Hearing Prof K Sivapalan Sound Waves Longitudinal vibration

Hearing Prof. K. Sivapalan

Sound Waves. • Longitudinal vibration- alternate phases of condensation and rarefaction of molecules. • Velocity in air- 344 m/s, 770 miles/h at 20°C, 1450 m/s in water, faster in salt water. • Loudness- Sound energyamplitude of the waves [A, B]. • Pitch- frequency and ? other factors. 2000 pitches can be identified by humans [B, C]. • Timbre- character- different instruments playing at the same pitch can be identified. • Music- sound waves with patterns [D]. • Noise- no pattern [E], 11/25/2020 Hearing ? Perception. 2

Measurements. • Amplitude is measured in decibel scale- logarithm of ratio of a standard sound. • Standard sound- 0 decibel = 0. 000204 dyne/cm 2, threshold for hearing by an average human. • Pressure at see level- I bar [15 lb/in 2]. • Range of sound pressure from threshold to damage to cochlea is 0. 0002 – 2000 μbar. • Audible frequency- 20 – 20, 000 Hz. • Male voice- 120 Hz, female voice- 250 Hz. • Greatest sensitivity for hearing – 1000 – 3000 Hz. 11/25/2020 Hearing 3

Structure of the Ear 11/25/2020 Hearing 4

External ear. • Function is funneling sound through external auditory meatus to the tympanic membrane [ear drum] • Funneling can be improved by adding surface. • Wax secreted protects from insects but blocks when excessive. • Foreign bodies inserted can damage ear drum which is located in safe place. 11/25/2020 Hearing 5

Tympanic Membrane. • Tympanic membrane divides the external ear from middle ear. • The membrane is pulled inwards by tensor tympani. • Tendon of the muscle is attached to the manubrium of the maleous which is attached to the membrane. 11/25/2020 Hearing 6

Middle Ear. • Air filled cavity in the temporal bone • Communicates with nasopharynx through Eustachian tube. • The tube is normally closed. • The tube is opened during swallowing, chewing, and yawning. • This keeps pressure on either side of the ear drum equalized. • The cavity extends posteriorly as mastoid andrum. 11/25/2020 Hearing 7

Three Ossicles. Incus. Malleus Stapes. 11/25/2020 Hearing 8

Attachments of ossicles. • Manubrium [handle of the malleus] is attached to tympanic membrane. • Short process is attached to incus – synovial joint. • Incus articulates with the head of the stapes – synovial joint. • The foot plate is attached to oval window. 11/25/2020 Hearing 9

Ossicular conduction. • Motion of the tympanic membrane is imparted to malleus. • Malleus rocks in the axis of the junction between long and short processes. • Malleus transmits the movement to the head of the stapes. • Foot plate of the Stapes moves like a door hinged to the posterior edge of the oval window. 11/25/2020 Hearing 10

Conduction in Ear. • Tensor tympani and Stapedius muscles keep the membranes tense, ready to vibrate. • Tympanic membrane acts as a resonator. [surface area is 55 mm 2. • Leaver action multiplies the pressure 1. 3 times. Surfaces of the oval window is 3. 2 mm 2. [17 times less than Tympanic Membrane. This increases sound pressure 17 times at oval window] • There is a small loss because of resistance. • Even though some energy is lost, sound pressure is increased [1. 3 x 17=22 times] to vibrate fluid in internal ear. • Bone conduction: Transmission through bones of the skull directly to inner ear. 11/25/2020 Hearing 11

Tympanic Reflex • Loud sounds cause constriction of the middle ear muscles. • It protects the ear from high amplitude vibrations. • Reaction time is 40 – 160 ms. • This reflex cannot prevent the effects of gun shot, explosions or beginning of thunder. 11/25/2020 Hearing 12

Inner ear [Labyrinth]. • Bony labyrinth is a system of channels in the petrus part of the temporal bone containing perilymph. • Membranous labyrinth is inside the bony labyrinth, containing endolymph. 11/25/2020 Hearing 13

Membranous Labyrinth • It has two components. • Cochlea with organ of Corti for hearing. • Vestibule with semicircular cannels, utricle and saccule for sense of balance. 11/25/2020 Hearing 14

Cochlea. • Coiled tube, 35 mm long, makes 2¾ turns. • The bony core is the modiolus. • Spiral lamina spirals around the modiolus. • Spiral ganglion found in the modiolus contains the cell bodies of afferent nerves- cochlear nerve. 11/25/2020 Hearing 15

Cochlea ctd. • Reissner’s membrane from the base of the spiral lamina maks the Scala vestibuli above. • Basilar membrane from the tip of the spiral lamina forms Scala tympani below. • Scala media is at the centre. 11/25/2020 Hearing 16

Transmission in Cochlea. • Scala vestibuli and scala tympani communicate at helicotrema, contain perilymph. • At the base, scala vestibuli ends in oval window [stapes] and scala tympani ends at round window. • Scala media continues as membranous labyrinth into vestibule. • Movements of stapes set up waves in perilymph in scala vestibuli. • The Raisner’s membrane is the only flexible part of the scala vestibule and through it the vibrations are transmitted to the basilar membrane. • The waves reach maximum height at a certain point: high pitch near base and low pitch sounds near apex. • The displacements of basilar membrane are dissipated by the round window. 11/25/2020 Hearing 17

Basilar Membrane. • Basilar membrane contains 20000 -30000 basilar fibers projecting from modiolus to outer wall. • Fibers are stiff, elastic, reed like. • They can vibrate like the reeds of harmonica. • The length increases towards the apex. 0. 04 to 0. 5 mm. • Diameter of the fibers decrease towards apex. • So, basal fibers vibrate best in high frequency and apical at low frequency. 11/25/2020 Hearing 18

Organ of Corti. • Located on the basilar membrane. • Extends from base to apex. • Hair cells – receptors. • One row of inner hair cells [3500] and three rows of outer hair cells [20000]. • Thin, viscus and elastic tectorial membrane covers the hair cells. Hairs of the outer cells are embedded in the membrane. 11/25/2020 Hearing 19

Perception in Organ of Corti. • Sound waves distort the basilar membrane and the location of maximum distortion is derermined by the frequency [pitch] • As the basilar membrane and the tectorial membrane are hinged in different axis, movements bend the hair in the organ of Corti. • Bending of the hair in one direction depolarizes the cell which sets up action potential in the axon. • Bending in opposite direction hyperpolarizes. • Inner hair cells are the primary sensory cells. • Role of outer hair cells not clear. 11/25/2020 Hearing 20

Innervation. • Spiral ganglion found in the modiolus contains the cell bodies of afferent nerves. • 90 -95 % of the afferents innervate inner hair cells and the balance outer hair cells. • Most of the efferent fibers end on the outer hair cells. 11/25/2020 Hearing 21

• Afferents end in dorsal and ventral cochlear nuclei in the medulla. • Central connections are bilateral at all levels. • Inferior colliculi auditory reflexes. • Medial geniculate body relays to auditory cortex. • Afferents to cerebellum and reticular system. • Olivocochlear fibers are the efferents. 11/25/2020 Central connections. Hearing 22

Cortical representation. • The primary auditory area is in the superior temporal gyrus. • Low frequency anteriorly and high frequency posteriorly. 11/25/2020 Hearing 23

Determination of loudness: • Louder sounds vibrate basilar membrane more, causing more bending of hair, and higher frequency of action potential in the nerve. • Number of nerves conducting impulseshair cells before and after the specific area for the frequency vibrate as the amplitude increases. • ? outer hair cells may also be involved. 11/25/2020 Hearing 24

Sound localization: • Difference in time between arrival in the two ears- 20μs difference can be detected. • Difference in loudness between the ears. • Turning the head to facilitate localization. 11/25/2020 Hearing 25

Masking. • Presence of one sound decreases ability to hear another sound. • Middle ear muscles could play a role by altering the tension in membranesespecially low frequency sounds could be masked. • Efferents could inhibit hair cells. • Cortical selection of preferred sound. 11/25/2020 Hearing 26

Deafness. • Conduction deafness- defect in conduction up to middle ear. • Wax, damage to tympanic membrane, stiffness of joints of ossicles. • Nerve deafness- damage to haircells or nerves. • Infection, injuries. 11/25/2020 Hearing 27

Tests for hearing. • Tuning fork test • Weber test- tuning fork at vertex. • Rinne test- tuning fork on mastoid process, when no sound before the ear. • Audiometry. • Pure tones produced at different frequencies and sent through ear phons. • The threshold amplitude is recorded for each frequency and plotted. 11/25/2020 Hearing 28