6.2 Sense of Hearing PDF

At the end of the session, the students should be able to:  describe the structure and function of the outer, middle and inner parts of the ear  explain the physiology of hearing  describe the auditory pathway  describe the physiology of equilibrium  The ear is the organ of hearing...

At the end of the session, the students should be able to:  describe the structure and function of the outer, middle and inner parts of the ear  explain the physiology of hearing  describe the auditory pathway  describe the physiology of equilibrium  The ear is the organ of hearing  It is also the organ for the sense of equilibrium  The receptors for hearing are mechanoreceptors  The organ of hearing are divided into external ear, middle ear & internal ear.  External/outer ear - collects sound waves and channels sound waves  Middle ear/tympanic cavity – uses a bony system to amplify sound vibrations  Internal/inner ear – generates action potentials to transmit sound and balance information to the brain  Consists of an auricle (pinna) & external auditory canal The auricle (pinna)  It is a fleshy part of the external ear  Function - collects and directs sound waves toward auditory meatus External auditory canal  Slightly ‘S’ shape extending from auricle to the tympanic membrane  Function - collecting and channelling sound waves into the ear canal  Contain ceruminous glands secrete cerumen (earwax)  Hair & cerumen – prevent foreign object from reaching the eardrum  External ears ends at the tympanic membrane  It is an air-filled cavity in the temporal bone  Begins at the tympanic membrane  Contains the auditory ossicles  Has opening to Eustachian tube  Oval window & round window – connect the middle ear to inner ear Auditory ossicles  Consists of the malleus(hammer), incus (anvil) and stapes (stirrup)  The skeletal muscles control the amount movement of these bones to prevent damage by excessively loud noises  Function – transmit sound vibrations from tympanic membrane to oval window of inner ear Eardrum/tympanic membrane  It is a thin, semitransparent partition between the external auditory canal and the middle ear  Separates the outer ear from the cavity of the middle ear  Convert sound waves to vibrations The tympanic membrane Eustachian tube  Connect each middle ear with the throat  Purpose - equalizes air pressure between the outside air & the middle ear cavity – necessary for normal hearing  Throat infections may spread to the middle ear through the Eustachian tube  Myringitis is an inflammation of the ear drum  Otitis externa (commonly called “swimmer’s ear”) is a dermatitis of the epithelium of the outer ear (infectious and noninfectious)  Consists of bony labyrinth and membranous labyrinth  Bony labyrinth - a series of interconnecting chambers in the temporal bone  Membranous labyrinth – located inside the bony labyrinth  Endolymph – fluid fills the membranous labyrinth  Perilymph - fluid fills the space between the bony labyrinth & membranous labyrinth  Divided into vestibule, cochlea & semicircular canals  Vestibule & semicircular canals – involved in balance  Cochlea – involved in hearing The cochlea  It is the coil portion of the bony labyrinth  Function - sound wave transduction  Contains 3 region - the scala vestibuli, the scala media or cochlear duct & the scala tympani – all filled with perilymph & endolymph  The cochlear duct – is a part of membranous labyrinth & triangular in shape  Basilar membrane – contain supporting cells & specialised cochlear hair cells containing auditory receptors Organ of corti/spiral organ  Contains the receptor for sound  Consists of supporting cell & hair cells  Hair cells - receptor for auditory sensations  Forms synapses (action potential) with sensory & motor neurons in the cochlear branch of vestibulocochlear (VIII) nerve  Meniere’s disease is a disorder of the inner ear that can affect hearing and balance, and is thought to be due to increased pressure in the cochlea and semicircular canals (extra endolymph)  Episodes of vertigo (the room spinning) and ringing in the ears (tinnitus) can be a mild annoyance, or a chronic, disabling disability  Sound waves enter the external auditory meatus  Sound waves strike the tympanic membrane, causing it to vibrate back and forth  The vibration conducts from the tympanic membrane through the ossicles (malleus →incus → stapes)  The stapes moves back and forth, pushing the membrane of the oval window in and out  Movement owindow membrane starts oscillations in the perilymph f the oval in the cochlea  Oscillations in the perilymph cause vibrations in the vestibular & basilar membrane  When the basilar membrane moves, the hairs on the hair cells in the organ of Corti rubs against the tectorial membrane & bend  Bending the hairs on the hair cells stimulates the formation of impulse  Impulses are transmitted to the auditory cortex of the temporal lobe on the cochlear branch of the cranial nerve VIII (vestibulocochlear nerve)  Sensory neurons in the cochlear branch of each vestibulocochlear (VIII) terminate in the medulla oblongata on the same side of brain. From the medulla, axons ascend to the midbrain, then to the thalamus, and finally to the auditory cortex of the temporal lobe  It is the combination of 2 different sense – the sense of static equilibrium & dynamic equilibrium  Static equilibrium – involved in evaluating the position of the head relatively to gravity & occurs when the head is motionless  Dynamic equilibrium – occurs when the head is moving in a rotational or angular direction  The organ involved located in the vestibule portion of the bony labyrinth of the inner ear  The labyrinth inside the vestibule divided into 2 saclike structure - utricle & saccule  Utricle & saccule - contains macula  Macula – it is organ of equilibrium consist of sensory hair cells & supporting cells  The projection of the hair cells are embedded in glutinous mass that covers the macula  Otoliths - it is a grains of calcium carbonate that embedded on the surface of the gelatinous mass  When the head is in upright position, the hairs are straight  When the head tilts or bend forward, the otoliths & gelatinuous mass move in response to gravity & bends the hair on the receptor cells  This action initiate impulse that travel CNS through vestibular branch of the vestibulocochlear nerve  CNS interpret information & send motor impulse for muscles to maintain balance  Dynamic equilibrium involves the maintenance of balance during sudden movements  The receptors located in the crista ampularis  The sense of organ are located in the semicircular canal  Controlled by the sensory hairs within the ampulla of the semicircular canals  Crista- a small elevation in the ampulla contain hair cells and supporting cells  Cupula - a mass of gelatinous material covering the crista  When the head move rapidly - the semicircular canals & the head move but endolymph remain stationary  The fluids push against the cupula & tilt to 1 side  As the cupula tilt, hairs on the hair’s cell bends which trigger a sensory impulse  Vestibulocochlear nerve - nerve CNS interpret information & send motor impulse for muscles to maintain balance  Impulse transmitted to the CNS on the vestibular branch of the vestibulocochlear nerve  Once generated, nerve impulse travel up the vestibular branch of CN VIII  Most of these axons synapse in the major integrating centers for equilibrium, in the medulla and pons, which also receive input from the eyes and proprioceptors  Ascending neurons continue  primary auditory area in the parietal lobe to provide us with conscious awareness of the position and movements of the head and limbs  Hair cells of utricle, saccule and semicircular ducts→Vestibular branch of the vestibulocochlear nerve →brain stem → cerebellum and thalamus→ cerebral cortex What is the structures acts to convert sound waves to vibrations?  Tympanic membrane What is the structures converts vibrations to action potentials?  Organ of Corti What is the structures carries action potentials caused by sound transduction?  Vestibulocochlear nerve What is the structure that collects and directs sound waves to move the tympanic membrane?  Auricle /pinna What is the structure that prevents damage by excessively loud noises?  Auditory ossicles