Question

Question 8 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.

Class - XI - Biology Chapter Neural Control and Coordination Page No. 329

Answer 1

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(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 inside of axoplasm. The electrical potential of the membrane changes from 70 mV towards zero and then the membrane is said to be depolarised.

(b) The photopigments of the retina are photosensitive compounds in the eye that are composed of retinal and opsin. Light induces dissociation of retinal from opsin which changes the structure of the opsin. It generates action potential in the bipolar neurons. These impulses/action potential are transmitted by the optic nerves to the visual cortex of the brain where the neural impulses are analysed and the erect image is recognised.

(c) When sound falls over the ear drum, it is then transmitted to the inner ear by ear ossicles. The vibrations are passed through the oval window onto the fluid of the cochlea, where they generate waves in the lymphs. The waves induce a ripple in the basilar membrane that bend the hair cells, pressing them against the techtonial membrane. As a result nerve impulses are generated in the associated afferent neurons and transmitted to auditory cortex of brain via auditory nerves, where the impulses are analysed and the sound is recognised.

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Answer 2

(a) The steady of a resting nerve fibre, i.e., the normal difference in the ionic concentrations and electric potential b/w its outside and inside, is maintained by the active transport of sodium and potassium ions against the concentration and electrochemical gradients. Thus, sodium ions are pumped out of and potassium ions are forced into the nerve fibre. The process of expelling out sodium ions and drawing in potassium ions against concentration and electrochemical gradients is termed sodium pump or better sodium-potassium-exchange pump. the active transport of sodium and potassium ions across the membrane requires energy, which is provided by metabolic processes within the nerve fibre itself with the help of an enzyme, sodium and potassium-dependent adenosine triphosphate, present in plasma membrane. The differential distribution of ions on the two sides of the membrane produces a potential difference of minus 50 to 100 millivolts across the membrane. This difference is the resting membrane potential referred to above.

(b) Retina has photopigments , now these photopigments are photosensitive compounds for eyes. It is composed of retinal and opsin.Now when the light comes it dissociates the Retinal from Opsin , which leads to the changes in the structure of opsin.It generates action potential in the bipolar neurons. These impulses are transmitted to visual cortex of the brain via optic nerves. Now in the brain neural impulses are analysised and an erect image is formed.

(c) The external ear receives the sound wave and it is then collected by ear pinna.The sound then entres and strikes the membranous structure called as ear drum. Now as soon as the ear drum persives the sound starts to vibrate.Now these vibrations ate send to ear ossicles. It is Malleus which receives the sound. Malleus is attached to tympanic membrane.Then vibrations of Malleus is received by incus.Now the vibrations of incus is received by stapes. Stapes is the smallest bone of the body.It is attached to oval window of the cochlea. The ossicles increases the efficiency of transmission of sound waves to the inner ear. Now the passed waves reaches lymphs in cochlea and generated waves there.The waves in the lymphs induce a ripple in the basilar membrane. Now the basilar membrane bends the hair cell and presses them against the tectorial membrane. As a result impulse are generated in the afferent neurons. Now the nerves are transmitted to auditory cortex of the brain and finally the sound is recognized.