LEARNING OBJECTIVES: L28 LEARNING OBJECTIVES: L28 Know the functional organisation of the external ear, middle ear & inner ear [organ of Corti, semicircular canals, utricle and saccule] Know the functional organisation of the external ear, middle ear & inner ear [organ of Corti, semicircular canals, utricle and saccule] Discuss the functions of the tympanic membrane and middle ear ossicles Discuss the functions of the tympanic membrane and middle ear ossicles Discuss the mechanisms of conduction of sound waves to the fluid of the inner ear [ossicular conduction; air conduction; bone conduction] Discuss the mechanisms of conduction of sound waves to the fluid of the inner ear [ossicular conduction; air conduction; bone conduction]

Review the functional anatomy of the ear Review the functional anatomy of the ear Describe the physical properties of sound Describe the physical properties of sound Explain the role of middle ear ossicles and the tympanic membrane as an Impedance matching device Explain the role of middle ear ossicles and the tympanic membrane as an Impedance matching device Describe the mechanism and significance of the tympanic reflex; Describe the mechanism and significance of the tympanic reflex; Describe the structure of inner ear and functions of hair cells Describe the structure of inner ear and functions of hair cells.

Describe the traveling wave theory of hearing; Describe the traveling wave theory of hearing; Describe the sequence of mechanisms leading to excitation of hair cells in the cochlea and sound transduction Describe the sequence of mechanisms leading to excitation of hair cells in the cochlea and sound transduction Explain how frequency of sound and intensity of sound are encoded ? Explain how frequency of sound and intensity of sound are encoded ? Describe the auditory pathway Describe the auditory pathway Briefely explain the principle involved in tests of hearing in assessing defects of hearing Briefely explain the principle involved in tests of hearing in assessing defects of hearing

Hearing & Physical properties It is the neural perception of sound energy/vibrations striking the ears 1. Identification of sound (“what?”) 2. Localization of sound (“where ?”)

Pitch (tone) – depends on the frequency of sound wave Pitch (tone) – depends on the frequency of sound wave Intensity (loudness) - depends on the amplitude of sound waves Intensity (loudness) - depends on the amplitude of sound waves Timbre (Quality) – depends on overtones Timbre (Quality) – depends on overtones It is responsible for characteristic differences in voices It is responsible for characteristic differences in voices

Human ear can detect, frequencies from 20-20,000 cycles/sec [CPS] Maximum sensitivity-1000 to 4000 Hz OR CPS One frequency - Simple wave “Tone” Many frequencies superimposed- Music / Noise

Audible sound range tolerated by Human ear – 0 to 120 dB [decibels] Comfortable hearing – 20 dB Decibel-Sound energy /threshold of sound pressure level tolerated by human ear [Pressure = Force applied/unit area]

PHYSIOLOGY OF HEARING EAR EAR Receptors for two sensory modalities are housed in the INTERNAL ear Receptors for two sensory modalities are housed in the INTERNAL ear 1. HEARING 1. HEARING 2. EQUILIBRIUM 2. EQUILIBRIUM Divided in to three distinct parts 1. External ear 1. External ear 2. Middle ear 3. Internal ear

1. External ear 1. Cartilaginous pinna – movable in some animals 2. External auditory meatus Functions 1. Pinna helps to collect sound 2. External auditory meatus conveys sound waves from the environment to the tympanic membrane 3. Lining membrane - Has hairs and ceruminous glands secreting ear wax

Sound waves enter the external ear It is amplified by the middle ear Transduced (sound energy converted to electrical signals) in the inner ear This signals sent to various brain areas Perception of sound

TYMPANIC MEMBRANE TYMPANIC MEMBRANE Thin membrane between outer and middle ear Thin membrane between outer and middle ear Functions as a Resonator that reproduces the vibrations of the sound waves Functions as a Resonator that reproduces the vibrations of the sound waves Converts acoustical energy into mechanical energy Converts acoustical energy into mechanical energy MIDDLE EAR MIDDLE EAR Air filled cavity internal to the ear drum Air filled cavity internal to the ear drum Opens to the nasopharynx via, auditory tube (Eustachian tube) Opens to the nasopharynx via, auditory tube (Eustachian tube)

Malleus Malleus The handle of the malleus (Manubrium) is attached to the back of the tympanic membrane The handle of the malleus (Manubrium) is attached to the back of the tympanic membrane Head is attached to the wall of the middle ear Head is attached to the wall of the middle ear Short process is attached by ligaments to the incus Short process is attached by ligaments to the incus [Malleus and incus together act as a single lever] [Malleus and incus together act as a single lever] Arrangement of the auditory ossicles

Incus in turn articulates with the stem/ head of the stapes Incus in turn articulates with the stem/ head of the stapes Foot plate of the stapes is attached by annular ligament to the walls of the oval window Foot plate of the stapes is attached by annular ligament to the walls of the oval window Oval window lies against the fluid filled Oval window lies against the fluid filled membranous labyrinth of cochlea in internal ear membranous labyrinth of cochlea in internal ear

Two small skeletal muscles also located in middle ear Two small skeletal muscles also located in middle ear 1. Tensor tympani Contraction of the this muscle pulls the manubrium of the malleus medially Contraction of the this muscle pulls the manubrium of the malleus medially Decreases the vibrations of the tympanic membrane [protection] Decreases the vibrations of the tympanic membrane [protection] Allows for transmission of vibration to centre of tympanic membrane Allows for transmission of vibration to centre of tympanic membrane

2. Stapedius Contraction of this muscle pulls the footplate of the stapes out of the oval window Contraction of this muscle pulls the footplate of the stapes out of the oval window [Protects inner ear] [Protects inner ear]

Hearing Basilar membrane Pivot Round window Oval window Tympanic membrane Air To pharynx Liquid Sound conduction through middle ear

Functions of middle ear 1. Transmission of sound wave 2. Impedance matching (Is the mechanisms by which middle ear amplifies the sound) 3. Small muscle protect the hair cells in the internal ear from loud sounds 4. Maintenance of air pressure on either side of the tympanic membrane 5. Preferential route of conduction

I.Surface area of TM & surface area of stapes Impedence matching The surface area of tympanic membrane is 50mm 2 and that of oval window is 3mm 2 Thus area is reduced by 17 times (50/3) Leads to increase in air pressure at oval window

II. By Ossicular lever system II. By Ossicular lever system Matches impedance between air in ME & fluid in inner ear Lever mechanisms by the bony ossicles Lever mechanisms by the bony ossicles Increases the sound pressure that arrives at the oval window Increases the sound pressure that arrives at the oval window Lever action multiplies the force 1.3 times Lever action multiplies the force 1.3 times

Thus, reduction in vibrating membrane surface area and impedance matching together - Pressure applied on oval window is increased by 22 times in the middle ear Thus, reduction in vibrating membrane surface area and impedance matching together - Pressure applied on oval window is increased by 22 times in the middle ear ( 17 x 1.3 = 22 ) ( 17 x 1.3 = 22 ) Example: Thus, whispering sound energy- gets amplified in middle ear-enhances audibility Sound amplification in middle ear

Tympanic/Attenuation reflex Loud sounds initiate reflex contractions of middle ear muscles Loud sounds initiate reflex contractions of middle ear muscles Contraction of tensor tympani and stapedius muscles- Pull the manubrium of the malleus inward and the footplate of the stapes outward Decreases sound transmission into inner ear Ossicle system thus made rigid by muscle contractions reduces transmission of sound into inner ear by decibels. Reaction time of reflex : ms

Thus tympanic reflex is Protective in function and also plays a role in sound discrimination Prevents strong sound waves from causing excessive stimulation/damage of the auditory receptors housed in the cochlea of inner ear This reflex does not protect if sound stimulation is brief & intense OR Loud and abrupt (Example-gunshot) –Why???

Mechanisms of conduction of sound waves to the fluid of the inner ear Ossicular conduction (Air conduction means what ?): Main mechanism of conduction; i.e. through TM, via the middle ear ossicles, into the inner ear to the cochlea & stimulate hair cells. Ossicular conduction (Air conduction means what ?): Main mechanism of conduction; i.e. through TM, via the middle ear ossicles, into the inner ear to the cochlea & stimulate hair cells. Bone conduction: Transmission of vibration by skull bones to the fluid of the inner ear; happens with extremely loud sounds, or persons own voice. Bone conduction: Transmission of vibration by skull bones to the fluid of the inner ear; happens with extremely loud sounds, or persons own voice.

THE INNER EAR TRANSFORMS MECHANICAL VIBRATIONS INTO ELECTRICAL IMPULSES

INNER EAR Bony labyrinth Bony labyrinth Membranous labyrinth Membranous labyrinth

High frequency Low Frequency Schematic diagram of cross section of cochlea

The inner ear (also called labyrinth) The inner ear (also called labyrinth) Bony labyrinth is a series of channels in petrous part of temporal bone; it is filled with perilymph Bony labyrinth is a series of channels in petrous part of temporal bone; it is filled with perilymph The membranous labyrinth (is inside the bony labyrinth) and duplicates the shape of bony labyrinth; filled with endolymph The membranous labyrinth (is inside the bony labyrinth) and duplicates the shape of bony labyrinth; filled with endolymph There is no communication between spaces filled with endolymph and those filled with perilymph There is no communication between spaces filled with endolymph and those filled with perilymph The Organ of Corti rests upon basilar membrane The Organ of Corti rests upon basilar membrane The Organ of Corti contains inner hair cells, outer hair cells and different types of supporting cells. The Organ of Corti contains inner hair cells, outer hair cells and different types of supporting cells.

Note that the tips of OHC (but not IHC) are embedded in the tectorial membrane;

To understand the physical mechanism of hearing, appreciate the following facts: To understand the physical mechanism of hearing, appreciate the following facts: 1. The bony walls of the scala tympani & scala vestibuli are rigid 2. The tectorial membrane is rigid since it is attached to the bony labyrinth 3. The Reissner’s membrane is flexible 4. The stereocilia of the outer hair cells (but not the inner hair cells) are attached to the tectorial membrane 5. The basilar membrane is not under tension 6. Thus, shear forces in endolymph brought about by vibration of basilar membrane can bend the stereocilia of inner hair cells.

Basilar membrane characters Apex Apex is wide Apex is wide Relatively flaccid Relatively flaccid Respond maximally to Low frequency sounds Respond maximally to Low frequency soundsBase Base is narrow Base is narrow Thick and Stiff Thick and Stiff Respond maximally to high frequency sounds Respond maximally to high frequency sounds

Differences B/W Endolymph and Perilymph Endolymph is similar to ICF Endolymph is similar to ICF Produced by stria vascularis Produced by stria vascularis High K + High K + Low in Na + Low in Na + Perilymph is similar to ECF Perilymph is similar to ECF Low K + Low K + High Na + High Na +

Organ of corti Organ of corti Hair cells in the OC are specialized receptor cells for hearing Hair cells in the OC are specialized receptor cells for hearing Processes of hair cells (stereocilia) pierce the reticular lamina Processes of hair cells (stereocilia) pierce the reticular lamina There are about 3 rows i.e 20,000 outer hair cells (OHC) There are about 3 rows i.e 20,000 outer hair cells (OHC) There is 1 row, i.e about 3500 inner hair cells (IHC) There is 1 row, i.e about 3500 inner hair cells (IHC) Note that the tips of OHC but not IHC are embedded in tectorial membrane Note that the tips of OHC but not IHC are embedded in tectorial membrane

Innervation of hair cells: They are innervated by sensory nerve fibers of vestibulocochlear nerve whose cell bodies are in the spiral ganglion; They are innervated by sensory nerve fibers of vestibulocochlear nerve whose cell bodies are in the spiral ganglion; 95% of the afferent neurons innervate IHC; 95% of the afferent neurons innervate IHC; There are efferent fibers in the auditory nerve which end mainly on OHC; these efferent fibers (olivocochlear bundle) originate in the superior olivary nucleus There are efferent fibers in the auditory nerve which end mainly on OHC; these efferent fibers (olivocochlear bundle) originate in the superior olivary nucleus

Sound transmission (summarized) Sound waves in external environment Vibrations of tympanic membrane Vibrations of ear ossicles Vibrations of foot plate of stapes Vibrations of perilymph Vibrations of basilar membrane Vibrations of endolymph Depolarization of inner hair cells Action potentials in afferent auditory nerve fibers