonance ---> amplify sound Protection of tymanic membrane by : Mucus,hair,antibacterial action,long & tortous anatomy

B) Middle ear Air filled cavity bounded laterally by tympanic membrane  Connected to nasopharynx by eustachian tube Content : 3 bony ossciles : Malleus, Incus, Stapes Tympanic membrane 2 Muscle : Tensor Tympani & Stapedius Muscle Function of middle ear :  Transfer sound from external ear to inner ear  Bony ossicles act as a lever system for amplication of sound Attenuation reflex (middle ear muscle) [reflex contraction of tensor tympani and stapedius muscle in response to loud sound] ---> both will more hearer to each other 

Eustachian tube : 4 cm  Connect middle ear to nasopharynx  Equalize the pressure on both sides of tympanic membrane  Normally closed Open during : swallowing,yawning,chewing,sneezing [ when there is pressure difference between middle ear and nasopharynx ] If it is always open, sound of breath and talking can be produce noise 

C) Inner ear  Vestibular apparatus ---> posture & equlibrium ---> vestibular nerve Cochlear ---> hearing ---> cochlear nerve Cochlear :  coiled tube 3.5cm makes 2-5 turns around bony axis start from base to narrow apex it has 3 coiled tube : scala vestibuli,scala media,scala tympani 2 membranes : vestibular membrane,basilar membrane

Organ of corti Sense organ of hearing  lies pn the basilar membrane extending from base to apex consist of : Outer hair cell (3 rows) inner hair cells (1 rows) bases and sides of hair cells are surrounded by a network of cochlear nerve ending which lie in the spiral ganglia f(x) of organ of corti : generate nerve impulse when the basilar membrane move in response to sound vibration 

Cochlear potential Endocochlear potential potential chnages inside the cochlea during rest potential diff across the membrane of hair cell (-140 mv) Cochlear microphic potential potential changes with basilar membrane vibration  during passage sound wave Mechanism of stimulation of hair cells  Vibration of basilar membrane in response to sound wave lead to bending of the hair cells  Vibration of basilar membrane produce cochlear microphonic potential The vibration is closely related to the frequency and amplitude of sound

Auditory pathway consist of 4-6 neurons signal from one ear is transmitted to both sides of the brain 3 cross over : 

Superior olivary nucleus (2nd ON) Nucleus of lateral leminiscus (3rd ON) Inferior colliculus (3rd ON)

Loud sound pathway : Reticular activating system send impulse to cerebral cortex, spinal cord & cerebellum.

Auditory cortexThe primary auditory area (area 41,41) Responsible for perception of sound (frequency & intensity) but without understanding the meaning of these soundThe auditory association area (area 22) It lies adjacent to the primary area It is responsible for understanding the meaning of various sound and words Lesion of this area leads to auditory aphasia (the person can hear the sound but he is unable to understand the meaning of the spoken words

1st Order Neuron nerves fibres at the sides & base of hair cells ---> bipolar cells in the spiral ganglia of the cochlea Axons of bipolar cells enter the dorsal & ventral nuclei in the upper part of the medulla 2nd Order Neuronpass to the superior olivery nucleus of both sides3rd Order NeuronFrom SON pass upward through the lateral leminiscus of both sides.Some fibers relay in it while other relays in inferior colliculus of midbrain4th Order Neuronfrom inferior colliculus ---> medial geniculate body5th Order neuronfrom medial geniculate body to the auditory cortex of both sides.

New Page

New Page