Ears functions for two purposes-

1. hearing and second is the maintaining the balance of head. For hearing, the organ of corti of cochlea has receptor hair cells.  second for equilibrium, the vestibule and semicircular canals have receptors.

Physiology of hearing

the following events takes place for hearing process:-

1. The pinna allows the sound waves into the auditory canal
2. When the sound waves reaches the tympanic membrane,  the alternating high and low pressure of air causes the membrane to vibrate.
3.  The eardrum is connected to the malleus, which in turn also starts to vibrate. these vibrations are transmitted from malleus to incus and then to the stapes.
4.  as the stapes vibrates back and forth, it also pushes the membrane of oval window in and out. The oval window vibrates 20 times more faster than the eardrum.
5.  the movement of oval window sets the fluid pressure waves inside the perilymph of cochlea. as the oval window moves inwards, it pushes the perilymph of scala vestibuli
6.  the pressure waves are then transmitted from the scala vestibuli to scala tympani and then to the round window.
7. as the pressure waves strikes the wall of scala vestibuli, it also pushes the vestibular membrane back and forth, which also strikes the endolymph of cochlear duct.
8.  the pressure waves of endolymph strikes the basilar membrane to vibrate which moves the hair cells of organ of corti. this produces the receptor potential which generates nerve impulses.

the nerve impulses are carried to the brain through the cochlear branch of vestibulocochlear nerve.

(Above figure is taken from G.Tortora 12 edition. And use only for educational purpose )

Physiology of Equilibrium

There are two types of equilibrium, the static equilibrium and dynamic equilibrium. The static equilibrium works for maintaining the position of body mainly the head in linear acceleration. While the dynamic equilibrium works for maintaining body position in rotational acceleration.

Static Equilibrium

The vestibule contains the receptors for static equilibrium. the walls of utricle and saccule consist of a small thick region known as macula. the two macula which are perpendicular to each other are the receptors for static equilibrium.

The macula consist of two types of cells- hair cells which are sensory receptors and the supporting cells. The hair cells have 40-80 microvilli and 1 kinocilium. Together the microvilli and kinocilium are known as hair bundle. Resting on the hair cells are the otolithic membrane. A layer of calcium carbonate crystals called as otoliths are resting on the otolithic membrane.

If we tilt our head forward, the otolithic membrane is pulled by the gravity and it also bends in the direction of tilt; which bends the hair bundles. Bending of hair bundles produces nerve impulses; which are carried  through the vestibular branch of vestibulocochlear nerve to the brain.

Dynamic Equilibrium

The semicircular canals contains the receptors for dynamic equilibrium. The positions of semicircular ducts function for rotational acceleration  and deacceleration. The ampulla present on one end of each duct contains a small elevation known as crista. the crista contains the hair cells and the supporting cells. surrounding the crista is a gelatinous  material known as cupula. when we move our head, the attached semicircular ducts and hair cells moves with it. Bending of hair cells produces the receptor potential which further generates the nerve impulses. the nerve impulses reaches the brain through the vestibular branch of vestibulocochlear nerve.

Multiple choice questions(MCQs)

1. What contains the receptors for hearing?
A. vestibule B. ampulla
C. organ of corti D. macula

2. what causes the tympanic membrane to vibrate back and forth?
A. regular high and low pressure of air
B. regular high and low pressure of currents
C. alternating high and low pressure of currents
D. alternating high and low pressure of air

3. Which of the following are involved in physiology of equilibrium?
A. crista                                    B. macula
C. saccula                                 D. all of the above

4. Match the following-
A. utricle                                        1. Surrounds the crista
B. macula                                      2. Small elevation on the ampulla
C. crista                                         3. Thick region on saccule and utricle
D. cupula                                      4. In the endolymph of vestibule

5. Why the oval window vibrates 20 times more faster than the eardrum?
A. large surface area of oval window
B. small surface area of oval window
C. large vibrations in oval window
D. both B and C

6. the otoliths are the crystals of —— carbonate?
A. calcium B. potassium
C. sodium D. magnesium

7. Which of the following statement is NOT true?
A. otolithic membrane sits on the hair bundles
B. ampulla has a large depression known as crista
C. two macula are perpendicular to each other
D. bending of hair bundles generates receptor potential

8. Where is the cupula present?
a. encloses the saccule B. encloses the crista
c. encloses the utricle D. encloses the macula

9. what makes the vestibular membrane vibrate back and forth?
A. scala vestibule B. basilar membrane
C. oval window D. round window

10. Which step is not involves in the hearing process?
A. pinna directs sound waves towards the auditory tube
B. vibration in eardrum causes the auditory bones to vibrate
C. vibrations in basilar membrane moves the hair cells of organ of corti
D. none of the above

 ASNWERS:-

1. organ of corti
2. alternating high and low pressure of air
3. all of the above
4. A – 4 B – 3 C – 2 D – 1
5. both B and C
6. calcium
7. ampulla has a large depression known as crista
8. encloses the crista
9. scala vestibule
10. none of the above

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REFRENCE:-Gerard J. Tortora -Principles of anatomy and physiology; edition twelfth ; page no.-: 626-631.