1. Which cranial nerves are associated with hearing?

2. The organ of hearing and equilibrium
The Ear
The organ of hearing and equilibrium

3. What is Sound?
Pressure disturbance originating from a vibrating object
Compressions and rarefactions of particles in a medium

4. What sound looks like

5. Pitch
Frequency
Number of waves that pass a given point in a given time
Hearing range is 20 Hz to 20,000 Hz
Higher frequency = higher pitch
Lower frequency – lower pitch

6. Which disturbance is a higher pitch?

7. Loudness Amplitude Height of the wave (crest)
Intensity of the sound = energy
Humans: 0.1 decibels to 120 dB (pain threshold = 130dB)

8. Which disturbance is louder?

9. Hearing Auditory area of temporal lobe cortex
Sound waves through the air must be perceived
What type of receptors are involved?

10. Intro video

11. Three Parts
External ear
Middle ear
Inner Ear

13. External ear: Auricle/Pinna

15. External ear: auditory canal
Lined with skin, bearing hairs, sebaceous glands, and modified sweat glands
Secretes sticky cerumen (earwax): purpose?
1 inch long

16. Impacted eardrum

17. Boundary between external and middle ear: tympanic membrane
Connective tissue, thin
Rich supply of nerves and blood vessels
Cone shaped, apex points into ear
Vibrates in response to sound waves entering the ear

18. Middle ear Small, air-filled chamber
Eardrum on one side, bone on other

19. Otitis media

20. Otitis media: middle ear inflammation

21. Or this…

22. Middle ear: eustachian tube
Linked to nasopharynx
Closed by a membrane most of the time
Opens briefly when yawn or swallow
Equalizes middle ear pressure and external air pressure

23. Middle ear: ossicles Smallest bones in body
Suspended by tiny ligaments, linked together by joints
Malleus secured to ear drum
Stapes – base fits into oval window (entrance to cochlea)

24. Middle ear: ossicles
Transmit vibratory motion of eardrum to oval window
Sets fluids of inner ear in motion
Eventually excites the hearing receptors

25. Problems… Chronic ear infections – ossicles can fuse
“glue ear”: stapes becomes fixed to the oval window
Protection:
Muscles help protect the eardrum and reduce sound transduction when accosted by loud noises

26. Inner ear Complicated structure Behind eye socket Secure site
Delicate receptor machinery

27. Inner ear: two divisions
Bony labyrinth: the cavity – filled with perilymph (fluid)
Membranous labyrinth – inside the cavity, floating in perilymph, filled with endolymph

28. Inner ear: vestibule Equilibrium receptor region
Otoliths – increase sensitivity
Respond to pull of gravity
Report changes in head position

29. Inner ear: semicircular canals
Oriented in 3 planes
Anterior
Posterior
Lateral
Equilibrium receptors
Conducts to vestibular nerve of 8th cranial nerve

30. Inner ear: cochlea
Converts the physical vibrations of sound waves and converts them into electrical impulses

31. Inside the cochlea

32. Inner ear: cochlea Lined with receptors for hearing
Stimulated by bending of hairs: mechanoreceptors
Organ of corti: receptor organ for hearing

33. Vestibulocochlear nerve (cranial nerve 8)
Cochlear branch
Conducts to primary auditory cortex of temporal lobe
Can damage this nerve when exposed to prolonged periods of loud noise
Drugs = ototoxic agents

34. Hearing test