THE EARS AND HEARINGTHE EARS AND HEARING

Equilibrium and hearing are provided by a Equilibrium and hearing are provided by a receptor complex called the inner earreceptor complex called the inner ear

- - the receptors are called HAIR CELLSthe receptors are called HAIR CELLS

- the complex structure of the inner ear - the complex structure of the inner ear and arrangements of accessory and arrangements of accessory structures account for abilities of hair structures account for abilities of hair cells to respond to different stimuli and cells to respond to different stimuli and to provide the input for 2 senses:to provide the input for 2 senses:

1. EQUILIBRIUM- informs us of the 1. EQUILIBRIUM- informs us of the position of the body in space by position of the body in space by monitoring gravity, linear monitoring gravity, linear acceleration, and rotationacceleration, and rotation

2. HEARING- 2. HEARING- enables us to detect enables us to detect and interpret sound waves and interpret sound waves

ANATOMY OF THE EARANATOMY OF THE EAR

The ear is organized into 3 parts:The ear is organized into 3 parts:

1. External ear- 1. External ear- visible portion of the visible portion of the ear, collects and directs sound ear, collects and directs sound waves toward the middle earwaves toward the middle ear

2. Middle ear- collects and amplifies sound 2. Middle ear- collects and amplifies sound waves and transmits them to inner earwaves and transmits them to inner ear

3. Inner ear- 3. Inner ear- contains the sensory contains the sensory organs for hearing and equilibriumorgans for hearing and equilibrium

EXTERNAL EAREXTERNAL EARThe external ear extends beyond the lateral The external ear extends beyond the lateral

surface of the headsurface of the head

PARTS:PARTS:

1. Auricle/Pinna- the visible part of the ear that 1. Auricle/Pinna- the visible part of the ear that has a flexible frame made of cartilage and has a flexible frame made of cartilage and covered with skincovered with skin

- - this is the part of the ear that you can see!this is the part of the ear that you can see!

2. Auditory Canal- opening in the center of the 2. Auditory Canal- opening in the center of the pinnapinna

- - the canal directs sound waves to the canal directs sound waves to the middle earthe middle ear

3. Tympanic membrane (tympanum)- 3. Tympanic membrane (tympanum)- closes the inner end of the auditory closes the inner end of the auditory canalcanal

- also called eardrum- also called eardrum

- - because it is S shaped, you cannot because it is S shaped, you cannot see the tympanic membrane see the tympanic membrane without a special instrumentwithout a special instrument

4. Ceruminous Glands- the outer portion of 4. Ceruminous Glands- the outer portion of the auditory canal has these wax-the auditory canal has these wax-producing glandsproducing glands

- EARWAX- - EARWAX- helps prevent foreign helps prevent foreign objects and insects from getting into objects and insects from getting into the earthe ear

- earwax also slows the growth of - earwax also slows the growth of microorganisms in the ear canal, and microorganisms in the ear canal, and reduces the chances of infectionreduces the chances of infection

MIDDLE EARMIDDLE EAR- Also called the TYMPANIC CAVITY- Also called the TYMPANIC CAVITY

- - small air-filled chamber located small air-filled chamber located within the spongy portion of the within the spongy portion of the temporal bonetemporal bone

- separated from auditory canal by the - separated from auditory canal by the tympanic membrane, but communicates tympanic membrane, but communicates with upper portion of pharynx with upper portion of pharynx (nasopharynx)(nasopharynx)

- - connected to nasopharynx with connected to nasopharynx with EUSTACHIAN TUBE (auditory tube)EUSTACHIAN TUBE (auditory tube)

OSSICLES- 3 small bones located OSSICLES- 3 small bones located between the tympanic membrane between the tympanic membrane and the inner wall of the middle earand the inner wall of the middle ear

- Vibrations of sound waves from the - Vibrations of sound waves from the tympanic membrane move to these bonestympanic membrane move to these bones

1. 1. Malleus- hammerMalleus- hammer

2. 2. Incus- anvilIncus- anvil

3. 3. Stapes- stirrupsStapes- stirrups

- these are responsible for transmitting - these are responsible for transmitting sound vibrations across the inner ear sound vibrations across the inner ear cavitycavity

The base of the stapes pulses against a The base of the stapes pulses against a membrane covering an opening called membrane covering an opening called the OVAL WINDOW, in the inner wall of the OVAL WINDOW, in the inner wall of the middle earthe middle ear

- - OVAL MEMBRANE- membrane OVAL MEMBRANE- membrane covering oval windowcovering oval window

- - the ossicles are completely formed the ossicles are completely formed at birth and do not change in sizeat birth and do not change in size

2 muscles are responsible for controlling 2 muscles are responsible for controlling the movement of the ossicles:the movement of the ossicles:

1. Tensor tympani- when this muscle 1. Tensor tympani- when this muscle contracts, the handle of the malleus is contracts, the handle of the malleus is pulled inward producing tension on the pulled inward producing tension on the tympanic membrane (reducing tympanic membrane (reducing movement)movement)

2. Stapedius- 2. Stapedius- acts in opposition to the acts in opposition to the tensor tympani, pulls on the stapestensor tympani, pulls on the stapes

Tension on either side of the tympanic Tension on either side of the tympanic membrane must be equal, or the membrane membrane must be equal, or the membrane will not vibrate properly and hearing will be will not vibrate properly and hearing will be impairedimpaired

EUSTACHIAN TUBE/ AUDITORY TUBE- EUSTACHIAN TUBE/ AUDITORY TUBE- equalizes the air pressure between the equalizes the air pressure between the middle and outer earmiddle and outer ear

- unfortunately can also allow microorganisms - unfortunately can also allow microorganisms to travel from the nasopharynx into the to travel from the nasopharynx into the tympanic cavity tympanic cavity INFECTION INFECTION

Swallowing or yawning causes the inner Swallowing or yawning causes the inner edges of the tube to open, and air is edges of the tube to open, and air is allowed to enter or leave allowed to enter or leave ears “pop”ears “pop”

- if the entrance to the Eustachian tube is - if the entrance to the Eustachian tube is inflamed because of infection, the edges inflamed because of infection, the edges may not open and the tympanic may not open and the tympanic membrane will not move properly membrane will not move properly temporary deafness or discomfort temporary deafness or discomfort occursoccurs

INNER EARINNER EAR

Contains receptors that initiate nerve Contains receptors that initiate nerve impulses which the brain interprets as impulses which the brain interprets as soundsound

- - most important part of auditory most important part of auditory devicedevice

- also contains parts concerned with - also contains parts concerned with balancebalance

- - divided into 3 canals called the BONY divided into 3 canals called the BONY LABYRINTHLABYRINTH

MEMBRANOUS LABYRINTH- fills the Bony MEMBRANOUS LABYRINTH- fills the Bony LabyrinthLabyrinth

- - separated from the bony wall by a separated from the bony wall by a fluid called PERILYMPHfluid called PERILYMPH

- the membrane itself is filled with another - the membrane itself is filled with another fluid called ENDOLYMPHfluid called ENDOLYMPH

The Bony Labyrinth is made up of 3 parts:The Bony Labyrinth is made up of 3 parts:

Vestibule, Cochlea, & Semicircular Vestibule, Cochlea, & Semicircular CanalsCanals

1. Vestibule- connected to the middle ear by the 1. Vestibule- connected to the middle ear by the oval windowoval window

- - acts as an entrance to the semicircular acts as an entrance to the semicircular canals and the cochleacanals and the cochlea

Inside the vestibule are 2 membranous sacs filled Inside the vestibule are 2 membranous sacs filled with endolymph called the UTRICLE and the with endolymph called the UTRICLE and the SACCULESACCULE

- - receptors in these sacs provide sensations receptors in these sacs provide sensations of gravity and linear accelerationof gravity and linear acceleration

2. Cochlea- snail-shaped organ of the 2. Cochlea- snail-shaped organ of the inner earinner ear

- - contains the COCHLEAR DUCT of contains the COCHLEAR DUCT of the membranous labyrinththe membranous labyrinth

- receptors in this duct provide the - receptors in this duct provide the sense of hearingsense of hearing

- - the duct is located between a pair the duct is located between a pair of perilymph-filled chambers of perilymph-filled chambers

3. Semicircular Canals3. Semicircular Canals

- - three looped tubes that are 90 degrees three looped tubes that are 90 degrees with one anotherwith one another

- enclose the semicircular ducts- tubular - enclose the semicircular ducts- tubular membranes in the semicircular canals membranes in the semicircular canals

- - receptors in these ducts are stimulated receptors in these ducts are stimulated by rotation of the headby rotation of the head

- the combination of the vestibule and the - the combination of the vestibule and the semicircular canals is called the VESTIBULAR semicircular canals is called the VESTIBULAR COMPLEXCOMPLEX

The bony labyrinth’s walls are dense The bony labyrinth’s walls are dense bone everywhere except at 2 small bone everywhere except at 2 small areas near the base of the cochlea: areas near the base of the cochlea:

Round window Round window

Oval windowOval window

- both of these are covered with - both of these are covered with membranes that separate perilymph in membranes that separate perilymph in the cochlea from air in the middle earthe cochlea from air in the middle ear

- the membrane of the oval window is firmly - the membrane of the oval window is firmly attached to the base of the stapesattached to the base of the stapes

- - when a sound vibrates the tympanic when a sound vibrates the tympanic membrane, the movements are membrane, the movements are conducted over the malleus and incus conducted over the malleus and incus to the stapesto the stapes

- movement of the stapes leads to - movement of the stapes leads to stimulation of receptors in the cochlear stimulation of receptors in the cochlear duct, and we hear the soundduct, and we hear the sound

RECEPTOR FUNCTION IN RECEPTOR FUNCTION IN INNER EARINNER EARReceptors of the inner ear are called HAIR Receptors of the inner ear are called HAIR

CELLSCELLS

- each of these hair cells communicates - each of these hair cells communicates with a sensory neuron by constantly with a sensory neuron by constantly releasing small quantities of releasing small quantities of neurotransmitterneurotransmitter

- - the free surface of hair cells are the free surface of hair cells are covered with about 80-100 finger-covered with about 80-100 finger-like STEREOCILIAlike STEREOCILIA

- hair cells don’t actively move these - hair cells don’t actively move these stereocilia; when an external force stereocilia; when an external force pushes the stereocilia, their movement pushes the stereocilia, their movement distorts the cell surface and alters the distorts the cell surface and alters the rate of neurotransmitter releaserate of neurotransmitter release

- - displacement of the stereocilia in displacement of the stereocilia in one direction stimulates the hair one direction stimulates the hair cells (increases neurotransmitter cells (increases neurotransmitter release)release)

- displacement in the opposite - displacement in the opposite direction inhibits hair cells direction inhibits hair cells (decreases neurotransmitter release)(decreases neurotransmitter release)

PHYSIOLOGY OF HEARINGPHYSIOLOGY OF HEARING

EQUILIBRIUM AND HEARINGEQUILIBRIUM AND HEARING

EQUILIBRIUMEQUILIBRIUM

2 types of equilibrium:2 types of equilibrium:

1. DYNAMIC- aids us in maintaining our 1. DYNAMIC- aids us in maintaining our balance when the head and body are balance when the head and body are moved suddenlymoved suddenly

- receptors are the semicircular ducts: - receptors are the semicircular ducts: provide info. about rotational provide info. about rotational movements of the headmovements of the head

2. STATIC- maintains our posture and 2. STATIC- maintains our posture and stability when the body is stillstability when the body is still

- receptors are the utricle and saccule: - receptors are the utricle and saccule: provide info. about your position with provide info. about your position with respect to gravityrespect to gravity

* if you stand with your head to the side, * if you stand with your head to the side, hair cells in these receptors report the hair cells in these receptors report the angle involved, and whether your head angle involved, and whether your head tilts forward or backwardtilts forward or backward

* * these receptors are also stimulated these receptors are also stimulated by sudden changes in velocityby sudden changes in velocity

Semicircular ducts- Semicircular ducts- Rotational movementRotational movementThe 3 semicircular ducts:The 3 semicircular ducts:

AnteriorAnterior

PosteriorPosterior

LateralLateral

are continuous with the utricleare continuous with the utricle

- - each duct contains a swollen area each duct contains a swollen area called the AMPULLA, which called the AMPULLA, which contains the sensory receptorscontains the sensory receptors

- hair cells attached to the walls of the - hair cells attached to the walls of the ampulla form a raised structure called a ampulla form a raised structure called a CRISTACRISTA

- the stereocilia of these hair cells are - the stereocilia of these hair cells are embedded in a gelatin-like substance embedded in a gelatin-like substance called the CUPULAcalled the CUPULA

- - when the head rotates, movement of when the head rotates, movement of the endolymph pushes against the the endolymph pushes against the cupula and stimulates the hair cellscupula and stimulates the hair cells

Each semicircular duct responds to one Each semicircular duct responds to one of three possible rotational of three possible rotational movements:movements:

- - shaking the head “no”shaking the head “no”

- - nodding “yes” nodding “yes”

- - tilting the head from side to sidetilting the head from side to side

Vestibule- Gravity and linear Vestibule- Gravity and linear accelerationacceleration

Hair cells of the utricle and saccule are Hair cells of the utricle and saccule are clustered in oval MACULAEclustered in oval MACULAE

- - as in the ampullae, stereocilia are as in the ampullae, stereocilia are embedded in a gelatinous materialembedded in a gelatinous material

- the macular receptors lie under a thin - the macular receptors lie under a thin layer of densely packed calcium layer of densely packed calcium carbonate crystals carbonate crystals the complex is the complex is called an OTOLITHcalled an OTOLITH

- when the head is upright, the crystals - when the head is upright, the crystals sit atop the macula, pushing the sit atop the macula, pushing the stereocilia downwardstereocilia downward

- when the head is tilted, the crystals - when the head is tilted, the crystals shift to the side, distorting the shift to the side, distorting the stereociliastereocilia this tells the CNS this tells the CNS that the head is no longer levelthat the head is no longer level

Otolith crystals are heavy, so when the Otolith crystals are heavy, so when the rest of the body makes a sudden rest of the body makes a sudden movement, they lag behindmovement, they lag behind

Ex: ElevatorEx: Elevator

- then an elevator starts downward, we - then an elevator starts downward, we know it right away because the crystals know it right away because the crystals are no longer pushing so forcefully are no longer pushing so forcefully against the surfaces of the hair cellsagainst the surfaces of the hair cells

- - once the crystals catch up and the once the crystals catch up and the elevator reaches constant speed, elevator reaches constant speed, we no longer feel like we are we no longer feel like we are movingmoving

- when the elevator slows, the crystals - when the elevator slows, the crystals press harder against the hair cells and press harder against the hair cells and we can feel the force of gravity we can feel the force of gravity increaseincrease

HEARINGHEARING

Receptors of the cochlear duct provide Receptors of the cochlear duct provide us with a sense of hearing that us with a sense of hearing that enables us to detect a quiet whisper, enables us to detect a quiet whisper, yet remain functional in a noisy roomyet remain functional in a noisy room

- receptors for auditory sensation are - receptors for auditory sensation are hair cells similar to those in the hair cells similar to those in the vestibular complexvestibular complex

In transferring vibrations from the In transferring vibrations from the tympanic membrane to the oval window, tympanic membrane to the oval window, the ossicles convert sound energy in air the ossicles convert sound energy in air to pressure pulses in the perilymph of the to pressure pulses in the perilymph of the cochleacochlea

- - these pulses stimulate hair cells these pulses stimulate hair cells along the cochlear spiralalong the cochlear spiral

- FREQUENCY of sound is determined by - FREQUENCY of sound is determined by which part of the cochlear duct is which part of the cochlear duct is stimulatedstimulated

- INTENSIY of sound is determined by - INTENSIY of sound is determined by how many hair cells are stimulated at how many hair cells are stimulated at that part of the cochlear ductthat part of the cochlear duct

Cochlear DuctCochlear Duct

If we look at the cochlear duct in cross If we look at the cochlear duct in cross section, we can see that it lies between section, we can see that it lies between a pair of perilymph filled chambers:a pair of perilymph filled chambers:

Vestibular ductVestibular duct

Tympanic ductTympanic duct

- the outer surfaces of these ducts are - the outer surfaces of these ducts are encased by the bony labyrinth encased by the bony labyrinth everywhere except at the oval windoweverywhere except at the oval window

ORGAN OF CORTIORGAN OF CORTI

The hair cells of the cochlear duct are found in The hair cells of the cochlear duct are found in the the ORGAN OF CORTIORGAN OF CORTI

- this structure sits above the - this structure sits above the BASILAR BASILAR MEMBRANEMEMBRANE, which separates the cochlear , which separates the cochlear duct from the tympanic ductduct from the tympanic duct

- in the organ of Corti, hair cells are arranged - in the organ of Corti, hair cells are arranged in a series of longitudinal rows with their in a series of longitudinal rows with their stereocilia in contact with the overhanging stereocilia in contact with the overhanging TECTORIAL MEMBRANETECTORIAL MEMBRANE

- - this membrane is attached to the inner this membrane is attached to the inner wall of the cochlear ductwall of the cochlear duct

- when a certain portion of the basilar - when a certain portion of the basilar membrane bounces up and down, the membrane bounces up and down, the stereocilia of the hair cells are distorted as stereocilia of the hair cells are distorted as they are pushed against the tectorial they are pushed against the tectorial membrane membrane

- - the basilar membrane moves in response the basilar membrane moves in response to pressure waves in the perilymphto pressure waves in the perilymph

- these waves are produced when - these waves are produced when sounds arrive at the tympanic sounds arrive at the tympanic membranemembrane

The Hearing ProcessThe Hearing Process

Some hearing terms:Some hearing terms:

CYCLES- a term used instead of wavesCYCLES- a term used instead of waves

HERTZ (Hz)- HERTZ (Hz)- number of cycles per second, number of cycles per second, represents frequencyrepresents frequency

PITCH- our sensory response to frequency; PITCH- our sensory response to frequency; how high or low a sound ishow high or low a sound is

INTENSITY- amount of energy or power of a INTENSITY- amount of energy or power of a sound; volumesound; volume

- - reported in DECIBELSreported in DECIBELS

Ex: Soft whisper = 30 decibelsEx: Soft whisper = 30 decibels

Jet plane = 140 decibelsJet plane = 140 decibels

6 BASIC STEPS OF HEARING:6 BASIC STEPS OF HEARING:

1. Sound waves arrive at tympanic membrane1. Sound waves arrive at tympanic membrane

- - waves enter the auditory canal and waves enter the auditory canal and travel toward the tympanic membranetravel toward the tympanic membrane

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

- - when the membrane vibrates, so does when the membrane vibrates, so does the malleus, incus, and stapesthe malleus, incus, and stapes

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

- since the rest of the cochlea is - since the rest of the cochlea is surrounded by bone, pressure can only surrounded by bone, pressure can only be relieved at the round window- be relieved at the round window- membrane bulges outwardmembrane bulges outward

4. Pressure waves distort the basilar 4. Pressure waves distort the basilar membrane on their way to the round membrane on their way to the round windowwindow

- the basilar membrane does not have the - the basilar membrane does not have the same structure throughout its length: near same structure throughout its length: near the oval window it is narrow and stiff; at the oval window it is narrow and stiff; at the other end it is wider and flexiblethe other end it is wider and flexible

- - the location of maximum stimulation the location of maximum stimulation varies with frequencyvaries with frequency

- high frequency sounds vibrate the - high frequency sounds vibrate the membrane near the oval windowmembrane near the oval window

- low frequency sounds vibrate the - low frequency sounds vibrate the membrane further away from the membrane further away from the oval windowoval window

- - the LOUDER the sound, the the LOUDER the sound, the greater the movement of the greater the movement of the membranemembrane

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

- - the resulting displacement of the the resulting displacement of the stereocilia stimulates sensory neuronsstereocilia stimulates sensory neurons

- the number of hair cells stimulated in a given - the number of hair cells stimulated in a given region of the organ of Corti gives information region of the organ of Corti gives information about the intensity of the sound about the intensity of the sound louder louder sound, more hair cells stimulatedsound, more hair cells stimulated

6. Information about the region and 6. Information about the region and intensity of stimulation is relayed to intensity of stimulation is relayed to the CNS over the cochlear branch of the CNS over the cochlear branch of the vestibulocochlear nervesthe vestibulocochlear nerves

- this information is carried to the - this information is carried to the medulla oblongata for distribution to medulla oblongata for distribution to other centers in the brainother centers in the brain

AUDITORY SENSITIVITYAUDITORY SENSITIVITYWe never use the full potential of our auditory We never use the full potential of our auditory

system because body movements and our system because body movements and our internal organs produce sounds that are tuned internal organs produce sounds that are tuned out by adaptationout by adaptation

- when other environmental noises fade away, - when other environmental noises fade away, the level of adaptation drops and the system the level of adaptation drops and the system becomes more sensitivebecomes more sensitive

- - if we relax in a quiet room, our heartbeat if we relax in a quiet room, our heartbeat gets louder as the auditory system gets louder as the auditory system adjusts to the level of background noiseadjusts to the level of background noise