question_answer 6)

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

Answer:

(a) Mechanism of Synaptic Transmission There are two types of synapses namely- electrical synapses and chemical synapses. At electrical synapses, electric current can flow directly from one neuron into the other across these synapses. Transmission of an impulse across electrical synapses is very similar to impulse conduction along a single axon. At a chemical synapse, the membranes of the pre- and post-synaptic neurons are separated by a fluid-filled space called synaptic cleft. The rise in the stimulus-induced permeability to Na+ is extremely short lived. It is quickly followed by arise in permeability to K+. Within a fraction of a second, K4' diffuses, outside the membrane and restores the resting potential of the membrane at the site of excitation and the fibre becomes once more responsive to further stimulation. (b) Mechanism of Vision The light rays in visible wavelength focuses on the retina through the cornea and lens generates potentials (impulses) in rods and cones. Light induces dissociation of the retinal from opsin resulting in changes in the structure of the opsin. This causes membrane permeability changes. As a result, potential differences are generated in the photoreceptor cells. This produces a signal that generates action potentials in the ganglion cells through the bipolar cells. These action potentials (impulses) are transmitted by the optic nerves to the visual cortex area of the brain, where the nerve impulses are analysed and the image formed on the retina. (c) Mechanism of Hearing The external ear receives sound waves and directs them to the ear drum. The ear drum vibrates in response to the sound waves and these vibrations are transmitted through the ear ossicles to the oval window. The vibrations are passed through the oval window on to the fluid of the cochlea, where they generate waves in the lymphs. The waves in the lymphs induce a ripple in the basilar membrane. These movements of the basilar membrane bend the hair cells, pressing them against the tectorial membrane. As a result, nerve impulses are generated in the associated afferent neurons. These impulses are transmitted by the afferent fibres via auditory nerves to the auditory cortex of the brain, where the impulses are analysed and the sound is recognized.