Physiology Of Hearing PDF

PHYSIOLOGY OF HEARING FUNCTIONAL ANATOMY OF HEARING Divided into outer, middle and inner ear OUTER EAR Pinna Tympanic membrane MIDDLE EAR Eustachian tube Ossicles – malleus, incus, stapes Tensor tympani and stapedius muscles Structure and Function — Peripheral Auditory...

PHYSIOLOGY OF HEARING FUNCTIONAL ANATOMY OF HEARING Divided into outer, middle and inner ear OUTER EAR Pinna Tympanic membrane MIDDLE EAR Eustachian tube Ossicles – malleus, incus, stapes Tensor tympani and stapedius muscles Structure and Function — Peripheral Auditory System INNER EAR BONY LABYRINTH Contains perilymph and membranous labyrinth structures MEMBRANOUS LABYRINTH Cochlea Semicircular canals Otolith organs COCHLEA 35mm coiled tube that directly abuts the bony labyrinth Has 3 chambers Superior Scala vestibuli Inferior Scala tympani Scala media/cochlear duct Scala vestibuli (upper) and scala tympani (lower) are separated from the cochlear duct by the vestibular membrane and the basilar membrane  Scala vestibuli and typani contain perilymph Continuous with CSF Low potassium concentration Cochlear duct contains endolymph which is rich in potassium Scala vestibuli and scala tympani are continuous at the cochlear apex through the helicotrema ORGAN OF CORTI Spiral shaped organ on the basilar membrane Extending from the apex to the base of the cochlea Contains hair cells which are highly specialized auditory receptors 20,000 outer hair cells in three rows 3500 inner hair cells in two rows At the tips of the hair cells are protrusions called stereocilia The tips of the outer hair cells are embedded in the tectorial membrane Processes of hair cells are within endolymph the bases are within perilymph Structure and Function — Peripheral Auditory System Structure and Function — Peripheral Auditory System  sensory neurons arborize around the bases of the hair cells. Cell bodies of which are located in the spiral ganglion. Most afferent fibers terminate at the inner hair cells Most afferent fibers terminate at the inner hair cells SOUND TRANSMISSION THROUGH THE TYMPANUM AND OSSICLES Sound waves travelling through the air canal cause the vibration of the tympanum The vibration is transferred to the ossicles. From the manubrium of the malleus to the footplate of the stapes. The ossicles multiply the force of the vibrations by about 1.3x The stapes transmits vibrations through the oval window to the perilymph filled scala vestibuli of the cochlea. The vibration is thus transferred as a wave through the  The wave amplitude of the wave generated depends on the intensity of the sound. Lower pitched sounds create waves that peak near the apex while higher pitched sounds generate waves that peak near the base A shearing force on the inner hairs is generated by the differential movement of the tectorial membrane and the basilar membrane The place along the basilar membrane which is the most stimulated is used to determine sound frequencies. This is called the place principle  The tips of the stereocilia have mechanically gated channels which open when they make contact Calcium and potassium ions enter and cause the membrane to depolarize. Myosin based motor at the base of the stereocilia cause them to return to their starting position, closing the channels and restoring the resting state There’s secondary release of glutamate which initiates depolarization of the afferent neurons TYMPANIC REFLEX The tensor tympani and stapedius muscles pull the manubrium of the malleus inwards and the footplate of the stapes outwards whenever there are loud sounds This prevents the transmission of excessive loud sounds from causing excessive stimulation of the auditory receptors  The polarization of the auditory nerve fibers depends on the intensity of the sound Signals are transmitted to the CNS via the central auditory pathway. Auditory divisions of CN VIII end up in the dorsal and ventral cochlear nuclei Then pass on to the inferior colliculus Through the medial geniculate body to the auditory cortex in the superior temporal gyrus of the temporal lobe  Collateral fibers from the auditory tracts pass directly to the reticular activating system. This allows loud sounds to activate the entire nervous system.  Olivocochlear bundle arises from the superior olivary complexes and end at the bases of the outer hair cells. They modulate the sensitivity of the hair cells via acetylcholine OTOSCLEROSIS A condition that causes hardening of the ossicles thus causing stiffness and impairing transmission of sound through the middle ear. OTITIS MEDIA AND MENINGITIS Infections from the middle ear can extend into the inner ear and subsequently cerebrospinal fluid PRESBYACUSIS Age related hearing loss as a result of degeneration of the hair cells in the cochlea