Study the given figure of human internal ear and answer the questions that follows: [2+2] [figure in the original paper] i) Label part A, B, C, D. ii) Write function of A and D.

Internal Ear: Structure and Function

The human internal ear (labyrinth) is housed within the petrous part of the temporal bone. It consists of the bony labyrinth and the membranous labyrinth, which contains the sensory receptors for equilibrium and hearing.

i) Labeling of Parts A, B, C, and D

Based on the standard anatomical structure of the internal ear, the parts are identified as follows:

ii) Functions of Part A and Part D

Function of Part A (Cochlea): The cochlea is the auditory portion of the internal ear. Its primary function is hearing. Inside the cochlea lies the Organ of Corti, which contains hair cells (sensory receptors). When sound vibrations are transmitted from the middle ear to the fluid (perilymph and endolymph) within the cochlea, these vibrations cause the basilar membrane to vibrate. This movement stimulates the hair cells, which convert mechanical energy into electrical nerve impulses. These impulses are transmitted via the cochlear nerve (part of the vestibulocochlear nerve, CN VIII) to the auditory cortex of the brain, where they are interpreted as sound. The cochlea also provides tonotopic organization, where different frequencies of sound stimulate different regions of the basilar membrane.

Function of Part D (Oval Window): The oval window is a membrane-covered opening that separates the middle ear from the inner ear. Its primary function is to transmit mechanical vibrations from the middle ear to the fluid-filled inner ear. The footplate of the stapes (the smallest bone in the human body) rests against the oval window membrane. When the stapes moves in and out due to sound waves, it pushes the oval window membrane inward and outward. This action creates pressure waves in the perilymph fluid of the cochlea, initiating the process of hearing. Without the oval window, sound vibrations could not effectively pass from the air-filled middle ear to the fluid-filled inner ear due to the impedance mismatch between air and fluid.

Summary of Key Points

1. Cochlea (A): Responsible for hearing; contains the Organ of Corti; converts mechanical vibrations to neural signals.
2. Oval Window (D): Acts as a transmission interface; allows stapes vibrations to enter the fluid-filled cochlea.
3. Semicircular Canals (B) & Vestibule (C): Responsible for balance and equilibrium (not asked for function, but labeled for completeness).

This structure ensures that the human ear can detect a wide range of sound frequencies and maintain body balance simultaneously.