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Physiology of the Ear - Structure, Function of External, Middle, Inner Ear & Hearing
- 1. Physiology of External, Middle and Inner Ear Dr. Karishma R Pandey Assistant Professor Dept. of Basic And Clinical Physiology BPKIHS
- 2. 1) Structure and Function of External , middle and inner ear 2) Organ of corti 3) Auditory transduction 4) Impedence matching 5) Cochlear amplification Learning objectives
- 4. The external ear funnels sound waves External auditory meatus Tympanic membrane (eardrum). External Ear
- 5. Air-filled cavity in the temporal bone Opens via the auditory (eustachian) tube into the nasopharynx and through the nasopharynx to the exterior. Middle Ear Three auditory ossicles: malleus, incus, and stapes Two small skeletal muscles: tensor tympani and stapedius The tube is usu. closed, opens during swallowing, chewing, and yawning. Thus keeps the air pressure on the two sides of the eardrum equalized.
- 6. Inner Ear The inner ear (labyrinth) is made up of two parts, one within the other. Bony labyrinth is a series of channels in the petrous portion of the temporal bone Inside these channels, surrounded by a fluid called perilymph, is the membranous labyrinth. It is filled with a K+-rich fluid called endolymph No communication
- 7. Throughout its length, the basilar membrane and Reissner's membrane divide it into three chambers or scalae Bony walls of the scala vestibuli are rigid, Reissner's membrane is flexible Cochlea Coiled tube 35 mm long Makes a two and three quarter turns.
- 8. Organ of Corti Located on the basilar membrane, contains the hair cells (auditory receptors). four rows of hair cells : three rows -- outer hair cells one row -- inner hair cells Processes of the hair cells pierce the tough, membrane-like reticular lamina that is supported by the pillar cells or rods of Corti Thin, viscous, but elastic tectorial membrane in which the tips of the hairs of the outer hair cells are embedded.
- 9. Modiolus, the bony core around which the cochlea is wound within it spiral ganglion is located comprises of cell bodies of the sensory neurons around the bases of the hair cells Axons of the afferent neurons that innervate the hair cells form the auditory (cochlear) division of the eighth cranial nerve.
- 10. Stereocilia, are present in all hair cells Inner hair cells -- primary sensory cells --generate action potentials in the auditory nerves, They are stimulated by the fluid movements Outer hair cells on depolarization becomes short and hyperpolarization becomes long Increases the amplitude and clarity of sounds.
- 12. Resting membrane potential of the hair cells = –60 mV Stereocilia are pushed toward the kinocilium = –50 mV. Processes is pushed in the opposite direction = hyperpolarized. Hair processes provide a mechanism for generating changes in membrane potential proportional to the direction and distance the hair moves.
- 13. The ionic composition of endolymph is same as intracellular fluid of hair = so no diffusion of ions When transduction channels open large potential difference of 150mV exist between the endolymph (80 mV) and the hair cell interior is -70 mV that drives the cation into the steriocilia The depolarization causes voltage gated Ca channels at the base of the hair cells to open and allow more influx of Ca which in turn causes release of neurotransmitter The hair cell depolarization also opens up voltage sensitive K channels at the base of hair cells, thus K ions diffuse out into perilymph which has low conc of K and thus restores the resting potential of the hair cells. Ca pump restores the Ca ions inside the cell
- 14. K+ that enters hair cells via the mechanically sensitive cation channels is recycled It enters supporting cells then to other supporting cells by way of tight junctions eventually reaches the stria vascularis secreted back into the endolymph Scala media is electrically positive by 85 mv relative to the scala vestibuli and scala tympani.
- 15. Impedence Matching brought about 2 ways 1. Force exerted by the sound on the tympanic membrane is concentrated on a much smaller area of the stapes of the footplate—it result in pressure gain 2. The mechanical advantage of the ossicle is higher than 1 which produces further pressure gain
- 16. Cochlear Amplification Outer hair cells amplify the sound Depolarization of OHC= causes them to contract which pulls the Basilar membrane and thus amplifies its movement
Editor's Notes
- Vel of sound in air= 340m/s
- Receptors for two sensory modalities, hearing and equilibrium, are housed in the ear. The external ear, the middle ear, and the cochlea of the inner ear are concerned with hearing. The semicircular canals, the utricle, and the saccule of the inner ear are concerned with equilibrium.
- manubrium (handle of the malleus) is attached to the back of the tympanic membrane. Its head is attached to the wall of the middle ear, and its short process is attached to the incus, which in turn articulates with the head of the stapes. The stapes is named for its resemblance to a stirrup. Its foot plate is attached by an annular ligament to the walls of the oval window. Contraction of the former pulls the manubrium of the malleus medially and decreases the vibrations of the tympanic membrane; contraction of the latter pulls the foot plate of the stapes out of the oval window.
- This membranous structure more or less duplicates the shape of the bony channels. It is filled with a K+-rich fluid called endolymph, and there is no communication between the spaces filled with endolymph and those filled with perilymph.
- upper scala vestibuli and the lower scala tympani contain perilymph and communicate with each other at the apex of the cochlea through a small opening called the helicotrema. At the base of the cochlea, the scala vestibuli ends at the oval window, which is closed by the footplate of the stapes. The scala tympani ends at the round window, a foramen on the medial wall of the middle ear that is closed by the flexible secondary tympanic membrane. The scala media, the middle cochlear chamber, is continuous with the membranous labyrinth and does not communicate with the other two scalae.
- four rows of hair cells : three rows -- outer hair cells lateral to the tunnel formed by the rods of Corti, and one row -- inner hair cells medial to the tunnel. This organ extends from the apex to the base of the cochlea and consequently has a spiral shape. 20,000 outer hair cells and 3500 inner hair cells in each human cochlea Ninety to 95% of these sensory neurons innervate the inner hair cells; only 5–10% innervate the more numerous outer hair cells, and each sensory neuron innervates several outer hair cells. most of the efferent fibers in the auditory nerve terminate on the outer rather than inner hair cells. The axons of the afferent neurons that innervate the hair cells form the auditory (cochlear) division of the eighth cranial nerve. basilar membrane is relatively permeable to perilymph in the scala tympani, and consequently, the tunnel of the organ of Corti and the bases of the hair cells are bathed in perilymph. the processes of the hair cells are bathed in endolymph, whereas their bases are bathed in perilymph.
- Stereocilia, are present in all hair cells increase progressively in height; along the perpendicular axis, all the stereocilia are the same height tight junctions between the hair cells and the adjacent phalangeal cells prevent endolymph from reaching the bases of the cells is embedded in an epithelium made up of supporting cells, with the basal end in close contact with afferent neurons. Projecting from the apical end are 30 to 150 rod-shaped processes, or hairs. have cores composed of parallel filaments of actin. The actin is coated with various isoforms of myosin outer hair cells respond to sound, like the inner hair cells, but depolarization makes them shorten and hyperpolarization makes them lengthen. They do this over a very flexible part of the basal membrane, and this action somehow increases the amplitude and clarity of sounds These changes in outer hair cells occur in parallel with changes in prestin, a membrane protein, and this protein may well be the motor protein of outer hair cells. The outer hair cells receive cholinergic innervation via an efferent component of the auditory nerve, and acetylcholine hyperpolarizes the cells. However, the physiologic function of this innervation is unknown
- Displacing the processes in a direction perpendicular to this axis provides no change in membrane potential, and displacing the processes in directions that are intermediate between these two directions produces depolarization or hyperpolarization that is proportionate to the degree to which the direction is toward or away from the kinocilium. Very fine processes called tip links tie the tip of each stereocilium to the side of its higher neighbor, and at the junction are cation channels in the higher process that appear to be mechanically sensitive. When the shorter stereocilia are pushed toward the higher, the open time of these channels increases. K+—the most abundant cation in endolymph—and Ca2+ enter via the channel and produce depolarization. one hypothesis is that a molecular motor in the higher neighbor next moves the channel toward the base, releasing tension in the tip link This causes the channel to close and permits restoration of the resting state. The motor apparently is myosin-based. Depolarization of hair cells causes them to release a neurotransmitter, probably glutamate, which initiates depolarization of neighboring afferent neurons. outer hair cells, on the other hand, have a different function. .
- The ionic composition of endolymph and intracellular fluid of hair cells is same= no diffusion of ions When transduction channels open large potential difference of 150mV exist between the endolymph (80 mV= endolymphatic potential) and the hair cell interior is -70 mV that drives the cation into the steriocilia This high pd increases the sensitivity of the system and helps in the formation of receptor potential by increasing conductance of cations thru the apical membrane of hair cells The depolarization cuses voltage gated ca channels at the base of the hair cells to open and allow more ca which in turn causes release of neurotransmitter hair cell depolarization also opens up voltage sensitive k channels at the base of hair cells, thus k ions diffuse out into perilymph which has low conc of k and thus restores the resting potential of the hair cells. Ca pump restores the ca ions inside the cell
- is arrangement is necessary for the normal production of generator potentials. perilymph is formed mainly from plasma. On the other hand, endolymph is formed in the scala media by the stria vascularis and has a high concentration of K+ and a low concentration of Na+ Cells in the stria vascularis have a high concentration of Na+–K+ pump. In addition, it appears that a unique electrogenic K+ pump in the stria vascularis accounts for the fact that the scala media is electrically positive by 85 mV relative to the scala vestibuli and scala tympani.
- the sound wave in air set up presuure waves in the cochlear fluid only if there is proper impedence matching’ The acoustic impedence of water is higher than that of the air an dwithout impedence matching most of the sound reaching the cochlea is reflected back instead being transmitted into the cochlear fluid It The pressure gain ensures that more than half the sound energy striking the tympanic membrane is transmitted to the cochlea To overcome the inertia of the ossicles some energy is lost, but the sound pressure is magnified in the middle ear in 2 ways: d/t lever action within the ossicular chain d/t smaller area of the oval window in comparison to the tympanic membranethis magnification of sound in the middle ear is called impedence matching It is also imp to overcome the impedence in the innerear so as to transmit the sound from the air to the liquid medium The middle ear is said to match the impedence of the external ear to the inner ear Middle ear magnifies the pressure of the sound by 3 mechanisms Major : area of tm is 20 times area of foot plate of stapes F=P*A When a sound impinging to the TM with a given force is transmitted to the stapes pressure is much higher at the stapes Ability to move the fluid is depends on the pressure than the force Thus the area diff amplifies the pressure Since the area of the footplate is const increase in force increases the pressure Leveraction of the osicles—malleus longer than incus, incus displaced less than malleus but with greater force This lever action provides amplification by 1.3 times Curved membrane mechanism: tm is rigidly fixed near its rim Not tightly stretched, hangs loose like a tent, thus sound produces less displacement at the center than at the periphery so the force exerted on the malleus is greater
- Low intensity sounds don’t have adequate energy to cause vibration of BM sfter overcoming the inertia of the cochlear fluids It seems that the outer hair cells amplify the sound Initial feeble movement of the BM stimulates both IHC and OHC Depolarizationnof IHC= AP