Hearing

Anatomy of the EarEars, Hearing, and Balance

The ears provide the sense of hearing. They also detect head position and motion, so they are essential to balance. The parts concerned with hearing and balance are located in different areas of the ear, but the function of both is based on “hair cell” receptors.

The Outer EarThe Outer EarThe outer ear is composed of the ear flap (pinna) and the outer ear canal (external auditory meatus). The Ear Flap (Pinna)The pinna collects sound by acting as a funnelThe pinna amplifies the sound and directs it to the auditory canalHelps to funnel sound waves into the outer ear canalThe filtering effect of the pinna selects sounds in the frequency range of human speech

The Outer EarAmplificationAmplification of sound is by the pinna, the tympanic membrane (eardrum) and the middle earFrequenciesLow frequencies and high frequencies are treated differentlyFor low frequencies; it directs sounds toward the ear canalFor high frequencies; they enter the ear canal at a very slight delayThese three parts cause an increase in amplification by about 10 to 15 dB

The Outer Ear Canal The Outer Ear Canal (External Auditory Meatus)

The outer ear canal (external auditory meatus) is slightly S-shapedThe external auditory meatus is a tube that runs from the outer ear to the middle ear. It extends from the pinna to the eardrum and is about 35mm in length and 5 to 10mm in diameter

The Middle Ear• The middle ear is the portion internal

to the eardrum.• The mammalian middle ear contains

three ossicles.• The primary function of the middle

ear is to transfer acoustic energy from compression waves in air to fluid membrane waves within the cochlea.

Ossicles•  The middle ear contains three tiny bones

known as the ossicles, the maleus, incus and stapes.

• They are also referred to as the hammer, anvil, and stirrup.

• The malleus and incus evolved from lower and upper jaw bones present in reptiles.

• The ossicles are supposed are supposed to mechanically convert the vibrations of the eardrum into amplified pressure waves in the fluid of the cochlea.

MusclesThe movement of the ossicles is controlled by two muscles, the stapedius and the tempor tympani muscle.The stapedius muscle is the smallest skeletal muscle in the body connects to the stapes and is controlled by the facial nerve.The tensor tympani muscle connects to the base of the malleus and is controlled by the trigeminal nerve.These muscles contract in response to loud noises, reducing the transmission to the inner ear, this is known as the acoustic reflex.

The Inner EarThe Inner Ear

Composed of the cochlea, semicircular canals, and vestibule, which are all linked togetherThe semicircular canals have three canals; the posterior, the superior, and the lateralAll are filled with fluid and all are protected by the temporal bone

Hair CellsHair Cells

Each hair cell is seen to have 40 to 100 hairs arranged in a curveNerve fibers run from the cell bases

The Process of BalanceA process involving a range of sensory inputs, analysis in the brain and motor outputsInputs arrive from the eyes, micro-receptors in muscle tendons, and skin pressure sensorsThe inner ear’s fluid-filled vestibule and semicircular canals play a key role They incorporate sensitive hair cells similar to the cochlea’sThe gravity (static equilibrium) react to the speed and direction of head movements (dynamic equilibrium)Both equilibriums respond to most head positions and movements

Parts of the Ear & Their Function

Pinna (ear flap)

Skin-covered flap with subcutaneous fat, cartilage, and connective tissue

Outer Ear Canal (External Auditory Meatus)

A tube running from the outer ear to the middle ear. It extends from the pinna to the eardrum and is about 35mm in length and 5 to 10mm in diameter.

Tympanum, or Tympanic Membrane (Eardrum)

About the size of the owner’s little fingernail; resembles thin skin

Cochlea

Containing the organ of hearing, barely larger than the little finger’s tip, it spirals for 2 ¾ turns

Parts of the Ear & Their Function

Semicircular Canals(Posterior, Superior, and Lateral)

Contain sense organs functioning in balance

Vestibule Contains organs of balanceVestibular Nerve Carries nerve signals from the

balance organs to the brain

Vestibulocochlear (Auditory) Nerve

Conveys nerve signals from the vestibule and cochlea to the brain

Parts of the Ear & Their Function

Canals Each semicircular canal has a bulge near one end called the ampulla. This houses a low mound of hair cells, their hair ends set into a taller jellylike mound, the cupula. As the head moves, fluid in the canal lags behind, swirls past the cupula and bends it. This pulls the hairs and triggers their cells to fire nerve signals

Eustachian Tube

Runs to an opening in the side of the upper throat, level with the soft palate

How we HearWhen we hear noises, our body is converting energy from sound waves into nerve impulses which are interpreted by the brain.Sound waves are produced when air is mechanically disturbed.Sound is measured in two different ways:1. Frequency:

also known as the pitch (high or low) Frequency is measured by the number of complete sound waves per second High pitch sounds can be damaging to hearing.

2. Intensity:Loudness

How we HearWhat happens when we hear a noise…1. Sound waves enter the ear canal, and

make the ear drum vibrate2. Vibrations travel through the 3 connected

bones located in the middle ear3. This motion forces fluid movement in the

inner ear4. The movement of the fluid bends

thousands of tiny, delicate hair-like cells. This process converts vibrations into nerve impulses

How we Hear5. Nerve impulses are transported to the brain via the auditory nerve6. Finally, the impulses are converted into what we hear as soundhttp://www.sc.edu/ehs/modules/Noise/hearing.htm

the ear is a delicate structuremakes it easy for the ear to get damagedwhen the eardrum is damaged or the tiny bones of the middle ear lose their ability to vibrate, the ear loses its ability to conduct vibrations

Hearing Loss

Hearing Lossif damage occurs to the cochlea’s hair cell receptors or their nerves, the damage can cause sensorineural hearing loss or nerve deafnessdiseases sometimes cause sensorineural hearing lossthe main reasons for sensorineural hearing loss are biological changes such as heredity, aging, and exposure to loud music or noise

Hearing Losswhen the tissues are destroyed they remain deadusing a hearing aid may develop enough sound to stimulate neighboring hair cellswhen digital hearing aids are used they improve hearing by:amplifying vibrations for frequenciescompressing sound

Hearing Lossa cochlear implant is a sort of bionic earit is the only way to repair hearing for those with nerve deafnessa cochlear implant translates sounds into electric signalsthese signals carry information about the sound to the brain