Question -
Answer -
(a) The action potentialis largely determined by Na+ ions. The action potential results from thefollowing sequential events
(i) Disturbance caused to the membrane of a nervefibre by a stimulus results in leakage of Na+ into the nerve fibre.
(ii) Entry of Na+ lowers the trans-membrane potentialdifference.
(iii) Decrease in potential difference makes themembrane more permeable to Na+ than to K+ ions so that more Na+ enter the fibre than K+ leave it.
(iv) Accumulation of Na+ in the nerve fibre initiates depolarisation(action potential), making the axonic contents positively charged relative tothe extracellular fluid.
(v) With continued addition of Na+ the potential reaches zero and then plus40-50 millivolts. This is the peak of action potential.
(vi) Permeability of a depolarised membrane to Na+ then rapidly drops, there are now as many Na+ on the inside of the membrane as on theoutside.
(b) Mechanism of vision: The light rays in visiblewavelength focused on the retina through the cornea and lens generatepotentials (impulses) in rods and cones. Light induces
dissociation of the retinal from opsin resultingin changes in the structure of the opsin. This causes membrane permeabilitychanges. As a result, potential differences are generated in the photoreceptorcells. This produces a signal that generates action potentials in the ganglioncells through the bipolar cells. These action potentials (impulses) aretransmitted by the optic nerves to the visual cortex area of the brain, wherethe neural impulses are analysed and the image formed on the retina isrecognised based on earlier memory and experience.
(c) Mechanism of hearing : The external earreceives sound waves and directs them to the ear drum. The ear drum vibrates inresponse to the sound waves and these vibrations are transmitted through theear ossicles (malleus, incus and stapes) to the oval window. The vibrations arepassed through the oval window on to the fluid of the cochlea, where theygenerate waves in the lymphs. The waves in the lymphs induce a ripple in thebasilar membrane. These movements of the basilar membrane bend the hair cells,pressing them against the tectorial membrane. As a result, nerve impulses aregenerated in the associated afferent neurons. These impulses are transmitted bythe afferent fibres via auditory nerves to the auditory cortex of the brain,where the impulses are analysed and the sound is recognised.