Hearing & Equilibriumphysiology

Abbas A. A. Shawka

Hearing • The mechanism of hearing is closely associated with the

mechanism of equilibrium; therefore, the inner ear acts as an organ of hearing and equilibrium.

• For hearing, the sound

• waves have to pass through the three subdivisions of the ear, which are :-

1. External ear

2. Middle ear.

3. Internal ear.

External ear • The pinna collects and

reflects the sound waves into the external auditory canal.

1. In lower animals, pinna is more important, in whom it can be moved by muscular action in the direction of sound source.

2. In humans, the pinna is not moveable, but its peculiar shape aids in discerning the source of sound (e.g. in front of versus behind the head).

Middle ear • Two impoatant openings in the

medial wall of midlle ear …

• Oval window is present above, in which foot plate (face plate or stapes) is attached. It leads to the vestibule of the internal ear and transmits the sound vibrations of the ossicles to the perilymph of scala vestibuli.

• Round window is present in the lower part and is closed by a thin membrane called secondary tympanic membrane.

• It accommodates the pressure waves transmitted to the perilymph of the scala tympani( get rid of energy )

Middle ear • Tensor tympani

- It constantly pulls the handle of malleus inwards and thus keeps the tympanic membrane tensed.

- Due to this, vibrations on any portion of the tympanic membrane are transmitted to the malleus.

• Stapedius

- it pulls the footplate of stapes out from the oval window.

• Both muscles of the middle ear act simultaneously and reflexly in response to loud sound and attenuate the sound

Internal ear • Bony labyrinth consists of three parts: vestibule, semicircular

canals and the cochlea.( cavities of the petrous bone )

• Membranous labyrinth is lodged within the bony labyrinth.

• It is filled with endolymph (which resembles intracellular fluid) and is surrounded by perilymph (which resembles extracellular fluid in its composition.

• The inner ear can be divided into two main parts:

• 1. Vestibular receptor apparatus. It consists of :-

• A. Utricle and saccule, which are lodged in the bony vestibule and are collectively called otolith organs.

• B. Semicircular ducts, which lie within the body of semicircular canals.

• Vestibular apparatus is concerned with equilibrium

• 2. Auditory receptor apparatus is formed by the duct of cochlea, which lies within the bony cochlea.

1. Bony labyrinth

2. Membranous labyrinth

Bony cochlea • Bony cochlea is a spiral tube, which

in humans has a two and three-fourth turns around a central bone called the modiolus.

• The base of the modiolus is directed towards internal acoustic meatus and transmits vessels and nerves to the cochlea.

• Around the modiolus and winding spirally, like the thread of a screw, is a thin plate of bone called osseous spiral lamina.

• It divides the bony cochlea incompletely and gives attachment to the basilar membrane.

• Two membranes (basilar membrane and Reissner’smembrane) divide the bony cochlea into three compartments

1. Scala vestibule

2. Scala media

3. Scala tympani

Bony chochlea• Scala vestibuli and scala

tympani are filled with perilymph and communicate with each other at the apex of cochlea through an opening called helicotrema.

• Scala vestibuli is separated from the scala media by Reissner’s membrane and is closed by the footplate of stapes, which separates it from the air-filled middle ear

• Scala tympani is separated from the scala media by the basilar membrane and is closed by secondary tympanic membrane. It is also connected with a subarachnoid space through the aqueduct of cochlea

Membranous chochlea• Scala media or cochlear duct or

membranous cochlea appears triangular on cross-section. Its three walls are formed by (Fig. 11.2-4):

• Basilar membrane, which is attached medially to the osseous spiral lamina and laterally to the fibrous spiral ligament (which lines the bony cochlea), forms the inferior wall of the cochlear duct. The basilar membrane supports the organ of Corti.

• Reissner’s membrane, which is attached medially to the wall of limbus and laterally to the upper margin of stria vascularis, forms the superior wall of the cochlear duct.

• Stria vascularis forms the lateral wall of the cochlear duct. It consists of vascular epithelium and is concerned with the secretion of endolymph.

Organ of corti• the sense organ of hearing.

• is situated on the top of the basilar membrane in the scala media.

• Contains peripheral hearing receptors ..

• 1- Rods of Corti. These are two projections (inner and outer rods) from the basilar membrane into the scalamedia. In between the two rods is the tunnel of Corti, which contains a fluid called cortilymph. The exact function of the rods and cortilymph is not known.

• 2. Hair cells. Hair cells are the receptor cells that transduce sound energy into electrical energy.

• Two groups of hair cells lie on the basilar membrane

• A ) inner hair cells

• These cells are probably more important in the transmission of the auditory impulses. These are responsible for fine auditory transmission

• B ) Outer hair cells

• These are responsible for detecting the presence of sound.

• 3- Tectorial membrane

• This membrane is attached to the upper surface of spiral lamina and its free edge extends just beyond the outermost neuroepithelial cells.

• The shearing force between the hair cells and tectorial membrane produces the stimulus to hair cells.

Auditory pathway • Auditory pathways comprise

following relay stations

1. Spiral ganglion.

2. Superior olivary nucleus complex, trapezoid nucleus and nucleus of lateral lemniscus.

3. Inferior colliculus,

4. Medial geniculate body and

5. Auditory cortex.

Spiral ganglion • Dendrites of these bipolar

cells constitute the afferent fibres innervating the hair cells.

• Axons of these bipolar cells form the cochlear division of eighth cranial nerve.

• The cochlear nerve ends in the cochlear nuclei in the medulla.

Cochlear nuclei • The axons of second-order

neurons from the cochlear nuclei pass medially in the dorsal part of pons.

• Most of them cross to the opposite side, but some remain uncrossed.

1. The crossing fibres of two sides form a conspicuous mass of fibres called the trapezoid body.

2. Some crossing fibres run separately in the dorsal part of pons and do not form part of the trapezoid body.

Lateral lemniscus • Third-order neurons have

their cell bodies mainly in the superior olivarycomplex (made up of a number of nuclei) and also in trapezoid nucleus and nucleus of lateral lemniscus.

• The fibres of lateral lemniscus ascend to the mid brain and terminate in the inferior colliculus.

Inferior colliculi & medial geniculate body • Fourth-order neurons have their

cell bodies in the inferior colliculus, where the fibres of lateral lemniscus terminate.

• Fibres arising in the inferior colliculus enter the inferior brachium to reach the medial geniculate body.

• Fifth-order neurons have their cell bodies in the medial geniculate body where most of the fibres arising in inferior colliculus terminate. Some fibresfrom the lateral lemniscus reach this body without relay in the inferior colliculus.

• Fibres arising in the medial geniculate body form the acoustic radiation, which ends in the acoustic area of the cerebral cortex.

Auditory cortex • Major areas constituting

auditory cortex present in the temporal lobe are:

1. Primary auditory cortex (areas 41 and 42).

2. Auditory association areas (areas 22, 21 and 20).

Physiology of hearing

• Physiology of audition can be discussed under following headings:

1. Stimuli or sound waves

2. Conduction of sound waves

3. Transduction of sound waves

4. Neural transmission of signals and

5. Encoding of signals.

Stimuli of sound wave

• 1- speed of sound

• 2- frequency of sound

• 3- Amplitude (intensity) of sound

• 4- pitch

Conduction of sound waves • 1- Role of the external ear

• Pinna :- collects and reflects the sound waves into the external auditory meatus. Its peculiar shape, in humans, aids in discerning the source of sound (e.g. in front versus behind the head).

• External auditory meatus :- conducts the sound waves to the tympanic membrane.

• It is S-shaped course:

• 1- Helps in amplifying the sound waves.

• 2- Prevents mechanical injury to the tympanic membrane

• 3- Helps in maintaining favourable temperature and humidity for normal functioning of the tympanic membrane.

Conduction of sound waves • 2- role of middle ear

• Tympanic membrane

• The vibrating tympanic membrane causes the ear ossiclesto vibrate.

• Thus, the tympanic membrane acts as:

• Pressure receiver, i.e. it is extremely sensitive to pressure changes produced by the sound waves.

• Resonator, i.e. starts vibrating with pressure changes produced by the sound waves.

• Critically dampens, i.e. the vibrations of tympanic membrane cease immediately after the end of sound.

Conduction of sound waves • Mechanism of conduction from middle to inner ears

• 1) impedance matching mechanism

• This fact can be appreciated by the observation that a person under water cannot hear any sound made in the air. This happens because 99.9% of the sound energy is reflected away from the surface of water because of the impedance offered by it.

• Exactly, a similar situation exists in the ear when the air-filled middle ear has to conduct the sound to the fluid-filled inner ear. Nature has compensated for it by providing impedance matching mechanism to the middle ear.

• The middle ear functions as an impedance matching device, primarily by amplifying the sound pressure.

Mechanisms to amplified sound by middle ear • 1- Lever action of ossicles.

• Handle of malleus is 1.3 times longer than the long process of incus, this provides a mechanical leverage advantage, due to which the middle ear ossicles increase the force of movement by 1.3 times.

• 2- Hydraulic action of the tympanic membrane

• is exerted because the effective vibratory area of the tympanic membrane (about 45 mm2) is much greater than the stapes—oval window surface area (about 3.2 mm2). This size difference, means the force produced by the sound is concentrated over a smaller area, thus amplifying the pressure exerted on the oval window (14 folds).

• 3- Curved membrane effect

• Movements of the tympanic membrane are more at the periphery than at the centre where malleus handle is attached. This too provides some leverage.

Mechanisms to amplified sound by middle ear • Thus, the above three mechanisms together increase the

sound pressure 18 folds (i.e. 14 × 1.3). In this way, the impedance mismatching between the air-filled middle ear and fluid-filled inner ear is mostly compensated.

• Therefore, when the tympanic membrane and the ossiclesare removed, and the sound waves strike the oval window directly, even very loud sounds are heard as whispers.

• Mechanism of conduction from middle to inner ears

• 2) phase difference between oval and round window

• If the sound waves were to strike both the windows simultaneously, they would cancel each other’s effect with no movement of perilymph and no hearing.

• This acoustic separation of windows is achieved by the presence of intact tympanic membrane and a cushion of air in the middle ear around the round window.

Conduction of sound waves • Mechanism of conduction from middle to inner ears

• 3) natural resonance of external and middle ear

• The external ear and middle ear, due to the inherent anatomic and physiologic properties, allow certain frequencies of sound to pass more easily to the inner ear.

• the greatest sensitivity of the sound transmission is between 500 and 3000 Hz, and these are the frequencies most important to human in day-to-day conversation.

• 4) attenuation reflex

• is a preventive reflex which reduces sound pressure amplitude by affecting the mobility and transmission properties of the auditory ossicles.

• Stimulus for this reflex is loud sound. The two muscles of the middle ear (tensor tympani and stapedius) contract reflexively in response to the intense sound

Transduction of sound waves • Transduction of mechanical sound wave into electrical

signal occurs in the organ of Corti of inner ear.

1. Vibration of basilar membrane

2. Stimulation of the hair cells

3. Membrane potential changes in the hair cells

1) Vibration of basilar memb• Sound waves from the middle ear are passed on to the

inner ear through the oval window by in-and-out motion of the stapes

• Sound waves entering the inner ear from the oval window spread along the scala vestibuli as a travelling wave.

• Most of the sound energy is transferred directly from the scala vestibuli to the scala tympani. Very little of the sound wave ever reaches the helicotrema at the apex of cochlea.

• As the sound energy passes from the scala vestibuli to the scala tympani, it causes the basilar membrane to vibrate. It is important to note that the part of the cochlea where height of pressure wave reaches its maximum varies with the frequency of sound

2) Stimulation of the hair cells • The up-and-down movements of the basilar membrane in turn

cause the organ of Corti to vibrate up and down. The tops of the hair cells in the organ of Corti are held rigid by the reticular lamina and the hair of the outer hair cells are embedded in the tectorial membrane.

• Because the tectorial and basilar membranes are attached at different points on the limbus, they slide past each other as they vibrate up and down.

• When the organ of Corti moves up, the tectorial membrane slides forward relative to the basilar membrane bending the stereocilia away from the limbus

• When the organ of Corti moves down, the tectorial membrane slides backwards relative to basilar membrane and bends the stereocilia towards the limbus

• The bending of stereocilia stimulates (excites) the hair cells.• Depolarization occurs when the stereocilia bend away from the

limbus and• Hyper-polarization occurs when the stereocilia bend towards

the limbus.

Movement of organ of corti and the sterocilia of the hair cells

Hyper-polarizing Depolarizing

NORMAL

3) Membrane potential changes in the hair cells

• The bending of the stereocilia produces a change in the membrane potential of the hair cells proportionate to the degree of displacement (generator potential). The electrical activity of the inner ear can be considered as under:

1. Resting condition.

2. During stimulation of ear.

• DETAILS WILL NOT DISUSSED

Neural transmission of signals • Discussed in the AUDITORY PATHWAY

• Some salient features of auditory pathway which need special emphasis are:

• 1- Bilateral representation.

- From medulla onwards each ear is bilaterally represented in the auditory pathway with only slight preponderance in the contralateral pathway.

- Because of the bilateral representation, lesion beyond medulla has a slight effect on the auditory acuity.

• 2- Descending pathway.

- There is not only an ascending auditory pathway, but also a significant descending pathwayforming feed-forward and feed-backward loops.

Neural transmission of signals • 3- The integration of visual and auditory information

occurs due to interconnection of the superior and inferior colliculi.

• 4- play a role in general arousal :

- The auditory pathways in the brain stem give collaterals to the reticular formation and the cerebellum and thus play a role in general arousal..

• 5- Tonotopic organization ( similar to somatotopicorganization )

- The different parts of organ of corti respond to tones of different frequencies from the basilar to the apical part of cochlea.

Neural transmission of signals • 6- features of other areas - Brodmann’s area 22 is concerned with the processing of

auditory signals related to speech.- During language processing, it is much more active on the left

side than on the right side. - Area 22 on the right side is more concerned with melody, pitch

and sound intensities.

• 7- auditory pathway spasticity - There exists a area plasticity in the auditory pathways, i.e. they

are modified by experience.- Individuals who become deaf before language skills are fully

developed, viewing sign language activates auditory association areas.

- Conversely, individuals who become blind early in life are demonstrably better at localizing sound than individuals with normal eyesight.

Neural process of auditory informations

1. Encoding of frequency (pitch determination),

2. Encoding of intensity (determination of loudness),

3. Feature detection ( respond to a certain condition is more than the other )

4. Localization of sound in space.

Applied aspects

• Noise and masking !

• Hearing loss and deafness

• Examination of hearing

Noise and masking • Masking refers to a phenomenon in which the presence of

one type of sound decreases the ability of the ear to hear another type of sound.

• In other words, masking represents the inability of the auditory mechanism to separate the simultaneous stimulation into separate components. Masking is more effective for sounds with similar frequencies than with sounds for widely different frequencies.

• Low frequency tones mask high-frequency tones more easily than the reverse.

• Example of masking observed is the difficulty in conversation in noisy surroundings.

Hearing loss and deafness

• 1. conductive hearing loss

• 2. Sensory neural hearing loss

• 3. Mixed loose

1) Conductive hearing loss • The causes of conduction hearing loss may lie in the:

• External ear: any obstruction in the ear canal, e.g. by wax, tumours, atresia etc.

• Tympanic membrane : e.g. perforation.

• Middle ear cavity, e.g. fluid in the middle ear (as in otitis media).

• Ear ossicles, e.g. disruption of ear ossicles and fixation of ear ossicles (otosclerosis).

• Eustachian tube obstruction as in retracted tympanic• membrane.

Conductive hearing loss • Characteristically, hearing loss is partial and never

complete because skull bones themselves conduct sound to the cochlea (bone conduction) and the basilar membrane can be set into vibrations.

Sensory-neural hearing loss• Sensorineural (SN) hearing loss results from lesions of cochlea

(sensory type) or eighth cranial nerve and its central connections (neural type).

• SN hearing loss can be congenital or acquired.

• Congenital SN hearing loss is present at birth. It may be due to anomalies of the inner ear or damage to the hearing apparatus by prenatal or perinatal factors.

• Acquired SN hearing loss appears later in life. Cause may be genetic (delayed onset) or non-genetic. Causes of nongenetic acquired SN deafness are:

Infection of labyrinth (viral, bacterial or spirochaetal).

Acoustic trauma, i.e. injury to labyrinth or eighth nerve.

Noise trauma or noise-induced hearing loss occurs due to prolonged exposure to industrial noise.

Ototoxicity. Certain drugs cause damage to inner ear, e.g. streptomycin, neomycin, quinine, chloroquine, etc.

Neoplasms, e.g. acoustic neuroma.

Systemic disorders, e.g. diabetes mellitus, hypertension etc.

Sensory-neural hearing loss

• Characteristic features

1. Usually loss of hearing is complete.

2. Speech discrimination is poor.

Mixed hearing loss

• Both conductive and sensorineural hearing loss is present in the same ear.

• Characterized by:

- Air-bone gap indicating conductive hearing loss and Impairment of bone conduction indicating sensorineural hearing loss.

Tinnitus

• Tinnitus refers to ringing sensation in the ear. It is caused by irritative stimulation of either the inner ear or the vestibulocochlear nerve.

Presbycusis. • The gradual hearing loss associated with aging in called

presbycusis. It occurs due to the gradual loss of hair cells and neurons.

Vestibular apparatus • Semicircular canals.

• The three semicircular canals are arranged at right angles to each other, so that all the three planes are represented as

1. Anterior semicircular canal is vertical and placed at right angles to the long axis of the petrous bone. Thus, it lies in a plane that points forward and outward at about 45° from the sagittal plane.

2. Posterior semicircular canal is also vertical but is placed parallel to the long axis of the petrous bone. Thus, it lies in a plane that points backward and outward at about 45° from the sagittal plane.

3. Lateral semicircular canal is set in a horizontal position making an angle of about 30° with the horizontal plane.

• It is important to note that the right anterior and left posterior canals lie in the one plane while the left anterior and right posterior canals lie in the other plane.

Vestibular apparatus • One end of each semicircular

canal is dilated and is called ampulla. The ampulla contains the receptor organ known as crista ampullaris.

• The semicircular canals open into the utricle by means of five orifices.

• The ampullary end of each canal and narrow end of horizontal canal open independently, while narrow ends of anterior and posterior canals open jointly by a common orifice.

Vestibular apparatus • Utricle and saccule

• Otolith organ refers combined to the two vestibular sacs called the utricle and saccule.

• Utricle is the larger of the two vestibular sacs in which open the three semicircuarcanals.

• Saccule is a globular sac which is connected to utricle indirectly and to cochlea

Vestibular receptors

• The receptor cells of the vestibular system are called hair cells which are slowly adapting mechanoreceptors:

1. The hair cells of the semicircular canals are located in a mass of tissue within the ampulla called crista ampullaris.

2. The hair cells of the utricle and saccule are located in a mass of tissue called the macula.

Hair cells of vestibular system • A large non-motile cilium located at one end of the cell is

called kinocilium.

• When stereocilia are bent toward the kinocilium the cell depolarizes

• When the stereocilia are bent away from the kinocilium,

• the cell hyperpolarizes.

• The changes in the activity of hair cells are conveyed to central nervous system by the afferent fibres, which form the vestibular part of eighth cranial nerve.

Receptors of the semicircular canals • Crista ampullaris

1. Neuroepithelium

2. Secretory epithelial cells

3. Cupula

- is dome-shaped large mass of gelatinous material in which are embedded the cilia arising from the hair cells.

- At its free end the cupula is in loose contact with the wall of ampulla.

- As a result, it forms a compliant seal that closes the lumen of the canal, preventing free circulation of endolymph.

How crista ampullaris work ?!

• The movements produced in the endolymph by the angular movements of head pushes the cupula backwards, causing the cilia of hair cells to bend.

• Depending upon whether the stereocilia are pushed towards or away from the kinocilium, the hair cell depolarizes or hyperpolarizes.

• It is important to note that cupula is unaffected by linear acceleration force, as it has the same specific gravity as the endolymph.

Receptor in otoliths organ • Macula

• 1- neuroepithelium

• 2- secretory epithelial cells

• 3- otoliths membrane

- It is a flat gelatinous membrane covering the hair cells.

- This contains crystals of calciu carbonate called otoliths or otoconia (ear dust), which increase its specific gravity as compared to endolymph.

- The cilia of hair cells project in the gelatinous membrane

How macula work ?!• The movements produced in the otolith membrane by

linear acceleration of the head cause the cilia of hair cells to bend.

• This leads to excitation of vestibular afferents supplying these cells. Orientation of the macula is :-

• Macula of utricle is directed horizontally, so its cilia are in a vertical plane, which are stimulated by horizontally directed linear acceleration, e.g. moving in a car.

• Macula of saccule is directed vertically, so its cilia are in a horizontal plane and are stimulated by vertically directed linear acceleration, e.g. moving in a lift.

Vestibular pathway

Mechanism of functioning of semicircular canals

• Receptors of semicircular canals are stimulated by rotatory movements or angular acceleration of the head.

• Semicircular canals are oriented in three different planes, so movement of the head in any direction generates an unique pattern of activity within the semicircular canals.

• The three axes of the semicircular canals are those activated while:

• 1- Nodding the head up and down (as in signifying yes). This movement occurs along transverse axis,

• 2- Shaking the head from side to side (as in signifying no). This movement occurs along the vertical axis and

• 3- Tilting the head so that ear touches the shoulders. This movement occurs along the anteroposterior axis.

Vetical axis No” movement

Transverse axis Yes” movement

Anteroposterior axis

Head rotation

Mechanism of functioning of semicircular canals

• Receptors of horizontal canals are stimulated during rotation of head in vertical axis while receptors of vertical canals are stimulated during rotation of head in anteroposterior or transverse axis. However, the mechanism of

• stimulation of receptors is same in all the canals.

• Receptors of semicircular canals are stimulated only at the beginning and at the stoppage of rotatory movements.

• During continued rotation at a constant speed, these receptors are not stimulated rather they are adapted as explained

Mechanism of functioning of utricle and saccule • General features of functioning of utricle and saccule are:

• These provide information about linear acceleration and change in head position relative to the force of gravity.

• Receptors (hair cells) present in the maculae of utricle and saccule act as the stretch receptors, the effective stimulus being the pull of gravity on the otolith membrane.

• These receptors discharge tonically even in the absence of head movement because of pull of gravity on the otolith. So, these receptors show little adaptation (of receptors of semicircular canals).

• During linear acceleration of the head the otolith membrane having more specific gravity lags behind due to inertia.

• This causes cilia of hair cells embedded in otolith membrane to bend. This leads to excitation of vestibular afferents supplying these cells

Utricle functioning

• As mentioned earlier, the macula of utricle is directed horizontally and so its cilia are in verticalplane

• These vertically oriented cilia are stimulated by horizontally directed linear acceleration, e.g. moving in a car.

• These hair cells are also stimulated during dorsiflexion or ventroflexion of the head, i.e. by nodding the head up and down (as in signifying yes).

Saccule functioning

• As mentioned earlier, macula of saccule is directed vertically and so its cilia are in horizontal plane

• These horizontally oriented cilia are stimulated by vertically directed linear acceleration,e.g. moving in a lift up or down. These hair cells are also stimulated when the head is tilted sideways, e.g. if the head is tilted laterally to the right the otolith membrane of macula of right saccule hangs downwards and pulls on its macula, which is maximally stimulated; and the otolith membrane of left saccule points upwards and rests on the macula. This being the position of minimal stimulation of the nerve endings.

Nystingmus and post rotatory nystagmus • Nystagmus. For example, when the head is rotated to the left, the

eyes move slowly toward the right in order to keep the image on the fovea. When the eyes have rotated as far as they can, they are rapidly returned to the centre of the socket.

• These reflex movements of the eyes are called nystagmus.

• Thus, nystagmus has two components of the movements:

• Slow components, i.e. slow movement of the eyes to maintain visual fixation is initiated by receptors in the semicircular canals. When the head rotates to the left, receptors in the left horizontal canal are stimulated. Their axons activate reflex movements of the eyes toward the right through the impulses reaching the nuclei of third, fourth and sixth cranial nerves.

• Quick component. When slow movement of eyeballs is limited, the eyeballs move to a new fixation point in the direction of rotation of head. This movement to a new fixation point occurs with a jerk. So, it is called the quick component. The quick component of nystagmus is due to impulses from the vestibular nuclei to the ocular muscle.

Nystingmus and post rotatory nystagmus

• Post-rotatory nystagmus occurs after the body has been rotated and the movement ceases. This is due to movement of cupula in the opposite direction caused by the endolymph when rotation is stopped.