The EarHearing and Balance Dr Spandana Charles Anatomy

The Ear-Hearing and Balance Dr. Spandana Charles

Anatomy The ear is divided into three major areas • External ear– The Pinna – External acoustic meatus • Middle Ear- – Tympanic membrane – Auditory ossicles • Malleus • Incus • Stapes • Inner Ear- Cochlea-Hearing – Semicircular canals-Balance –

Cochlea

Top of the Reissner’s membrane lies the scala vestibuli containing ‘perilymph’ Between the Reissner’s membrane & the basilar membrane lies the scala media containing ‘endolymph’ Below the basilar membrane the perilymph filled cavity called scala tympani The organ corti rests on the basilar membrane

• Scala Vestibuli & Scala Tympani-Peri lymph Ultra filtrate of Blood Resembles ECF-Rich in Na • Scala Media- Endo lymph. Secreted by Stria Vascularis. Rich in Potassium

Basilar Membrane • Fibrous membrane separating scala media & scala tymphani • Contains 20, 000 – 30, 000 basilar fibres • Stiff, elastic reed like structures, fixed at basal ends • Length from base to apex (0. 04 – 0. 5 mm) • Diameter tapers towards apex • Stiff short - high frequency –base • Long limber - low frequency - apex

Organ of Corti • The end organ of hearing • Rests on the basilar membrane • Contains 2 rows of hair cells- inner and outer • Hair cells contain Stereocilli • They are capped by a membrane called tectorial membrane • At the bottom of the hair cells , nerve fibers emerge and later on form the cochlear division of the 8 th nerve

• Tiplink • Animation Ear

Figure 15. 30 a Pathway of sound waves and resonance of the basilar membrane. Slide 1 Auditory ossicles Malleus Incus Stapes Cochlear nerve Oval window Scala vestibuli Helicotrema 4 a Scala tympani Cochlear duct 2 3 4 b Basilar membrane 1 Tympanic membrane Round window 4 a Sounds with frequencies below hearing travel through the helicotrema and do not excite hair cells. Route of sound waves through the ear 4 b Sounds in the hearing 1 Sound waves 2 Auditory ossicles 3 Pressure waves range go through the vibrate the tympanic vibrate. Pressure is created by the stapes cochlear duct, vibrating the amplified. membrane. pushing on the oval basilar membrane and window move through deflecting hairs on inner hair © 2013 Pearson Education, Inc. fluid in the scala vestibuli. cells.

Figure 15. 32 The auditory pathway. Medial geniculate nucleus of thalamus Primary auditory cortex in temporal lobe Inferior colliculus Lateral lemniscus Superior olivary nucleus (ponsmedulla junction) Midbrain Cochlear nuclei Vibrations Medulla Vestibulocochlear nerve Vibrations Spiral ganglion of cochlear nerve Bipolar cell Spiral organ © 2013 Pearson Education, Inc.

Auditory Processing • Pitch perceived by impulses from specific hair cells in different positions along basilar membrane • Loudness detected by increased numbers of action potentials that result when hair cells experience larger deflections • Localization of sound depends on relative intensity and relative timing of sound waves reaching both ears © 2013 Pearson Education, Inc.

Equilibrium and Orientation • Vestibular apparatus – Equilibrium receptors in semicircular canals and vestibule – Vestibular receptors monitor static equilibrium – Semicircular canal receptors monitor dynamic equilibrium © 2013 Pearson Education, Inc.

Figure 15. 33 Structure of a macula. Macula of utricle Macula of saccule Kinocilium Stereocilia Otoliths membrane Hair bundle Hair cells Vestibular nerve fibers Supporting cells © 2013 Pearson Education, Inc.

Maculae • Sensory receptors for static equilibrium in utricle and saccule • Monitor the position of head in space, necessary for control of posture • Respond to linear acceleration forces, but not rotation • Contain supporting cells and hair cells • Stereocilia and kinocilia are embedded in the otolith membrane studded with otoliths (tiny Ca. CO 3 stones) © 2013 Pearson Education, Inc.

The Crista Ampullares (Crista) • Sensory receptor for rotational acceleration – One in each semicircular canal – Major stimuli are rotational movements • Each crista has supporting cells and hair cells that extend into gel-like mass called ampullary cupula • Dendrites of vestibular nerve fibers encircle base of hair cells © 2013 Pearson Education, Inc.

Figure 15. 35 a–b Location, structure, and function of a crista ampullaris in the internal ear. Ampullary cupula Crista ampullaris Endolymph Hair bundle (kinocilium plus stereocilia) Membranous labyrinth Crista ampullaris Fibers of vestibular nerve Hair cell Supporting cell Anatomy of a crista ampullaris in a semicircular canal Section of ampulla, filled with endolymph Cupula Fibers of vestibular nerve At rest, the cupula stands upright. Scanning electron micrograph of a crista ampullaris (200 x) Flow of endolymph During rotational acceleration, As rotational movement slows, endolymph moves inside the endolymph keeps moving in the semicircular canals in the direction of rotation. Endolymph flow opposite the rotation (it lags behind due bends the cupula in the opposite to inertia). Endolymph flow bends the direction from acceleration and cupula and excites the hair cells. inhibits the hair cells. © 2013 Pearson Education, Inc. and deceleration Movement of the ampullary cupula during rotational acceleration

Figure 15. 36 Neural pathways of the balance and orientation system. Input: Information about the body’s position in space comes from three main sources and is fed into two major processing areas in the central nervous system. Somatic receptors (skin, muscle and joints) Vestibular receptors Visual receptors Cerebellum Vestibular nuclei (brain stem) Central nervous system processing Oculomotor control (cranial nerve nuclei III, IV, VI) Spinal motor control (cranial nerve XI nuclei and vestibulospinal tracts) (eye movements) (neck, limb, and trunk movements) Output: Responses by the central nervous system provide fast reflexive control of the muscles serving the eyes, neck, limbs, and trunk. © 2013 Pearson Education, Inc.

Applied aspects • • • Motion Sickness Conduction deafness Sensorineural deafness Menierre s disease Nystagmus

Properties of Sound Waves • Frequency – Number of waves that pass given point in given time – Pure tone has repeating crests and troughs • Pitch – Perception of different frequencies – Normal range 20– 20, 000 hertz (Hz) – Higher frequency = higher pitch • Quality – Most sounds mixtures of different frequencies – Richness and complexity of sounds (music) • Amplitude - loudness – Subjective interpretation of sound intensity – Normal range is 0– 120 decibels (d. B) – Severe hearing loss with prolonged exposure above 90 d. B • Amplified rock music is 120 d. B or more © 2013 Pearson Education, Inc.

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