9.6 Hearing and EquilibriumPages 445-448

The Ear

•Two separate functions: hearing and equilibrium

•Cilia: tiny hair cells that respond to mechanical stimuli

•Movement of cilia causes a nerve cell to generate an impulse

The Ear

Can be divided into 3 sections:

1.The outer ear

2.The middle ear

3.The inner ear

The Outer EarThe Pinna• External flap that collects and funnels the sound

waves from a large area to a canal

The Auditory Canal• Carries sound to the eardrum (tympanic

membrane) and makes earwax that traps, collects, and pushes out foreign invading particles

The Middle Ear

•Eardrum or tympanic membrane – receives sound waves and resonates, causing the ear bones to move

•3 small bones: malleus, incus and stapes

•The malleus receives the vibrations from the eardrum moves vibration to the incus moves to the stapes

The Middle Ear

•Ear bones are small and dense. They amplify the vibrations and transfer the mechanical energy efficiently to the oval window

•The oval window receives sound waves from the ear and transfers it to the fluid in the inner ear

Inner Ear•Made up of three distinct areas – the

vestibule, the semi-lunar canals, and the cochlea

•The vestibule and semi-lunar canals are involved with balance, while the cochlea is related to hearing

•The hairs in the cochlea respond to different frequencies▫Short hairs high frequency▫Long hairs low frequency

Eustachian Tube

•Extends from the middle ear to the mouth and the chambers of the nose

•It functions to equalize the air pressure on both sides of the eardrum

•This is the reason why throat infections often lead to ear infections, or vice versa

Hearing and Sound•Waves push against the eardrum and

vibrations are passed to the three bones of the middle ear

•The bones transfer, amplify, and concentrate the mechanical wave to an oval window

•The oval window pushes inward, the round window, located immediately below the oval window, moves outward, triggering waves of fluid within the inner ear

Hearing and Sound•The cochlea receives these movements, the

hairs are stimulated, and the mechanical pulse is converted into an electrical signal

•When the hairs move, the sensory nerves in the basilar membrane are stimulated

•Auditory signal is then sent to the temporal lobe of the cerebrum via auditory nerves, and the sound is banked

Safety Mechanism

•A Reflex Arc ▫Restrict the movement of the malleus and

reduce the intensity of movement▫Muscle contracts, pulling the stapes away

from the oval window protects the inner ear from powerful vibrations

•If the sound is too sudden – like a bomb or fire cracker – the protective reflex doesn’t kick in fast enough

Equilibrium (Static)•Involves movement along one plane

•Head position monitored by fluid-filled sacs saccule and utricle which contain cilia in a gelatinous material with otoliths

•Normal position: otoliths do not move

•Head tilts: otoliths cause gelatinous material to shift, the hair receptors to bend and the sensory nerve to relay message to brain.

Head Erect Cilia remain erect

Head Moves Cilia move

Equilibrium (Dynamic)•Provides information about movement

•Balance is maintained by 3 semicircular canals equipped with a pocked called the ampullae

•Rotational stimuli causes fluid to move, bending cilia attached to the hair cells in the ampullae

•Once hair cells bend, information is relayed to the brain