Explain the following:
$(a)$ Role of $Na ^{+}$ in the generation of action potential
$(b)$ Mechanism of generation of light-induced impulse in the retina.
$(c)$ Mechanism through which a sound produces a nerve impulse in the inner ear.

(N/A) The action potential is determined by $Na ^{+}$ ions. The $Na ^{+}$ channels,which are closed in the resting state,open and cause the inflow of $Na ^{+}$ ions by diffusion into the axoplasm. The electrical potential of the membrane changes from $-70 \, mV$ towards zero,and the membrane is said to be depolarized.
$(b)$ The photopigments of the retina are photosensitive compounds composed of retinal and opsin. Light induces the dissociation of retinal from opsin,which changes the structure of the opsin. This generates an action potential in the bipolar neurons. These impulses are transmitted by the optic nerves to the visual cortex of the brain,where the neural impulses are analyzed and the image is recognized.
$(c)$ When sound waves strike the eardrum,they are transmitted to the inner ear by the ear ossicles. The vibrations pass through the oval window into the fluid of the cochlea,generating waves in the lymph. These waves induce a ripple in the basilar membrane,which bends the hair cells,pressing them against the tectorial membrane. As a result,nerve impulses are generated in the associated afferent neurons and transmitted to the auditory cortex of the brain via the auditory nerves,where the impulses are analyzed and the sound is recognized.