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Renin Angiotensin Mechanism.

Special Sense Organs: Ear

 

Introduction:

  • The organ of hearing is the ear. 

  • It is supplied by VIIIth cranial nerve (Vestibulocochlear Nerve), which is stimulated by vibrations of sound waves.

  • The ear is divided into three main regions.

1) External (outer ear)    

2) Middle ear        

3) Internal ear

1) External (Outer ear):  

  • The external ear consists of the auricle (pinna), external auditory canal and eardrum (tympanic membrane).

  • The auricle is made up of elastic cartilage which is covered by skin. 

  • The external auditory canal is a curved tube about 2.5 cm long and extends from the auricle to the eardrum. 

  • The eardrum is a thin, semi transparent part present in between the external auditory canal and middle ear.


2) Middle ear:

  • The middle ear is the small cavity present in the temporal bone. 

  • The anterior wall of the middle ear contains an opening that directly connects with the nasopharynx called the auditory tube (Eustachian tube). 

  • Middle ear consists of three smallest bones present in the body called auditory ossicles. 

  • The three bones are the malleus, incus and stapes. 

  • The one end of the malleus is attached to the internal part of the eardrum. 

  • The other end of the malleus articulates with the body of incus. 

  • The incus articulates with the head of the stapes. 

  • The base of the stapes fits into the oval window. 

  • Below the oval window is another opening called the round window.

Internal structure of Ear


3) Internal (Inner) ear

  • Structurally the internal ear consist of two main divisions,

a) The bony labyrinth :

  • It is a series of cavities present in the temporal bone. 

  • It consists of a fluid called perilymph. 

  • It is divided into three areas.

i) The semicircular canals - 

  • They are anterior, posterior and lateral semicircular canals. 

  • Each canal consists of a swollen enlargement at one end called the ampulla. 

  • It contains receptors for equilibrium. 

ii) The vestibule - 

  • It is the oval central portion of the bony labyrinth. 

  • It also contains receptors for equilibrium.

iii) The cochlea - 

  • It continues with the vestibules. 

  • It is a bony spinal canal like a snail shell and makes almost three turns around a central bony core. 

  • The cochlea contains receptors for hearing.

b) Membranous labyrinth –

  • These are the series of tubes present inside the bony labyrinth. 

  • It is lined by epithelium and contains a fluid called endolymph.

Physiology of hearing:

  • The auricle directs the sound waves into the external auditory canal. 

  • These sound waves strike the eardrum and cause the eardrum to vibrate back and forth. 

  • The vibration is transmitted from the eardrum to the malleus to the incus and then to the stapes. 

  • The vibrations from the middle ear are transferred to the perilymph. 

  • From the perilymph the vibrations are transmitted to the endolymph, which leads to the generation of nerve impulses. 

  • The generated nerve impulses pass to the auditory portion of the cerebral cortex. 

  • These impulses of hearing are interpreted by the brain.


Commonly Asked questions:

  1. With a well labelled diagram describing the structure of the ear.

  2. Write a short note on “Physiology of hearing”.
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