1. Mechanoreception – Audition and Equilibrium

2. Hearing and Equilibrium
Figure 10.37

3. The Middle Ear

4. Inner ear Membranous labyrinth contains endolymph
Bony labyrinth surrounds and protects membranous labyrinth
Vestibule
Semicircular canals
Cochlea

5. The Inner Ear

6. Components of the inner ear
Vestibule contains the utricle and saccule
Semicircular canals contain the semicircular ducts
Cochlea contains the cochlear duct

7. Windows
Round window separates the perilymph from the air spaces of the middle ear
Oval window connected to the base of the stapes
Basic receptors of inner ear are hair cells
Provide information about the direction and strength of stimuli

8. Hearing Sounds are waves of compressed air traveling through space
- sound intensity wave height
- pitch  wave frequency

9. Organ of hearing (and equilibrium) – inner ear
Cochlea
Vestibular apparatus

10. Hearing
1- The sound waves enter the external auditory canal and trigger vibrations of the tympanic membrane
2- The tympanic membrane induces a vibration of the ossicles
3- the last ossicle, the stapes, transmits amplified vibrations to the oval window
4- The vibrations induce waves in the perilymph of the various inner ear chambers
5- the round window absorbs excess energy and prevent wave reverberation
6- the fluid wave is transduced into an electrical signal by the auditory receptors, the organs of Corti located on the basilar membrane

11. Receptors for Sound: The Organ of Corti
The hair cells of the organ of Corti transduce fluid wave into an electrical signal
The energy of the wave causes the basilar and vestibular membrane to move, thus displacing the cilia from the organ of Corti

12. The Organ of Corti

13. Hearing
Cochlear duct lies between the vestibular duct and the tympanic duct
Hair cells of the cochlear duct lie within the Organ of Corti
Intensity is the energy content of a sound
Measured in decibels

14. The Cochlea

15. Pathway of Sound
Sound waves travel toward tympanic membrane, which vibrates
Auditory ossicles conduct the vibration into the inner ear
Tensor tympani and stapedius muscles contract to reduce the amount of movement when loud sounds arrive
Movement at the oval window applies pressure to the perilymph of the cochlear duct
Pressure waves distort basilar membrane
Hair cells of the Organ of Corti are pushed against the tectoral membrane

16. Sound and Hearing

17. Sound and Hearing

18. Neural pathway
Sensory neurons of hearing are located in the spiral ganglion of the cochlea
Afferent fibers form the cochlear branch of cranial nerve VIII
Synapse at the cochlear nucleus

19. Signal Transduction
Movements of the cilia open or close potassium channels  changes in the state of polarization of the hair cell
Changes in potassium leakage due to cilia bending trigger changes in neurotransmitters exocytosis
The neurotransmitters send an electrical signal to an afferent neuron of the cochlear nerve
The louder the sound, the more the cilia bend, the more numerous are the APs produced

20. Coding for Pitch
The location of the organs of Corti on the basilar membrane codes for pitch
- Organs of Corti located near the oval window are more sensitive to high pitch sounds while the ones located toward the tip of the cochlea respond more readily to low pitch sound

21. Coding for sound intensity

22. Neural Pathway for Sounds
Cochlear nerve  nucleus in medulla oblongata  thalamus  auditory cortex in the temporal lobe
So, how do we perceive the direction from which a sound is coming from?

23. Equilibrium
Anterior, posterior and lateral semicircular ducts are continuous with the utricle
Each duct contains an ampulla with a gelatinous cupula and associated sensory receptor
Saccule and utricle connected by a passageway continuous with the endolymphatic duct
Terminates in the endolymphatic sac
Saccule and utricle have hair cells clustered in maculae
Cilia contact the otolith (statoconia)

24. Equilibrium
Ability to detect head position and movement (or acceleration)
Change of speed = linear acceleration (utricle and saccule)
Turning = rotational acceleration (semi-circular canals)

25. Utricle and saccule
Sensory cells have cilia extending into a gelatinous material topped by otoliths
Saccule detects backward-frontward movement
Utricle detects changes relative to gravity

26. The Vestibular Complex

27. Semi-circular canals
The receptors in the ampulla are hair cells with cilia extruding into a gelatinous mass (cupula)
When the head rotates, the cupula moves  cilia pulled APs (vestibular nerve  cerebellum …)

28. The Vestibular Complex

31. The Vestibular Complex

33. Vestibular Neural Pathway
Vestibular receptors activate sensory neurons of the vestibular ganglia
Axons form the vestibular branch of cranial nerve VII
Synapses within the vestibular nuclei

34. Pathways for Equilibrium Sensation

35. So why does a person become dizzy after he/she stops spinning?