1. The Special Senses Hearing
Chapter 16
The Special Senses
Hearing

2. Outer ear Anatomy of the ear – External Ear
Auricle or pinnae surrounds the ear
External acoustic meatus (ear canal or auditory canal)
ends on Tympanic membrane (ear drum)
Ceruminous glands - produce cerumen (ear wax)

3. Middle ear Starts with the tympanic membrane
Communicates with pharynx via pharyngotympanic tube (auditory tube, Eustachian tube)
Middle ear encloses and protects the auditory ossicles
malleus, incus, stapes
oval window
tensor tympani muscle - stiffens the malleus and tympanic membrane
stapedius muscle - reduces movement of the stapes on the oval window

4. Anatomy of the Ear

5. Middle and Inner Ear
Balance
Hearing
Figure 15.25b

6. The Middle Ear - Frontal View
Lateral Medial
Figure 17.21

7. The Middle Ear - Medial View
Figure 15.26

8. Inner ear
Membranous labyrinth contains endolymph (similar to intracellular fluid)
Bony labyrinth surrounds and protects membranous labyrinth
Perilymph lies between the two labyrinths (similar to cerebrospinal fluid)
Vestibular complex
Vestibule - contains the utricle and saccule , sense of gravity, linear acceleration
Semicircular canals - contains semicircular ducts, rotation, equilibrium
Cochlea contains the cochlear duct, sense of hearing

9. The Inner Ear
green = perilymph blue = endolymph

10. Chambers of the Cochlea
The cochlea is divided into three chambers:
Scala vestibuli
Scala media (chochlear duct)
Scala tympani

11. Perilymph and Endolymph
Scala vestibuli
Scala media
Scala tympani
Figure 17.28a

12. Vibrations of the inner ear
Scala vestibuli
(perilymph)
Scala media
(endolymph)
Basilar membrane
Tectoral membrane
Scala tympani
(perilymph)
Marieb Figure 15.34

13. Sound
Cochlear duct lies between the vestibular duct and the tympanic duct
Hair cells of the cochlear duct lie within the Organ of Corti
Frequency (cycles per second (Hz)) is determined by which region is stimulated
Intensity is the energy content of a sound, how many hair cells are activated and their degree of movement
Measured in decibels

14. 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
Contain stereocilia
Provide information about the direction and strength of stimuli

15. Organ of corti Hair cells sit on basilar membrane
Steriocilia are in contact with the tectorial membrane
thin membrane attached to the cochlear duct

16. The Organ Of Corti (scala media)

17. 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 (in the vestibular duct), and then to the endolymph of the cochlear duct.
Pressure waves distort basilar membrane
Hair cells of the Organ of Corti are pushed against the tectoral membrane

18. Sound and Hearing
Figure 17.28a

19. Signal amplification

20. Receptor cell depolarization
High K+ solution
High K+ solution K+

21. Pitch
Figure 15.32

22. Neural pathway
Sensory neurons of hearing are located in the spiral ganglion of the cochlea (first order neuron)
Afferent fibers form the cochlear branch of cranial nerve VIII
Synapse at the cochlear nucleus (second order neuron)
Goes to the inferior colliculus of the mesencephalon (third order neuron) Collaterals go to the olivary nucleus (that fine tunes the ear and adjusts for volume
then to the thalamus (fourth order neuron)
temporal lobe of the cerebral cortex

23. Neural Pathway (also see fig 16.18)4
Note that signals from each ear go to both sides of the brain! 3 2 1
Figure 15.34