Hearing

Anatomy

1.) Outer earPinna/Auricle:

- Visible ear- Contains many folds and ridges- Made up of Cartilage and fat

Ear (Auditory)Canal: Pathway to middle ear- Outer ⅔ = cartilaginous- Inner ⅓ = bony- Contains hair cells and cells that produce a wax (cerumen)

- lubricates ear and protects ear along with hairs

Anatomy2.) Middle Ear

Eardrum (tympanic membrane): - Thin, delicate membrane - Stretched across end of the ear canal

- separates environment from middle ear

- Vibrates as sound waves impact it

Anatomy 3 Auditory Bones (ossicular chain)

- Smallest bones- Suspended by small ligaments

1.) Malleus (hammer)- First in chain- Articulates with eardrum and incus

2.) Incus (anvil)- Articulates with Malleus and sends vibration to Stapes

Anatomy

3.) Stapes (stirrup)

- Articulates with Incus and a projection called the footplate of the stapes lays in the oval window

- Connects with small muscle (stapedius) that contract when very loud sounds are detected

Oval Window:

- Pathway to inner ear- Footplate of stapes vibrates and transfers vibrations to cochlea

Anatomy **Eustachian tube:

- Muscular tunnel that opens and closes - Provides fresh air, drains debris, and equalizes pressure- Opening located at the front wall of the middle ear cavity

- other end opens in the back part of the throat.

Anatomy 3.) Inner Ear:

○ Cochlea:- Snail-shaped, bony structure- Three fluid-filled compartments - Membrane in middle contains Basilar Membrane

- Contains small fibers (Cilia)

- hair cells are spread along the structure to sense the amplitude/ pitch

AnatomyOrgan of corti has small hairs that pick up fiber vibrations and connect to the auditory nerve

- release message to Auditory Nerve

Auditory Nerve:

- Nerve that carries the message from cochlea to the brain - Fast message, highly myelinated axons

Thalamus:

- Relay station for sensory impulses passing upward to cerebral cortex- Crude recognition of pleasant or not

Auditory Cortex:- Part of brain dealing with hearing- Located in temporal lobe

Anatomy

Thalamus:

- Relay station for sensory impulses passing upward to cerebral cortex- Crude recognition of pleasant or not

Auditory Cortex:- Part of brain dealing with hearing- Located in temporal lobe

Physiology: The Stimuli

● Sound Waves ○ Frequency: the length of waves○ Amplitude: the high of waves (loudness) ○ Pitch: strength of waves

● Long Waves have low frequency and pitch● Short Waves have high frequency and pitch

○ Ex. Violins produce faster and shorter waves than a cello

Physiology: The Stimuli

● Sound measured in decibels ● Threshold:

○ Absolute Threshold- point at which your hearing sense detects a stimulus half of the time■ 0 decibels

○ Sound Intensity increases every 10 decibels.

Physiology: Converting Sound Waves

1. Outer Ear channels the sound waves into the auditory canal of the Eardrum (Tympanic Membrane)

2. Middle Ear transmits eardrum vibrations through a piston made of 3 bones (Malleus, Incus, & Stapes) a. Vibration causes the eardrum to move back &

forth which changes the sound wave to mechanical vibration

Physiology: Converting Sound Waves

3. Bones of the Middle Ear amplify & relay eardrums vibration through the Oval Window into the Cochlea

a. Incoming vibrations cause the Oval Window to vibrate the fluid that fills the Inner Ear

4. Fluid stimulates hair cells (receptor cells) covering the Basilar Membrane

b. Bundles of cilla trigger the release of protein which trigger neural response

Physiology: Converting Sound Waves

5. Movement of hair cells triggers impulses in adjacent nerve cells

a. “Hairs” on hair cells (Stereocilia & Kinocilium) are stiffened by actin filaments & linked together by tip-links

b. Bending the Stereocilia towards Kinocilium puts tension on the tip-links, which opens cation channels

c. Sound stimuli occurs after trapped stereocilia of hair cells are deflected by localized movement of Basilar Membrane

Physiology: Converting Sound Waves

a. Results in an inward K+ and Ca2+ and graded depolarization i. Depolarization: increases intracellular Ca2+ & hair cells release of

neurotransmitter (glutamate) 1. Allows afferent cochlear fibers to transmit a faster stream of

impulses to the brain for auditory interpretation

b. Bending Stereocilia away from Kinocilium relaxes the tip-links & closes the mechanically gated ion channels which then allows for repolarization to occur (sometimes hyperpolarization occurs)

Physiology: Converting Sound Waves

6. Axons of Nerve Cells converge to form the Cochlear Nerve

7. Cochlear Nerve then sends the neural message created to the Thalamus

8. Thalamus converts the neural message into electrical impulses which are sent to the Auditory Cortex and Temporal Lobe

9. Processing of the electrical impulses allow us to hear the sound waves

Physiology: Locating Sounds

● Two ears allow stereophonic hearing ○ Stereophonic- “Three dimensional”

● Intensity difference of sound processed by brain to determine rough location of sound○ More difficult to determine when stimuli is equidistant from both

ears

Physiology: Overview Video

Homeostatic Imbalances: Deafness

● Any sort of hearing loss is considered deafness of some sort

● Two types, Conduction deafness, and sensorineural deafness

● Conduction deafness- Occurs when something hampers sound conduction to the fluids of the internal ear. ○ Causes include: perforated eardrums, inflammation, and otosclerosis

● Sensorineural deafness- Results from nerve damage

Homeostatic Imbalances: Tinnitus

● Ringing or clicking sounds in ears in absence of auditory stimuli

● Caused by cochlear nerve degeneration, inflammation of the middle or interior ears, and a side effect of aspirin

Sources

http://www.betterhearing.org/hearingpedia/how-we-hear

https://faculty.washington.edu/chudler/chhearing.html

https://www.youtube.com/watch?v=Ie2j7GpC4JU

https://books.google.com/books?id=avztel9pWGUC&printsec=frontcover&source=gbs_ge_summary_r&cad=0#v=onepage&q&f=false