Nerve Impulse

The membranes of the nerve cells have electrical potentials. Due to ionic changes it might become polarized. These changes in the electrical potential of the membrane of the nerve cell results into generation of nerve impulse. It begins with change in the permeability of the membrane and give rise to electrochemical events. These are known as nerve impulses. Normally these impulses start off in the axon hillock instead of dendrites or soma as the membrane is not excitable in these regions. With the generation of impulse in the axon hillock the surrounding membrane gets depolarized thus generating impulse in the adjacent part of the axon. These impulses do not travel backward because the membrane of soma is not excitable. The magnitude of these impulses do not depend upon the magnitude of the sensation, rather they follow all-or-none principle. This principle states that the nerve fibers either respond to the limit of their capability or do not get excited at all. A weak stimulation not capable of generating an impulse might end up resulting into local excitatory process.

The nature of nerve impulse is chemical. It consists of sodium-potassium (Na+-K+) pump, ion size, and diffusion pattern. In the resting state more sodium ions (Na+) are located outside the membrane. Potassium (K+) ions available in the fluid environment are actively transported into the neuron. However, they are insufficient to affect other ions. So, the outer membrane remains positively charged. Many negatively charged acid molecules (acid anions) are also present inside the cell. They are large enough to pass through the cell membrane. This also contributes in making the inner membrane negatively charged. The principle of diffusion is evident in the flow of ions and hence substances move from region of high concentration towards low concentration zone. Chlorine ions (Cl-) which carry negative charge freely move back and forth through the membrane.