Special Senses: The Ear

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 ossiclesthat 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!