NTA Level 4 • Semester 1 • PST04103

Structure and Functions of the Ear

Human Anatomy and Physiology • Source Session/Topic 10

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Session 10: Structure and Functions of the Ear

Total Session Time: 120 minutes + 60 minutes assignment

• None

Learning Tasks

By the end of this session students are expected to be able to:

• Define Ear

• Describe Structure and Function of the Ear

• Explain Mechanism of Hearing and Balance

Resources Needed:

• Flip charts, marker pens, and masking tape

• Black/white board and chalk/whiteboard markers

• Overhead Projector and Transparencies or Slide Projector/LCD and Computer

• Handout 10.2: External and Middle Ear

• Handout 10.3:Position of the Middle Ear

• Handout 10.4: The Inner Ear

SESSION OVERVIEW

SESSION CONTENTS

STEP 1: Presentation of Session Title and Learning Tasks (5 minutes)

READ or ASK students to read the learning tasks and clarify

STEP 2: Definition of the Ear (5minutes)

|Activity: Buzzing (5 minutes) |

| |

|ASK students to pair up and buzz on the following question for 2 |

|minutes |

| |

|What is the ear? |

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|ALLOW few pairs to respond and let other pairs to add on points not |

|mentioned |

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|WRITE their response on the flip chart/board |

| |

|CLARIFY and SUMMARIZE by using the content below |

• Ears are paired sensory organs comprising of:

o The auditory system, involved in the detection of sound.

o The vestibular system, involved with maintaining body balance/

equilibrium.

STEP 3: Structure and Functions of the Ear (50minutes)

|Activity: Small Group Discussion ( 20 minutes) |

| |

|DIVIDE students into small manageable groups |

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|ASK students to discuss on the following question |

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|What are the Structure and Functions of the Ear |

|ALLOW students to discuss for 10 minutes |

| |

|ALLOW few groups to present and the rest to add points not mentioned |

| |

|CLARIFY and SUMMARIZE by using the contents below |

• The ear divides anatomically and functionally into three regions

o The external ear

o The middle ear

o The inner ear

• All three regions are involved in hearing.

• Only the inner ear functions in the vestibular system.

• The external ear (or pinna, the part you can see) serves to protect the

tympanic membrane

• (eardrum), as well to collect and direct sound waves through the ear

canal to the eardrum.

• About 1¼ inches long, the canal contains modified sweat glands that

secrete cerumen, or earwax. Too much cerumen can block sound transmission

External Acoustic Meatus (External Ear)

• The external acoustic meatus extends from the deepest part of the concha

to the tympanic membrane (eardrum), a distance of approximately 1 inch

(2.5 cm).

• Its walls consist of cartilage and bone.

• The lateral one-third is formed from cartilaginous extensions from some

of the auricular cartilages and the medial two-thirds is a bony tunnel in

the temporal bone.

• Throughout its length the external acoustic meatus is covered with skin,

some of which contains hair and modified sweat glands producing cerumen

(earwax).

• Its diameter varies, being wider laterally and narrow medially.

• The external acoustic meatus does not follow a straight course.

• From the external opening it passes upwards in an anterior direction,

then turns slightly posteriorly still passing in an upwards direction,

and finally, turns again in an anterior direction with a slight descent.

Tympanic Membrane

• The tympanic membrane separates the external acoustic meatus from the

middle ear.

• It is at an angle, sloping medially from top to bottom and posteriorly to

anteriorly.

• It consists of a connective tissue core lined with skin on the outside

and mucous membrane on the inside.

• Around the periphery of the tympanic membrane a fibrocartilaginous ring

attaches it to the tympanic part of the temporal bone.

• At its center, a concavity is produced by the attachment on its internal

surface of the lower end of the handle of malleus, part of the malleus

bone in the middle ear.

[pic]Refer to Handout 10.2: External and Middles Ear

The Middle Ear

• The middle ear is an air-filled, mucous membrane-lined space in the

temporal bone between the tympanic membrane laterally and the lateral

wall of the internal ear medially.

• It is described as consisting of two parts:

o The tympanic cavity immediately adjacent to the tympanic membrane

o The epitympanic recess superiorly

• It connects to the throat/nasopharynx via the Eustachian tube.

• This ear-throat connection makes the ear susceptible to infection (otitis

media).

• The eustachian tube functions to equalize air pressure on both sides of

the eardrum.

• Normally the walls of the tube are collapsed.

• Swallowing and chewing actions open the tube to allow air in or out, as

needed for equalization.

• Equalizing air pressure ensures that the eardrum vibrates maximally when

struck by sound waves.

• The middle ear communicates with the mastoid area posteriorly.

• Adjoining the eardrum are three linked, movable bones called ossicles,

which convert the sound waves striking the eardrum into mechanical

vibrations.

• The smallest bones in the human body, the ossicles are named for their

shape.

o The hammer (malleus) connected to the tympanic membrane.

o The anvil (incus), the middle bone, connects to the hammer.

o The stirrup (stapes) connected to the incus and the lateral wall of

the internal ear at the oval window.

o The base of the stirrup, the footplate, fills the oval window which

leads to the inner ear.

• Its basic function is to transmit vibrations of the tympanic membrane

across the cavity of the middle ear to the internal ear.

o It accomplishes this through three interconnected but movable bones

that bridge the space between the tympanic membrane and the internal

ear.

• Posterior to the epitympanic recess of the middle ear is the aditus to

mastoid antrum, which is the opening to the mastoid antrum.

• The mastoid antrum is a cavity continuous with collections of air-filled

spaces (the mastoid cells), throughout the mastoid part of the temporal

bone, including the mastoid process.

• The mastoid antrum is separated from the middle cranial fossa above by

only the thin tegmen tympani.

• The mucous membrane lining the mastoid air cells is continuous with the

mucous membrane throughout the middle ear.

• Therefore, infections in the middle ear can easily spread into the

mastoid area.

Auditory Ossicles

• The bones of the middle ear consist of the malleus, incus, and stapes.

• They form an osseous chain across the middle ear from the tympanic

membrane to the oval window of the internal ear.

• Muscles associated with the auditory ossicles modulate movement during

the transmission of vibrations.

Malleus

• The malleus is the largest of the auditory ossicles and is attached to

the tympanic membrane.

• Identifiable parts include the head of malleus, neck of malleus, anterior

and lateral processes, and handle of malleus.

• The head of malleus is the rounded upper part of the malleus in the

epitympanic recess.

• Its posterior surface articulates with the incus.

• Inferior to the head of malleus is the constricted neck of malleus, and

below this are the anterior and lateral processes:

o The anterior process is attached to the anterior wall of the middle

ear by a ligament.

o The lateral process is attached to the anterior and posterior

malleolar folds of the tympanic membrane.

Incus

• The second bone in the series of auditory ossicles is the incus.

• It consists of the body of incus, and long and short limbs:

o The enlarged body of incus articulates with the head of malleus and

is in the epitympanic recess.

o The long limb extends downward from the body, paralleling the handle

of the malleus, and ends by bending medially to articulate with the

stapes.

o The short limb extends posteriorly and is attached by a ligament to

the upper posterior wall of the middle ear.

Stapes

• The stapes is the most medial bone in the osseous chain and is attached

to the oval window.

• It consists of the head of stapes, anterior and posterior limbs, and the

base of stapes:

o The head of stapes is directed laterally and articulates with the

long process of the incus.

o The two limbs separate from each other and attach to the oval base.

[pic]Handout 10.3: Position of the Middle Ear

The Inner Ear

• The internal ear consists of a series of bony cavities (the bony

labyrinth) and membranous ducts and sacs (the membranous labyrinth)

within these cavities.

• All these structures are in the petrous part of the temporal bone between

the middle ear laterally and the internal acoustic meatus medially.

• The bony labyrinth consists of the vestibule, three semi-circular canals,

and the cochlea.

• These bony cavities are lined with periosteum and contain a clear fluid

(the perilymph).

• •This membranous labyrinth contains the actual hearing cells, the hair

cells of the organ of Corti.

Bony Labyrinth

• The vestibule, which contains the oval window in its lateral wall, is the

central part of the bony labyrinth.

• It communicates anteriorly with the cochlea and poster superiorly with

the semicircular canals.

• A narrow canal (the vestibular aqueduct) leaves the vestibule, and passes

through the temporal bone to open on the posterior surface of the petrous

part of the temporal bone.

• Semi-circular canals projecting in a poster superior direction from the

vestibule are the anterior, posterior, and lateral semi-circular canals.

• Each of these canals forms two-thirds of a circle connected at both ends

to the vestibule

Cochlea

• Projecting in an anterior direction from the vestibule is the cochlea,

which is a bony structure that twists on itself two and one-half to two

and three-quarter times around a central column of bone (the modiolus).

• This arrangement produces a cone-shaped structure with a base of cochlea

that faces posteromedially and an apex that faces anterolaterally.

• This positions the wide base of the modiolus near the internal acoustic

meatus, where it is entered by branches of the cochlear part of the

vestibulocochlear nerve [VIII].

[pic]Refer to Handout 10.4: The Inner Ear

STEP 4: Mechanism of Hearing and Physiology of Balance (50minutes)

Physiology of hearing

• The following events are involved in hearing

o The auricle directs sound waves into the external auditory canal.

o 2When sound waves strike the tympanic membrane, the alternating high-

and low-pressure of the air causes the tympanic membrane to vibrate

back and forth. The distance it moves, which is very small, depends

on the intensity and frequency of the sound waves. The eardrum

vibrates slowly in response to low-frequency (low-pitched) sounds

and rapidly in response to high-frequency (high-pitched) sounds.

o The central area of the eardrum connects to the malleus, which also

starts to vibrate. The vibration is transmitted from the malleus to

the incus and then to the stapes.

o As the stapes moves back and forth, it pushes the membrane of the

oval window in and out. The oval window vibrates about 20 times more

vigorously than the eardrum because the ossicles efficiently

transmit small vibrations spread over a large surface area (eardrum)

into larger vibrations of a smaller surface (oval window).

o The movement of the oval window sets up fluid pressure waves in the

perilymph of the cochlea. As the oval window bulges inward, it

pushes on the perilymph of the scalavestibuli.

o Pressure waves are transmitted from the scalavestibuli to the scala

tympani and eventually to the round window, causing it to bulge

outward into the middle ear.

o As the pressure waves deform the walls of the scalavestibuli and

scala tympani, they also push the vestibular membrane back and

forth, creating pressure waves in the endolymph inside the cochlear

duct.

o The pressure waves in the endolymph cause the basilar membrane to

vibrate, which moves the hair cells of the spiral organ against the

tectorial membrane. This leads to bending of the hair cell stereo

cilia, which produces receptor potentials that ultimately lead to

the generation of nerve

Physiology of Balance

• The semi-circular canals and vestibule function to sense movement

(acceleration and deceleration) and static position.

• The three semi-circular canals lie perpendicular to each other, one to

sense movement in each of the 3 spatial planes.

• At the base of the canals are movement hair cells, collectively called

the crista ampullaris.

• Depending on the plane of movement, the endolymph flowing within the

semicircular canals stimulates the appropriate movement hair cells.

• Static head position is sensed by the vestibule, specifically, its

utricle and saccule, which contain the position hair cells.

• Different head positions produce different gravity effects on these hair

cells.

• The utricle, or utriculus, along with the saccule is one of the two

otolith organs located in the vertebrate inner ear.

• These use small stones and a viscous fluid to stimulate hair cells to

detect motion and orientation.

• While the semi-circular canals respond to rotations, the otolithic organs

sense linear accelerations.

• We have two on each side, one called utricle, and the other saccule.

• The utricle contains mechanoreceptors called hair cells that distinguish

between degrees of tilting of the head, thanks to their apical cilia set-

up.

• These are covered by otolith which, due to gravity, pull on the cilia and

tilt them.

• The hair cells for both position and movement create nerve impulses.

• These impulses travel over the vestibular nerve to synapse in the brain

stem, cerebellum, and spinal cord.

• No definite connections to the cerebral cortex exist.

• Instead, the impulses produce reflex actions to produce the corrective

response.

• For example, a sudden loss of balance creates endolymph movement in the

semi-circular canals that triggers leg or arm reflex movements to restore

balance.

• The cavity of the utricle communicates behind with the semi-circular

ducts by five orifices.

• The ductus utriculosaccularis comes off of the anterior wall of the

utricle and opens into the ductus endolymphaticus.

STEP 5: Key Points (05minutes)

• Ears are paired sensory organs comprising of:

o The auditory system, involved in the detection of sound.

o The vestibular system, involved with maintaining body balance/

equilibrium.

• The external ear meatus extends from the deepest part of the concha to

the tympanic membrane (eardrum), a distance of approximately 1 inch (2.5

cm).

• The middle ear is an air-filled, mucous membrane-lined space in the

temporal bone between the tympanic membrane laterally and the lateral

wall of the internal ear medially.

• The internal ear consists of a series of bony cavities (the bony

labyrinth) and membranous ducts and sacs (the membranous labyrinth)

within these cavities.

STEP 6: Evaluation (05 minutes)

• What is ear

• What are the structure of the ear

• What are the functions of the ear

Step 7: Assignment (60 minutes)

|Activity: Take Home Assignment (10 minutes) |

| |

|DIVIDE learners in groups or individual. |

| |

|ASK the learners to work on the following assignment |

| |

|Explain mechanism of hearing and physiology of balance |

|ALLOCATE time for learners to do the assignment and submit |

| |

|REFER learners to recommended references |

References

Seeley, R. R., Stephens, T. D., &Tate, P. (2003). Anatomy and Physiology.

New York:

McGraw-Hill.

Shier, A., Butler, J., & Lewis, R. (2004). Hole’s Human Anatomy &

Physiology. New York:

McGraw-Hill.

Standring, S. (2008). Grays’s Anatomy The anatomical basis of clinical

practice. United

Kingdom: Churchill Livingstone Elservier.

Thibodeau, G. A., & Patton, K. T. (1999). Anatomy & Physiology. Saint

Louis: Mosby, Von

Hoffman Press, Inc.

Waugh, A., & Grant, A. (2006). Ross and Willson Anatomy and physiology in

Health and

illness. United Kingdom: Churchill Livingstone Elservier.

|[pic] |Handout 10.2:External and Middles Ear |

[pic][pic]

Source: Drake, 2005

|[pic] |Handout 10.3: Position of the Middle Ear |

[pic]

Source: Source: Drake, 2005

|[pic] |Handout 10.4: The inner Ear |

[pic]

Source: Source: Drake, 2005

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