Role of Na+ in the Generation of Action Potential:
(i) Stimulation of an axon immediately enhances manifold its membrane permeability to Na+.
(ii) As a result, Na+ ions diffuse across the j membrane from the extracellular fluid (ECF) where their concentration is higher, to the interior of the fibre where the concentration is much lower.
(iii) But the membrane permeability to K+ start
rising somewhat later only. So there is simultaneous rise in the outward diffusion of K+ from the cell interior having a higher K+ concentration.
(iv) These effects lower the overall cation concentration outside and enhance that inside I the membrane.
(v) The membrane is, thus deposited, with its
interior becoming electropositive to the exterior.
(vi) The depolarization spreads producing a local current. It induces nearby passive Na+ channels to open and so to depolarize the nearby site.
(vii) Hence, the initial depolarization passes , outward over the membrane and spread out in
all directions from the site of stimulation.
(b) Mechanism of generation of light induced impulse
(i) Light induces dissociation of retinal from opsin and results into change in structure of opsin.
(ii) It causes change in membrane permeability. As a result potential differences are generated in photoreceptor cells.
(iii) It produces a signal to generate action potential in ganglion cells.
(iv) These impulses are transmitted by optic nerves to visual cortex area of brain where impulses are analysed.
(c) Mechanism through which a sound produces a
nerve impulse in the inner ear:
(i) The vibrations produced in ear drum are transmitted to ear ossicles, then to ovil window, to inner ear.
(ii) Nerve impulse are generated and transmitted by afferent fibres to auditory cortex of brain.