1. Special Senses: The Ear

2. By the end of this class you should understand:
The properties of sound waves as they relate to hearing
The three regions of the ear and their functions
How the inner ear creates the senses of hearing and balance

3. Sensory Neurons
Recall there are five types of neurons, classified by the type of stimulus they respond to
Mechanoreceptor
Thermoreceptor
Nociceptor (Pain receptor)
Chemoreceptor
Photoreceptor

4. Mechanoreceptors
Mechanoreceptors depend on mechanically gated ion channels
When the cell is deformed, they open and allow sodium into the cell
There are many types of mechanoreceptors
The type of interest today is called the hair cell

5. Hair Cell Hair cells are unique in that they do not have an axon
The “hair” of the hair cell (also known as stereocilia) bend when there are vibrations
The bending allows positively charged ions into the cell
This depolarizes the cell membrane causing neurotransmitters to be released

6. Vibrations
The ear is responsible for detecting two different types of movements using hair cells
Sound: vibrations of the air
Vestibular sense: acceleration of the head
Both of these occur in the inner ear

7. Parts of the Ear

8. Vestibular Sense Sense of balance or equilibrium
Can sense both linear and angular acceleration
The vestibule detects linear acceleration
The semicircular canals detect angular acceleration

9. Acceleration Types Linear acceleration is in a straight line
Detectable in a car that is braking or gunning the engine
Also tells you which way is up
Angular acceleration is spinning or rolling
Detectable doing somersaults or spinning in a circle
In both cases the hair cells are stimulated by the linear or angular movements of heavy crystals called otoliths

10. Acceleration and Nausea
The brain must integrate signals from the different senses
A combination of forebrain and midbrain work
When these signals do not match the brain struggles to integrate them
Nausea (“carsickness” and “seasickness”) may result from feeling acceleration in the vestibular sense but not visually detecting any acceleration

11. Angular Acceleration
Spinning rapidly for an extended period of time can cause the semicircular canals to build up a lot of angular momentum
When you stop spinning you experience dizziness because the semicircular canals are still spinning
Causes the world to “spin” because your eyes are trying to follow the movement of the canals

12. Nature of Sound
The hair cells for the vestibular sense respond to the movement of heavy crystals
The hair cells for the sense of sound respond instead to the vibrations caused by sound
To understand this, it is important to understand the nature of sound

13. Pressure Waves
Sound is caused by pressure waves moving through a medium (air, water, etc)
A single pressure wave will only sound like a crack or boom, not a tone
Back-and-forth differences in air pressure at a certain frequency produce a constant tone
The faster the waves of air pressure arrive at the ear, the higher the perceived pitch

14. Pitch and Volume
The bigger the waves (the more energy is carried), the higher the volume
Higher amplitude, measured in decibels
Its all relative! The closer, the louder the sound
The faster the waves arrive, the higher the pitch of the sound
Higher frequency, measured in hertz
Same unit used for light frequency but light waves have millions of times higher frequencies

15. Resonance
A complication is that any object capable of producing or containing sound waves has a resonance frequency
The best example of resonance is making waves in a bathtub slowly bigger and bigger by moving your own body in time with the waves
Sound waves can have this happen as well!
Note the different lengths on harp/piano strings

16. Resonance in the Cochlea
There is a spiral-shaped section of the temporal bone called the cochlea
The cochlea serves as a resonating chamber
Vibrations from the ear are transmitted to pressure waves in the cochlea
Much like piano strings, there is a high-frequency to low-frequency resonance of the hair cells in the cochlea

17. Parts of the Ear

18. Pressure Waves Pressure waves are transmitted through the outer ear
Pinna (the part of the ear you can touch)
Auditory canal (the part of the ear you can put a Q-tip in)
The Tympanic membrane/eardrum forms the border between the outer and middle ear

19. Middle Ear
The middle ear is a cavity with three tiny bones called ossicles that attach to the tympanic membrane at one end and the inner ear at the other
The ossicles are unique to mammals
When air vibrations cause the eardrum to vibrate, the three bones transmit these vibrations to the inner ear

20. Middle Ear Ossicles The three ossicles are malleus, incus and stapes
Hammer, anvil, and stirrup, named for their shapes
Malleus is attached to the eardrum, stapes pounds on the oval window
The auditory tube connects this space to the nasal passage
If blocked due to illness, can result in headaches/earaches and reduced hearing

21. Oval Window
Just as the tympanic membrane forms the border between the outer and middle ear, the oval window forms the border between the middle and inner ear
The oval window is where pressure waves enter the cochlea
The faster the vibrations transmitted onto the eardrum, the faster the vibrations transmitted into the oval window

22. The Cochlea
The cochlea is filled with a liquid called perilymph that transmits vibrations from the oval window
The hair cells plus the membrane they attach to and the sensory neurons form the Spiral Organ, or Organ of Corti
The axons from these sensory neurons bundle together to become the cochlear nerve

23. The Nerves!
The cochlear nerve leaves the cochlea heading for the brain
The vestibular organs produce another nerve called the vestibular nerve
These two nerves actually join to become the vestibulocochlear nerve which is processed by the thalamus and midbrain before transmitting to the temporal lobe

24. That’s our show! Next Monday is review day! Bring your questions!
One week from today is Exam #2!