Anatomy and Physiology of Hearing

Introduction

The ears are paired sensory organs comprising the auditory system, involved in the detection of sound, and the vestibular system, involved with maintaining body balance/ equilibrium. The ear divides anatomically and functionally into three regions: the external ear, the middle ear, and the inner ear. All three regions are involved in hearing. Only the inner ear functions in the vestibular system


OUTER EAR AURICLE-/ framework of cartilaginous fibers except lobule  -capture sound & funnel it 
E.A.M. 2.5cm. Histology : - cart.2/3 X bone 1/3 - hair /skin - seb.&cerumucinous gl. channel , tubal resonator amplifying sound pr.

Tympanic membrane

Three layers : - ecto . - fibrous - mucosal
* cone-shaped /tension by T.T.M for better reception of vibrations of high frequency

Middle ear

Walls 15x13x2mm. Ossicles : -malleus -incus -stapes Muscles : - tensor tympani m. - stapedius m.

Middle Ear Muscles

Tensor tympani Attached to malleus Innervated by V, trigeminal nerve Stapedius Attached to stapes Innervated by VII, facial nerve Middle Ear Muscle Function: Help maintain ossicles in proper position Protect inner ear from excessive sound levels When ear exposed to sound levels above 70 dB, the muscles contract, decreasing amount of energy transferred to inner ear This protective reflex termed "acoustic reflex"


1-CONDUCTIONCONDUCT SOUND FROM THE OUTER EAR TO THE INNER EAR2 ) PROTECTIONCREATES A BARRIER THAT PROTECTS THE MIDDLE AND INNER AREAS FROM FOREIGN OBJECTSMIDDLE EAR MUSCLES MAY PROVIDE PROTECTION FROM LOUD SOUNDS3) TRANSDUCERCONVERTS ACOUSTIC ENERGY TO MECHANICAL ENERGYCONVERTS MECHANICAL ENERGY TO HYDRAULIC ENERGY4) AMPLIFIERTRANSFORMER ACTION OF THE MIDDLE EAR (TRAVELING WAVE )
Function of Middle Ear

Eustachian Tube

The eustachian tube connects the front wall of the middle ear with the nasopharynx The eustachian tube also operates like a valve, which opens during swallowing and yawning This equalizes the pressure on either side of the eardrum, which is necessary for optimal hearing.Without this function, a difference between the static pressure in the middle ear and the outside pressure may develop, causing the eardrum to displace inward or outward and this reduces the efficiency of the middle ear and less acoustic energy will be transmitted to the inner ear.

Inner ear

Bony labyrinth ___ perilymph Membranous labyrinth ___endolymph Cochlea Vestibule(utricle ,saccule ,s.c.c.)

Function of Inner Ear

Convert mechanical sound waves to neural impulses that can be recognized by the brain for: Hearing Balance


CochleaThe cochlea is a spiral structure, like a snail shell containing two and one half turns from its base at the oval window to its apex taken along the central pillar or modiolus,
*Small opening Scala v.&scala t. communicate through, at apex called helicotrema

Organ of corti

Located on the basilar membrane Contains the hair cells covered by tectorial membrane -Outer h. c. -Inner h. c.

OHC vs. IHC Function

Sound conduction pathways
1. Through Ossicular chain to oval window 11. Directly cross middle ear to round window (large perforation) III. Bone conduction (vibration of skull bones)

traveling waves

The impedance of the fluid in the cochlea is about 30 times greater than that of air, and if the sound were applied directly to the oval window, most of it (~97%) would be reflected, leaving only 3% transmission.

Transformer/Amplifier

. The middle ear enhances the transfer of acoustical energy in two ways: The area of the eardrum is about 17 times larger than the oval window The effective pressure (force per unit area) is increased by this amount. The ossicles produce a lever action that further amplifies the pressure Without the transformer action of middle ear energy in air transmitted to (about 30 dB loss). Malleus and incus vibrate together, transmitting the sound waves from the eardrum to the footplate of the stapes (this pushes the oval window in and out)(mechanical energy)

Lever system

Fig. 8-5. Schematic drawing of ossicle system to illustrate the lever arms and the position of the fulcrum. Relative areas of the tympanic membrane and the membrane of the oval window are shown.

When the stapes moves inward at the oval window pressure waves are transmitted to the perilymph of the scala vestibuli and thence through Reissner's membrane and the basilar membrane to the scala tympani. In the scala tympani, the vibrations pass again through perilymph to the round window at the base of the cochlea. The membranous covering of the round window bulges into the middle ear and forms the ultimate "give".The "give" at the round window is necessary to prevent pressure-wave reflections within the cochlea.


Movement of the cilia to kinocillium side results in a depolarization of the hair cell`s receptor that in turn releases a transmitter substance that finally depolarizes the afferent fibers that contact it.resulting in generation of action potential & transmission of the impulses

Central auditory pathways

Sound localization
Differences in the phase of the signals at each ear help in localization the source of low-frequency sound. Differences in intensity are used to localize the source of high-frequency sound. The difference in time of arrival of a sound at the two ears Central system function .

Question 1

What is the purpose of the pinna? A. Cosmetics B. Sound collector C. Same side localization D. A and B E. A, B and C

Question :2 The pars tensa portion of the TM:

Consists of 2 layers of tissue Consists of 4 layers of tissue Consists of 1 layer of tissue Consists of 3 layers of tissue Consists of 5 layers of tissue

Question :3

The Eustachian tube: Opens when one yawns Opens when one smiles Opens when one blinks It is always open Never opens

Question :4

The middle ear: Converts acoustic energy to hydraulic Converts hydraulic energy to mechanical Converts acoustic energy to electrical Converts mechanical to electrical

Question : 5

The middle ear amplifies sound: About 15 dB About 25 dB About 35 dB About 20 dB About 30 dB



Question 6:
The function of the inner ear: Balance Hearing Touch All the above A and B

QUESTION : 7

The channel that houses the organ of Corti: Scala tympani Scala media Scala vestibuli Semicircular canals B and D

Vestibular anatomybony&membranous labyrinths

Semicircular canals

Sensory epithelial cells

1. The sensory epithelia of the utricle, and saccule are macula. 2. The sensory cell of s.c.c. is crista ampullare

Macula &crista

Vestibular physiology
Utricular macula signals position of head& linear acceleration Saccule / exact function is not known S.c.c. angular rotation

Sense of balance – Semicircular Canals

Vestibular sensory cells


Sense of balance – Generation of receptor potential 1) K+ influx through the cilia for depolarization 2) Ca++ influx through voltage-gated channels 3) K+ outflux for repolarization


Movement of the cilia to kinocillium side results in a de polarization of the hair cell, a receptor potential that in turn releases a transmitter substance that finally depolarizes (the generator potential) the afferent fibers that contact it.

*The neural pathways of the reflex controlled by the horizontal semicircular canal

If the head is moved to the left, the eyes are moved* conjugately to the right, in order to maintain the gaze fixed on an object as the head moves
The vestibulo-ocular reflex has an important clinical use. As a result of the reflex, when the head is moved from side-to-side, the eyes remain fixed on an object by moving from side-to-side but in the opposite direction.
Vestibulo-occular reflux