Hearing

Maddie, Emma, Kelly, Meg

Inner Ear4 layers

Outer layer Bony Labyrinth – made up of dense bone

2nd layer Perilymph – liquid in between the bony and membranous labyrinths

3rd layer Membranous labyrinth – delicate, interconnected network of fluid filled tubes (receptors of the inner ear are found within these tubes)

Inner layer Endolymph – a fluid with electrolyte concentrations

KEY

Lateral

Semicircular canal

Cristae within ampullae

Maculae

Endolymphatic sac

Cochlea

Vestibular duct

Cochlear duct Organ of

CortiTympanic

duct

Posterior

(a)

(b)

AnteriorSemicircular

ductsVestibule

Saccule

Utricle

Endolymph

Perilymph

Membranouslabyrinth

Bony labyrinth

Membranous labyrinth

Bony labyrinth

Inner EarBony labyrinth – subdivided into…1. Vestibule – consists of the saccule and the utricle

(membranous sacs)2. 3 semicircular canals – enclose semicircular ducts• Combination of vestibule and semicircular canals is

called the vestibular complex3. Cochlea – spiral shaped bony chamber that contains the

cochlear duct

Inner Ear• Bony labyrinth consists of dense bone

everywhere except the round window and oval window

Equilibrium

• Equilibrium sensations provided by receptors of the vestibular complex

• Semicircular Ducts (Anterior, Posterior, Lateral semicircular)• Sensory receptors in the semicircular ducts respond to rotation movements

of the head• Each semicircular duct contains an ampulla (expanded region that contains

the receptors)• Crista – region in the wall of the ampulla that contains the receptors

• Bound to cupula

Equilibrium

• Each hair cell in the vestibule contains a kinocilium (single large cilium)

Equilibrium• Hair cells (receptors) are active during a movement, quiet

when the body is motionless• Free surface of each hair cell supports 80-100 long

stereocilia (resemble microvilli)• Hair cells provide information about the direction and

strength of mechanical stimuli• Stimuli involved varies by hair cell’s location• Gravity or acceleration in the vestibule• Rotation in the semicircular canals• Sound in the cochlea

Equilibrium• Movement of receptors controlled by three rotational

planes• Horizontal rotation (ex. Shaking your head no)

stimulates the hair cells of the lateral semicircular duct

• Vertical movement (ex. Nodding “yes”) excites the anterior duct

• Tilting your head from side to side activates receptors in the posterior duct

• Function – provide equilibrium sensations• Utricle and Saccule are connected by a slender

passageway that is continuous with the narrow endolymphatic duct, which ends in the endolymphatic sac

The Utricle and Saccule

• Hair cells of utricle and saccule are clustered in oval structures called maculae

• Hair cell processes are embedded in a gelatinous mass (contains densely packed calcium carbonate crystals known as statoconia)

• Otolith Whole complex (gelatinous matrix + statoconia)

The Utricle and Saccule

Gelatinous material

Statoconia

Nerve fibers

Otolith

Gravity

Gravity

Receptor output increases

“Otolith moves

downhill,” distorting

hair cell processes

(b) Structure of a macula

STEP

1

STEP

2

Head in the anatomical position

Head tilted posteriorly

(a)

(c)

Macula of Saccule

• When your head is in the normal, upright position, the statoconia sit atop the macula (their weight pushes the hair cell processes down rather than one side or another)

• When your head is tilted, the pull of gravity on the statoconia shifts them to the side, distorting the hair cell processes (alerts the central nervous system that the head is no longer level)

Macula of Saccule• Under normal circumstances, body can distinguish

between sensations of tilting and linear acceleration through visual information (amusement park rides confuse your sense of equilibrium because of the change in position and acceleration with restricted/misleading visual information)

Pathways for Equilibrium Sensations• Sensory fibers contained within the vestibular nuclei 4 functions

of the 2 vestibular nuclei• Integrating sensory information about balance and equilibrium

that arrives from both sides of the head• Send information to cerebral cortex and cerebellum of brain

• Reflexive motor commands issued by vestibular nuclei are distributed to motor nuclei for cranial nerves involved with eye, head, and neck movements

• Automatic movements of eye that occur in response to sensations of motion • directed by the superior colliculi of the mesencephalon (in

an attempt to keep your gaze focused on a specific point, despite changes in body position and orientation)

• Nystagmus condition in which people have trouble controlling their eye movements

Pathways for Equilibrium Sensations

Vestibular ganglion

Vestibule

Semicircularcanals

Cochlearbranch

Vestibularbranch

XI

VI

IV

III

Red nucleus

To ipsilateral superior colliculusand relay to cerebral cortex

Vestibular nucleus

To cerebellum

Vestibulospinal tracts

Vestibulocochlearnerve (VIII)

• Receptors responsible for hearing are hair cells in the cochlear duct

• Auditory ossicles convert pressure fluctuation in the air into fluctuation in the perilymph of the cochlea (outside pressure to inside pressure)

Hearing

• Frequency of sound determined from which part of cochlear duct is stimulated

• Volume is determined from how many hair cells are stimulated

Hearing

• Cochlear duct is between perilymph ducts: vestibular duct and tympanic duct

• Outer surfaces encased by bony labyrinth everywhere except bases of ducts

• Ducts are connected and actually form one long duct

The Cochlear Duct

• Hairs are located in the organ of Corti in longitudinal rows

• When the basilar membrane (which the hairs are located on) bounces, the hair cells are distorted by pressing against the upper membrane (tectorial membrane)

The Cochlear Duct

• Hearing is perception of sound• Sine waves: S-shaped curves created by high and

low pressure, travel in cycles• Travel at about 768 mph: speed of sound

An Introduction to Sound

• Wavelength inversely related to frequency (number of waves that pass through reference point for certain amount of time)

• Pitch=sensory response to frequency• Amplitude=intensity of sound, energy content• Cycles per second=hertz, Hz• Sound energy reported in decibels

An Introduction to Sound

• With the right combination of frequency and amplitude, object will vibrate at same frequency as sound: called resonance

• To hear sound, tympanic membrane must vibrate in resonance with sound waves

An Introduction to Sound

The Hearing Process Sound waves arrive at the tympanic membrane…

1. Enter external acoustic canal and travel to tympanic membrane

2. Movement of the tympanic membrane causes displacement of the auditory ossicles

a) Tympanic membrane is the surface for sound collectionb) Resonate with frequencies ~20-20,000 Hzc) When tympanic membrane vibrates, inner ossicles also vibrate= amplify the sound

3. Movement of the stapes at the oval window establishes pressure waves in the perilymph of the vestibular duct

a) Because liquid is incompressible, pressure can only be relieved at the round windowb) Stapes vibrate and creates pressure waves in the perilymph

The Hearing Process

4. The pressure waves distort the basilar membrane on their way to the round window of the tympanic duct

a) Pressure waves travel around perilymph and reach round windowb) As they do this, they disrupt the basilar membranec) High frequencies vibrate the basilar membrane near oval windowd) Lower the frequency, longer wavelength and further from oval window is the maximum distortione) Frequency translated to position along basilar membranef) Amount of movement depends on force of sound

The Hearing Process

5. Vibration of the basilar membrane causes vibration of hair cells against the tectorial membrane

a) Vibration of basilar membrane moves hair cells against tectorial membraneb) Ion channels open, depolarizes hair cellsc) Leads to release of neurotransmitters/ stimulates sensory organsd) Hairs are stimulated in rowse) Number of cells responding indicates intensity of sound

The Hearing Process

Region and intensity of stimulated area is relayed to the CNS over the Cochlear branch of the vestibulocochlear nerve

a) Cell bodies of sensory neurons located in spiral ganglion

b) Vestibulocochlear nerve is responsible for transmitting sound and equilibrium to the brain for further distribution

The Hearing Process

Auditory pathways• Vestibulocochlear nerve formed by neurons

• Info then goes to opposite side of brain to processing center which coordinates reflexes such as turning your head from a loud noise

• Auditory cortex in temporal lobe maps out the organ of Corti

• Frequency to position of basilar membrane is projected onto auditory cortex

• Creates sensation of pitch

• Damaged auditory cortex-responds to sound, but cannot interpret sounds or find patterns

Auditory pathways

Auditory sensitivity• Difficult to assess the absolute sensitivity of the system

• We could, in theory hear air molecules, but full potential is never reached because of our own body and other peripheral sounds

• We adapt to environment which affects hearing i.e. Relaxing in a quiet room

• Young children have the greatest hearing range

• Declines with age due to damage or other accumulated injuries

• Tympanic membrane is less flexible, articulations between ossicles stiffen and round window may begin to ossify

• Result: older individuals exhibit hearing loss

Auditory sensitivity

• Audiologist- treat hearing and balance problems• Otoplasty surgeon- a plastic surgeon that corrects deformities in ears

Occupations associated with hearing

•Deafness- loss of hearing•Tinnitus- perception of abnormal ear or head noises, ringing in your ears•Cholesteatoma-type of cyst found in the middle ear behind the eardrum, noncancerous tumor

Diseases/ Conditions associated with hearing

http://www.youtube.com/watch?v=Jk-4YiiPwBc&safe=active