Hearing and the Ears

The Anatomy of the Ear

The ear consists of 3 different sections1. The External Ear2. The Middle Ear

- The Auditory Ossicles3. The Inner Ear

Anatomy of the Ear

Vestibulocochlear nerve (VIII)

EXTERNAL EAR MIDDLE EAR INNER EAR

Auricle

Auditory ossicles Semicircularcanals

Petrous partof temporal

bone

Facial nerve (VII)

Bony labyrinth of inner ear

To pharynx

Tympanicmembrane

Cartilage

Externalacoustic

canal

Cochlea

VestibuleRound window

Oval window

Auditory tube

The External Ear

The External Ear

• Made up of fleshy and cartilaginous auricle, which surrounds the external acoustic canal

• Sounds coming from the back of the head are blocked by auricle, and sounds coming from the sides of the head are collected and channeled into the external acoustic canal.

• The external acoustic canal ends where the tympanic membrane is located.

The External Ear

• Tympanic Membrane- separates the external ear from the middle ear-is very delicate, and is protected by external acoustic canal and the auricle

• Foreign objects are denied access by the ceruminous glands which secrete a waxy secretion, cerumen.

The Middle Ear

The Middle Ear

• Air filled chamber• Communicates with the nasopharynx through

the auditory tube and mastoid air cells• The auditory tube permits the equalization of

pressures on either side of the tympanic membrane

-On the down side, the auditory tube can also let in microorganisms causing an unpleasant middle ear infection, aka otitis media.

Video!!!!!!

• http://www.youtube.com/watch?v=M3znkf_OEt8

• *Warning*real ear endoscopy! Don’t get grossed out!

The Auditory Ossicles

Stapediusmuscle

Malleusattached to

tympanicmembrane

Innersurface of tympanic

membrane

Malleus

Stapes

(b)(a)

Externalacoustic

canal

Tympanicmembrane

Auditory tube

Stapedius muscle

Stapes

Tensor tympani muscle

Footplate of stapes in oval window

IncusMalleus

Branch ofcranial

nerve VII(cut)

Round window

Connectionsto mastoid

air cellsTemporal

bone

Tendon of tensortympani muscle

Footplateof stapes at

oval window

Incus

The Auditory Ossicles • The collection of the three tiny bones in the middle ear. The

malleus, the incus, and the stapes

•Vibration of the tympanic membrane converts arriving sound waves into mechanical movements, the auditory ossicles acts as a lever for these vibrations to arrive to the inner ear.

The Auditory Ossicles

• There are two muscles located in the middle ear that protect the tympanic membrane and ossicles from violent movements under noisy conditions.

1. Tensor tympani muscle, stiffens the tympanic membrane, this increased stiffness reduces the amount of movement possible.

2. Stapedius muscle, contraction of the stapedius pulls the stapes, reducing movement of the stapes at the oval window

The Inner Ear

The Inner Ear

• Provides senses of equilibrium and hearing• The superficial contours are made up of a

dense bone layer called the bony labyrinth, which can be subdivided into the vestibule, three semicircular canals and the cochlea.

The Inner Ear

• The Vestibule- Consists of two membranous sacs: the uricle

and the saccule- The receptors in the uricle and the saccule

provide sensations of gravity and linear acceleration

The Inner Ear

• Semicircular canals- Enclose semicircular ducts- Are stimulated by the rotation of the head• The cochlea- Senses within the cochlear duct provide the

sense of hearing - Looks like a snail shell

The Inner EarKEY

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

EquilibriumThe state of

physical balance

Equilibrium

• Equilibrium sensations are provided by receptors of the vestibular complex

• Semicircular ducts convey information about rotational movements of the head

• The saccule and utricle convey information about your position with respect to gravity, this also stimulated by rapid acceleration

Equilibrium

• The anterior, posterior, and lateral semicircular ducts are continuous with the utricle

• Each semicircular duct contains– an ampulla: the expanded region that contains the

receptors – Crista: area where receptors are located in the ampulla.

Each crista is bound to a cupula– A cupula being a gelantinous structure that extends the

entire ampulla• Semicircular Ducts:– Contain sensory receptors and hair cells– Active during movement – Are quite when motionless

Endolymphaticduct

Endolymphaticsac

Vestibular branch (VIII)

Semicircularducts

Utricle

Ampulla

Ampulla

Saccule Maculae(a) Right semicircular ducts, anterior view

Direction of duct rotation

Direction ofduct rotation

Direction ofrelative

endolymphmovement

Semicircularduct

Ampulla

At rest

(c)

Hair cells

Crista

Cupula

Supporting cells

Sensory nerve

(b) Cross section through the ampulla

StereociliaKinocilium

Hair cell

Sensory nerve ending

(d) Hair cell

Displacement in thisdirection stimulates

hair cell

Displacement in this direction inhibits hair cell

Supporting cell

Anterior

PosteriorLateral Cochlea

Introduction to Sound

Introduction to Sound• The sound we hear consists of pressure waves

through the air• Each pressure wave consists of a certain

wavelength and each wavelength has a certain frequency (# of waves that pass a fixed reference point in given time)

Sound• Sound is measures in hertz (Hz)• Amplitude of sounds determines how

loud something seems to be. –Greater the energy = Greater the

amplitude

Sound• When sounds waves strike an object, their

energy is a physical pressure • For you to be able to hear any sound, your

thin tympanic membrane must vibrate in resonance– Resonance is the phenomenon that with the right

combination of frequencies and amplitudes, objects will vibrate at the same frequency.

The Hearing Process

The Hearing Process1. Sound waves arrive at the tympanic

membrane– Sound waves have direct access to the tympanic

membrane on the side of the head they enter

The Hearing Process2. Movement of the tympanic membrane

causes displacement of the auditory ossicles– the tympanic membrane provides a surface to

collect sound – It vibrates in resonance to sound waves with

frequencies between 20 and 20,000 Hz

The Hearing Process3. Movement of the stapes at the oval window

establishes pressure waves in the perilymph of the vestibular ducts – When the stapes moves inward, the round

window bulges outward into the middle ear– Stapes move in and out, vibrating at the

frequency of sound arriving at the tympanic membrane

– This creates pressure waves within the perilymph

The Hearing Process4. The pressure waves distort the basilar

membrane on their way to the round window of the tympanic duct– The louder the sound = the more the basilar

membrane moves

The Hearing Process5. Vibration of basilar membrane causes

vibration of hair cells against the tectorial membrane– Movement of hair cells causes there to be a rush

of ions to be sent to neurotransmitters causing the simulation of neurons

– More intense sound= hair cells become more active

– Hair cells are located on the organ of Corti

The Hearing Process6. Information about the region and intensity of

stimulation is relayed to the CNS over the cochlear branch of the vestibulocochlear nerve (VIII)– bipolar sensory neurons that monitor cochlear

hair cells are located at the center of the bony cochlea, the spiral ganglion

– From there information is carried to the cochlear nuclei of the medulla oblongata for distribution to other centers of the brain

The Hearing Process

Externalacoustic

canal

Movementof sound

waves

Malleus Incus Stapes Oval window

Cochlear branch of cranial nerve VIII

Vestibular duct (perilymph)

Cochlear duct (endolymph)

Tympanic duct (perilymph)

Roundwindow

Tympanicmembrane

Vestibular membrane

Basilar membrane

1

2

3

4

5

6

Professions

• Audiologist- Help people with hearing or balance problems

• Otolaryngologists (ENT)- treat anything wrong with the ears, nose, or throat

• Pediatrician- treat children with ear infections. Children are more likely to contract an ear infection

Diseases

• Middle Ear Infection- In the pocket of air behind the eardrum, if germs get into the middle ear, it fills with germ-fighting fluid called pus. The build up this fluid is what causes pain.

Diseases

• Nystagmus- The inner part of the ear senses movement and position helps control eye movements, if injured it can cause eyes to shake. If the shaking is so bad, then it can prevent the person from having 20/20 vision.

Diseases

• Deafness- occurs when sound vibrations don't go from the air around a person to the moving bones of the inner ear as well as they should. Causes: hereditary disorders, genetic disorders, prenatal exposure to disease, prolonged exposure to loud noises, trauma

• Swimmers Ear- Germs get into the outer part of the ear and cause an outer ear infection. Can often be brought on by water sitting in the outer ear, allowing for bacteria to grow.