(a) Role of Na⁺ in the generation of action potential. 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 siad to be depolarised. (b) Role of Ca⁺⁺ ions in the release of neurotransmitters at a synapse. When an impulse arrived at presynaptic cell, Ca⁺⁺ ions from the synaptic cleft enter the presynaptic cell. The Ca²⁺ ions cause the movement of the synaptic + vesicles to the surface of the cell and then get fused with the membrane. Rupturing of the vesicles and release of neurotransmitters of exocytosis follow this into the synaptic cleft. (c) Mechanism of generation of light induced impulse in the retina. The photopigments of the retina are photosensitive compounds in the eye that are composed of retinal (an aldehyde of vitamin A) and opsin (a protein). Light induces dissociation of retinal from opsin that leads to a change in membrane permeability. Consequently membrane potential differences are generated producing 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 areas of the brain where neural impulses are analysed and image is recognised based on earlier memory and experience. (d) Mechanism through which a sound produces a nerve impulse in the inner ear. 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.