1. 9.6 Hearing and Equilibrium
The Ear:
Used for hearing and equilibrium
The inner ear contains sensory cells for both functions
Each hair cells has between 30 and 150 cilia… responding to stiumuli

2. The ear can be divided into three main sections: 1) The outer ear
→The pinna
→The auditory canal
2) The middle ear
→Eardrum (or tympanic membrane)
→Ossicles
→Oval window
→Eustachian tube
3) The inner ear
→Vestibule
→Semicircular canals
→Cochlea

4. The Outer Ear The pinna: outer ear flap that collects the sound
The auditory canal: carries the sound to the eardrum and is lined with specialized sweat glands that produce earwax (used to trap foreign particles, preventing them from entering the ear.)

5. The Middle Ear (Tympanic Membrane)
Air filled chamber that holds the three small bones: the ossicles.
The three ossicles are the malleus (hammer), the incus (anvil) and the stapes (stirrup)
Sound vibrations that strike the eardrum are first concentrated withing the malleus, transmitted to the incus and then the stapes.
The stapes strikes the membrane covering the oval window in the inner wall of the middle ear.
The oval window amplifies the sound
The Eustachian tube goes from the middle ear to the mouth and the chambers of the nose and helps to equalize pressure. This is also the site that builds up with fluids in an ear infection.

6. The Inner Ear
The vestibule is connected to the middle ear by the oval window and has two small sacs: the utricle and the saccule. They help establish head position.
There are three semicircular canals that are arranged at different angles that helps identify body movement with the fluid inside them.
The cochlea is shaped like a snail’s shell and contains two rows of specialized hair cells that respond to sound waves of different frequencies and intensities and changes them into nerve impulses.
Anatomy of the Ear - Learning Activity

7. Hearing and Sound
Sound is a form of energy that must be converted into an electrical impulse before you can interpret it.
Sound must travel through a medium to be heard
Air most commonly, but also water and solids

8. Hearing and Sound
Sound travels fastest in solids, then liquids, then gases.
Muscles joining the bones of the middle ear protect the inner ear from excessive noise
Loud noises cause these muscles to contract, constricting the malleus, while a second muscle contracts, pulling the stapes away from the oval window.

9. The oval window receives vibrations from the ossicles and is pushed inwards
The cochlea receives fluid waves and converts them into electrial impulses (sound)
BBC Science & Nature - Human Body and Mind - Nervous System Layer

10. Organ of Corti The hearing apparatus in the cochlea
A single inner row, and three outer rows of specialized hair cells
The hair cells are covered in a gelatinous coating
Hair bending sends a message through sensory nerves
Effect of Sound Waves on Cochlear Structures

11. Pitch and Loudness
Identified by the cochlea structure narrowing and hairs getting shorter as it coils
Receptor hair cells on the basilar membrane trigger an action potential that is carried to the brain

12. Bass
Low frequency waves move to the wider more elastic area of the cochlea to vibrate
Animation: Effect of Sound Waves on Cochlear Structures (Quiz 1)

13. Mechanical Stimulation
The basilar membrane will respond to jarring blows … particularly to the head
The vibrations resonate through the skull, and pass onto the cochlea

14. Equilibrium Static Equilibrium: Movement along one plane
Eg. head position
Controlled by the saccule and utricle. Cilia from hair cells are suspended in a gelatinous material containing CaCO3 granules called otoliths.
When the head is in a normal position, otoliths do not move but when the head moves, gravity acts on otoliths, causing gelatinous material to shift and hair receptors to bend. Movement of hair receptors stimulates nerve that relays info about head position to brain.

15. Dynamic Equilibrium:
Provides information during movement
While moving, balance is maintained by three semi-circular canals…each of which has a small pocket called an ampulla
Rotational stimuli cause the fluid in these canals to move