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Question

Give a brief account on:
(a) Mechanism of synaptic transmission
(b) Mechanism of vision
(c) Mechanism of hearing


Solution

(a) Synapse is a junction between two neurons. It is present between the axon terminal of one neuron and the dendrite of next neuron separated by a cleft.
There are two ways of synaptic transmission.
(1) Chemical transmission
(2) Electrical transmission
1. Chemical transmission: When a nerve impulse reaches the end plate of the axon, it releases a neurotransmitter (acetylcholine) across the synaptic cleft. This chemical is synthesized in the cell body of the neuron and is transported to the axon terminal. The acetylcholine diffuses across the cleft and binds to the receptors present on the membrane of next neuron. This causes depolarization of membrane and initiates an action potential.
2. Electrical transmission: In this type of transmission, an electric current is formed in the neuron. This electric current generates an action potential and leads to transmission of a nerve impulse across the nerve fibre. This represents a faster method of nerve conduction than the chemical method of transmission.

(b) Mechanism of vision
Retina is the innermost layer of the eye. It contains three layers of cells – inner ganglion cells, middle bipolar cells, and outermost photoreceptor cells. A photoreceptor cell is composed of a protein called as opsin and an aldehyde of vitamin A called as retinal. When light rays are focused on the retina through the cornea, it leads to the dissociation of retinal from opsin protein. This changes the structure of opsin. As the structure of opsin changes, the permeability of membrane changes, generating a potential difference in the cells. This generates an action potential in the ganglionic cells and is transmitted to the visual cortex of the brain via optic nerves. In the cortex region of the brain, the impulses are analysed and the image is formed on the retina.

(c) Mechanism of hearing
The pinna of the external region collects the sound waves and directs it towards ear drum or external auditory canal. These waves strike the tympanic membrane and vibrations are created. Then, these vibrations are transmitted to the oval window, fenestra ovalis, through three ear ossicles, named as malleus, incus, and stapes. These ear ossicles act as a lever and transmit the sound waves to internal ear. These vibrations from fenestra ovalis are transmitted into the cochlear fluid. This generates sound waves in the lymph. The formation of waves generates a ripple in the basilar membrane. This movement bends the sensory hair cells present on the organ of Corti against tectorial membrane. As a result of this, sound waves are converted into nerve impulses. These impulses are then carried to the auditory cortex of brain via auditory nerves. In cerebral cortex of the brain, the impulses are analysed and the sound is recognized.

Biology
NCERT
Standard XI

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