Physiology of Hearing

Otoacoustic Emissions (OAEs)

They are low intensity sounds produced by outer hair cells of a normal cochlea and can be elicited by a very sensitive microphone placed in the external ear canal and an analysis by a computer. Sound produced by outer hair cells travels in a reverse direction:

Outer hair cells→basilar membrane→ perilymph→ oval window→ ossicles→ tympanic membrane→ ear canal.

160 dB

Increased Aldosterone is associated with metabolic alkalosis and not metabolic acidosis. Other findings associated with it includes hypokalemia (potassium excretion), hypernatremia (sodium conservation) and hypertension (characteristically diastolic).

Ref: Endocrine Physiology, 3rd Edition, Chapter 6; CURRENT Diagnosis and Treatment in Family Medicine, 3rd Edition, Chapter 36

Otoacoustic emissions are low intensity sounds produced by outer hair cells of a normal cochlea and can be elicited by a very sensitive microphone placed in the external ear canal and an analysis by a computer.

Neurons are ‘permanent cells’ that do not undergo mitotic division in post natal life. The production of neurons stop shortly after birth.

Ref: Neurons and Networks: An Introduction to Behavioral Neuroscience By John E. Dowling; Ganong’s Review of Medical Physiology, 22nd Edition, Page 259, 270; Robbins Pathologic Basis of Disease, 7th Edition, Page 91

Ans. is ‘d’ i.e., 85 

i. Conduction of sound:

• It is done mainly by middle ear. Middle ear not just simply conducts the sound but converts sound of great amplitude and less force to that of less amplitude and greater force.
• This function of the middle ear is called as impedance matching mechanism or the transformer action.

ii. Transduction of mechanical energy to electrical impulse:

• Movements of the stapes footplate causes vibrations in scala vestibuli followed by scala tympani and is transmitted to the cochlear fluids which brings about movement of the basilar membrane.
• This sets up shearing force between the tectorial membrane and the hair cells.
• The distortion of hair cells gives rise to electrical nerve impulse.

A sound wave, depending on its frequency, reaches maximum amplitude on a particular place on the basilar membrane, and stimulates that segment (traveling wave theory of von Bekesy).

Higher frequencies are represented in the basal turn of cochlea and the progressively lower one toward the apex.

The area of tympanic membrane is much larger than area of stapes footplate, the average ratio being 21:1.

As the effectice vibratory area of tympanic membrane is only two thirds, the effectives areal ratio is reducted to 14:1 which helps in impedance matching/ transformer action.

Hair cells: are important receptor cells of hearing and transduce sound energy into electrical energy.