1. Physiology of hearing. Vestibular analyzer

2. Main Components of the Hearing Mechanism:
Divided into 4 parts (by function):
Outer Ear
Middle Ear
Inner Ear
Central Auditory Nervous System

3. Structures of the Outer Ear
Auricle (Pinna)
Gathers sound waves
Aids in localization
Amplifies sound approx. 5-6 dB

4. External Auditory Canal:
Approx. 1 inch long
“S” shaped
Outer 1/3 surrounded by cartilage; inner 2/3 by mastoid bone
Allows air to warm before reaching TM
Isolates TM from physical damage
Cerumen glands moisten/soften skin
Presence of some cerumen is normal

5. Tympanic Membrane Thin membrane
Forms boundary between outer and middle ear
Vibrates in response to sound waves
Changes acoustical energy into mechanical energy
(From Merck Manual)

6. Eustachian Tube (AKA: “The Equalizer”)
Mucous-lined, connects middle ear cavity to nasopharynx
“Equalizes” air pressure in middle ear
Normally closed, opens under certain conditions
May allow a pathway for infection
Children “grow out of” most middle ear problems as this tube lengthens and becomes more vertical

7. Stapedius Muscle Attaches to stapes
Contracts in response to loud sounds; (the Acoustic Reflex)
Changes stapes mode of vibration; makes it less efficient and reduce loudness perceived
Built-in earplugs!
Absent acoustic reflex could signal conductive loss or marked sensorineural loss

8. Structures of the Inner Ear: The Cochlea
Snail shaped cavity within mastoid bone
2 ½ turns, 3 fluid-filled chambers
Scala Media contains Organ of Corti Converts mechanical energy to electrical energy

9. Central Auditory System
VIIIth Cranial Nerve or “Auditory Nerve”
Bundle of nerve fibers (25-30K)
Travels from cochlea through internal auditory meatus to skull cavity and brain stem
Carry signals from cochlea to primary auditory cortex, with continuous processing along the way
Auditory Cortex
Wernicke’s Area within Temporal Lobe of the brain
Sounds interpreted based on experience/association

10. Summary: How Sound Travels Through The Ear
Acoustic energy, in the form of sound waves, is channeled into the ear canal by the pinna. Sound waves hit the tympanic membrane and cause it to vibrate, like a drum, changing it into mechanical energy. The malleus, which is attached to the tympanic membrane, starts the ossicles into motion. The stapes moves in and out of the oval window of the cochlea creating a fluid motion, or hydraulic energy. The fluid movement causes membranes in the Organ of Corti to shear against the hair cells. This creates an electrical signal which is sent up the Auditory Nerve to the brain. The brain interprets it as sound!

11. Organ Of Corti The end organ of hearing
Contains stereocilia & receptor hair cells
3 rows OHC, 1 row IHC
Tectorial and Basilar Membranes
Cochlear fluids
(From Augustana College, “Virtual Tour of the Ear”)

12. Vestibular Apparatus
Vestibular apparatus and cochlea form the inner ear
Vestibular apparatus – provides sense of equilibrium
consists of otolith organs (utricle and saccule) and semicircular canals

13. Vestibular Apparatus
Sensory structures located within membranous labyrinth
filled with endolymph and located within bony labyrinth

14. Vestibular Apparatus
Utricle and saccule provide info about linear acceleration
Semicircular canals, oriented in 3 planes, give sense of angular acceleration

15. Vestibular Apparatus Hair cells – receptors for equilibrium
Each contains stereocilia (hair-like extensions)
1 of these is a kinocilium---a true cilium

16. Vestibular Apparatus
Stereocilia bend toward kinocilium – hair cell depolarizes
releases NT that stimulates CN VIII
When bent away from kinocilium – hair cell hyperpolarizes
In this way, frequency of APs in hair cells carries information about movement

19. When stereocilia are bend away from kinocilium, hair cell is hyperpolarized, i.e. inhibited. It occurs because acceleratory force acts to flow of fluid in semicircular canals during circular motion of the head or whole the body.
Hair cells are located along crista ampularis and protect their cilia in cupula. Hair cells are secondary sensor cells, which synapse with neurons. Axons of these nerve cells compose vestibular nerve.

20. Utricle and Saccule Have a macula that contains hair cells
Hair cells embedded in gelatinous otolithic membrane
contains calcium carbonate crystals (otoliths) that resist change in movement

21. Utricle and Saccule Utricle sensitive to horizontal acceleration
Hairs pushed backward during forward acceleration
Saccule sensitive to vertical acceleration
Hairs pushed upward when person descends

22. Semicircular Canals Provide information about rotational acceleration
Project in 3 different planes
Each contains a semicircular duct
Crista ampullaris – where sensory hair cells are located
10-42

23. Semicircular canal function
Ampula is enlargement at one end of semicircular canal. It has a small crest on top of which is a gelatinous mass known as cupula. Hair cells have two kinds of cilia – kinocilium and stereocilia.
Kinocilium is large cilium located at one end of hair cell. Stereocilia are small. When stereocilia are bent towards kinocilium, hair cell is depolarized, i.e. stimulated.

24. Semicircular Canals
Hair cell processes embedded in cupula of crista ampullaris
When endolymph moves cupula moves
Sensory processes bend in opposite direction of angular acceleration

28. Neural Pathways for Equilibrium and Balance

29. Nystagmus and Vertigo
Vestibular nystagmus – involuntary oscillations of eyes
occur when spinning person stops
Eyes continue to move in direction opposite to spin, then jerk rapidly back to midline
Vertigo – loss of equilibrium
Natural response of vestibular apparatus
Pathologically, may be caused by anything that alters firing rate of CN VIII
Often caused by viral infection

30. Vestibular System Consists of three semi-circular canals
Monitors the position of the head in space
Controls balance
Shares fluid with the cochlea
Cochlea & Vestibular system comprise the inner ear

31. Vestibular System Consists of three semi-circular canals
Monitors the position of the head in space
Controls balance
Shares fluid with the cochlea
Cochlea & Vestibular system comprise the inner ear