physiology of hearing a chapter of special senses

1. The Special Senses:
Hearing

2. The Ear
a. External ear
The auricle directs sound waves into the external auditory meatus to the tympanic
membrane
b. Middle ear
Air filled chamber ,Contains the auditory ossicles:-
1. Malleus
2. Incus
3. Stapes
Connected to throat by the eustachian tube
c. Inner ear
The inner ear functions in both hearing and equilibrium
Receptors for hearing and balance:
 Respond to separate stimuli
 Are activated independently

4. External Ear
 The auricle (pinna) is composed of:
 The helix (rim)
 The lobule (earlobe)
 External auditory canal
 Short, curved tube filled with ceruminous glands
 Tympanic membrane (ear drum)
 Thin connective tissue membrane that vibrates in
response to sound
 Transfers sound energy to the middle ear ossicles
 Boundary between outer and middle ears

5. Middle Ear (Tympanic Cavity)
• A small, air-filled, mucosa-lined cavity
– Flanked laterally by the eardrum
– Flanked medially by the oval and round windows
• Epitympanic recess – superior portion of the middle ear
• Pharyngotympanic tube – connects the middle ear to
the nasopharynx
– Equalizes pressure in the middle ear cavity with the external
air pressure

6. Ear Ossicles
• The tympanic cavity contains three small
bones: the malleus, incus, and stapes
– Transmit vibratory motion of the eardrum to the
oval window
– Dampened by the tensor tympani and stapedius
muscles

8. Inner Ear
• Bony labyrinth
– Tortuous channels worming their way through the
temporal bone
– Contains the vestibule, the cochlea, and the
semicircular canals
– Filled with perilymph
• Membranous labyrinth
– Series of membranous sacs within the bony labyrinth
– Filled with a potassium-rich fluid

10. The Cochlea
 The scala tympani terminates at the round window
 The scalas tympani and vestibuli:
 Are filled with perilymph
 Are continuous with each other via the helicotrema
 The scala media is filled with endolymph
 Cochlea contains the organ of Corti, the organ of hearing.
 The “floor” of the cochlear duct is composed of:
 The bony spiral lamina
 The basilar membrane, which supports the organ of Corti
 The cochlear branch of nerve VIII runs from the organ of Corti
to the brain

12. The Organ of Corti
 Detects sound waves
 Consists of hair cells on a
basement membrane
 Tips of hairs touch the
tectorial membrane
 When the basement
membrane vibrates, the
hair cells are bent, sending
a nerve impulse

14. Physiology of Hearing
The following events are involved in hearing
1. The auricle directs sound waves into the external auditory canal.
2. When sound waves strike the tympanic membrane, the alternating
waves of high and low pressure in the air cause the tympanic
membrane to vibrate back and forth.
3. The central area of the tympanic membrane connects to the malleus,
which vibrates along with the tympanic membrane. This vibration is
transmitted from the malleus to the incus and then to the stapes.
4. As the stapes moves back and forth, its oval-shaped footplate, which
is attached via a ligament to the circumference of the oval window,
the vibrations of oval window are about 20 times more vigorous
than the tympanic membrane because the auditory ossicles.

16. 5. The movement of the stapes at the oval window sets up fluid pressure waves
in the perilymph of the cochlea. As the oval window bulges inward, it pushes
on the perilymph of the scala vestibuli.
6. Pressure waves are transmitted from the scala vestibuli to the scala tympani
and eventually to the round window, causing it to bulge outward into the
middle ear.
7. The pressure waves travel through the perilymph of the scala vestibuli, then
the vestibular membrane, and then move into the endolymph inside the
cochlear duct.
8. The pressure waves in the endolymph cause the basilar membrane to vibrate,
which moves the hair cells of the spiral organ against the tectorial membrane.
This leads to bending of the stereocilia and ultimately to the generation of
nerve impulses in first-order neurons in cochlear nerve fibers.
9. Sound waves of various frequencies cause certain regions of the basilar
membrane to vibrate more intensely than other regions.

17. Auditory Pathway

18. Properties of Sound
• Sound is:
– A pressure disturbance (alternating areas of high and low pressure)
originating from a vibrating object
– Composed of areas of rarefaction and compression
– Represented by a sine wave in wavelength, frequency, and amplitude.
• Frequency – the number of waves that pass a given point in a
given time
• Pitch – perception of different frequencies (we hear from 20–
20,000 Hz)

19. Transmission of Sound to the Inner
Ear
• The route of sound to the inner ear follows this
pathway:
– Outer ear – pinna, auditory canal, eardrum
– Middle ear – malleus, incus, and stapes to the oval window
– Inner ear – scalas vestibuli and tympani to the cochlear
duct
• Stimulation of the organ of Corti
• Generation of impulses in the cochlear nerve

20. Transmission of sound waves

21.  Deafness:- loss of hearing
 Tinnitus:- is a ringing or clicking sound in the ears in the absence of auditory stimuli.
 Meniere’s syndrome is a labyrinth disorder that seems to affect all three parts of the internal
ear. The afflicted person has repeated attacks of vertigo, nausea, and vomiting.
 Otoscope:- An instrument used to examine ear.
 Labyrinthitis Inflammation of the labyrinth.
 Otalgia:- Earache.
 Otitis externa Inflammation and infection of the external acoustic meatus, caused by bacteria
or fungi that enter the canal from outside, especially when the canal is moist (e.g., after
swimming).
 Otoacoustic emissions Sounds generated by the movement of outer hair cells in the cochlea.
Stimulated otoacoustic emissions are an inexpensive way to screen new borns for hearing
defects.

22. References
 Tortora, Gerard J, and Bryan Derrickson. Principles of
Anatomy and Physiology. 2nd ed., Milton, Qld, John Wiley &
Sons Australia, Ltd, 2019.
 Garg, Krishna, and Medha Joshi. Anatomy and Physiology
for Nurses. New Delhi, Cbs Publishers & Distributors Pvt.
Ltd, 2015.
 Kendall, Elward, et al. Anatomy and Physiology.
Lincolnwood, Ill., Ntc Learning Works, 1998.
 Scanlon, Valerie C, and Tina Sanders. Essentials of Anatomy
and Physiology. Philadelphia, Pa, F.A. Davis Co, 2011.

23. THANK YOU