Explainthe following processes:
(a)Polarisation of the membrane of a nerve fibre
(b)Depolarisation of the membrane of a nerve fibre
(c)Conduction of a nerve impulse along a nerve fibre
(d) Transmission of a nerve impulseacross a chemical synapse
(a)Polarisation of the membrane of a nerve fibre
Duringresting condition, the concentration of K+ ions is moreinside the axoplasm while the concentration of Na+ ions ismore outside the axoplasm. As a result, the potassium ions movefaster from inside to outside as compared to sodium ions. Therefore,the membrane becomes positively charged outside and negativelycharged inside. This is known as polarization of membrane orpolarized nerve.
(b)Depolarisation of the membrane of a nerve fibre
When anelectrical stimulus is given to a nerve fibre, an action potential isgenerated. The membrane becomes permeable to sodium ions than topotassium ions. This results into positive charge inside and negativecharge outside the nerve fibre. Hence, the membrane is said to bedepolarized.
(c)Conduction of a nerve impulse along a nerve fibre
There aretwo types of nerve fibres – myelinated and non-myelinated. Inmyelinated nerve fibre, the action potential is conducted from nodeto node in jumping manner. This is because the myelinated nerve fibreis coated with myelin sheath. The myelin sheath is impermeable toions. As a result, the ionic exchange and depolarisation of nervefibre is not possible along the whole length of nerve fibre. It takesplace only at some point, known as nodes of Ranvier, whereas innon-myelinated nerve fibre, the ionic exchange and depolarization ofnerve fibre takes place along the whole length of the nerve fibre.Because of this ionic exchange, the depolarized area becomesrepolarised and the next polarized area becomes depolarized.
(d)Transmission of a nerve impulse across a chemical synapse
Synapse isa small gap that occurs between the last portion of the axon of oneneuron and the dendrite of next neuron. When an impulse reaches atthe end plate of axon, vesicles consisting of chemical substance orneurotransmitter, such as acetylcholine, fuse with the plasmamembrane. This chemical moves across the cleft and attaches tochemo-receptors present on the membrane of the dendrite of nextneuron. This binding of chemical with chemo-receptors leads to thedepolarization of membrane and generates a nerve impulse across nervefibre.
Thechemical, acetylcholine, is inactivated by enzymeacetylcholinestrase. The enzyme is present in the post synapticmembrane of the dendrite.
Ithydrolyses acetylcholine and this allows the membrane to repolarise.
