Neurons are excitable cells because their membranes (neurolemma) are in a polarised state. It is because of the different types of the ion channels that are present on the neural membrane (neurilemma). These ion channels are selectively permeable to different ions e.g., Na+ , K+ and proteins. 1. When the neuron is not conducting any impulse it is in resting state: It is known as resting potential. During this shape the axonal membrane is comparatively more permeable to potassium ions (K+) and it is nearly impermeable to sodium ion (Na+ ). Similarly, it is also impermeable to negatively charged protein (-) present in the axoplasm of the axon. And consequently or as a result of it, the axoplasm of axon has high concentration of K+ and negatively charged proteins and low concentration of Na+. In contrast to it there is high concentration of Na+ and low concentration of K+ in the fluid outside the axon. Thus, it forms a concentration gradient. These ionic gradients across the resting membrane are maintained by the active transport of ions by the Na - K pump (sodium potassium pump). It transports 3Na+ outwards for 2K+ into the cell. img 11 As a result of it the outer surface of the axonal membrane becomes (+) charged and therefore the membrane is polarised. This electrical potential difference that occurs across the resting plasma membrane is called as resting potential. 2. When a stimulus is applied/given to neuron at a point then action potential is created. The generation of nerve impulse and its propagation/condition along an axon is due to conversion of resting potential to action potential. (i) When the stimulus is applied at a site - A on the polarised membrane then the side - A becomes freely permeable to Na+ ions. (ii) This lead to rapid influx of Na+ inside the axon and reversal of polarity takes place at site - A. i.e., the outer surface of membrane becomes (+) and inner surface becomes ( - ). This is depolarisation. This difference across the plasma membrane at site - A is called action potential. This is actually the nerve impulses generated. (iii) Now the impulses (action potential) generated at site - A propagates towards site B. Where the membrane is (+) charged outside and ( - ) charged inside. On the outer surface current flows from site B to site A to complete, the circuit of current flow. The polarity is reversed at the site. At point site B the action potential is generated. Resulting in the shift of action potential from site A to site B. This takes place repeatedly along the axon and then the impulse is propagated. (iv) The time for rise in the structures induced permeability to K+ is very short. It is quickly followed by a rise in the permeability to K+ into the neurilemma. The time taken is fraction of a second K+ diffusion outside the membrane. Due to this the resting potential of the membrane at the site of excitation is restored. The fibre again becomes more responsive to further stimulation.