(a) Polarisation of the membrane of a nerve fibre: The neurons exist in a state of excitability called the polarised state. This is a state of rest when the neuron is not conducting an impulse. At this point, the neurilemma (plasma membrane of a nerve fibre) is more permeable to potassium ions and nearly impermeable to sodium ions. As a result of this, the axoplasm has high concentration of K⁺ and negatively charged proteins and low concentration of Na⁺. In contrast, the fluid outside the axon contains low concentration of K⁺, high concentration of Na⁺ leading to existence of a concentration gradient. Such a state of the fibre where there exists an electrical potential difference across the resting plasma membrane is the state of depolarisation and the potential is called the resting membrane potential. (b) Depolarisation of the membrane of a nerve fibre:The impulse spreads through the length of the nerve fibre in the form of a wave of depolarisation. Depolarisation is a state of reverse polariy. Depolarisation occurs as a result of the opening of Na⁺ channels whereas K⁺ channels remain closed. As a result there is an entry Na⁺ ions into the neuroplasm which leads to reversed potential difference as inside of the membrane starts turning positive i.e. from – 70 mV towards zero. At zero mV, the membrane is said to be depolarized (c) Conduction of a nerve impulse alone a nerve fibre:Electrical events in the nerve conduction. The electrical events involve the following changes in the electrical potential of nerve fibre. Resting potential state. The nerve cells remain bathed in a fluid called interstitial fluid. In this fluid remain dissolved sodium (Na⁺) and potassium (K⁺). During the resting phase, the neurilemma is comparatively 30 times more permeable to potassium ions (K⁺) than to sodium ions (Na⁺). As a result Na⁺ ions are present in high concentration outside the membrane and in low concentration on the inner side. Thus other surface whose (+ve) electric charge and inner surface (-ve) charge due to organic anions. The membrane at this state is said to be in polarised state and the resting potential remains at a value of 70 mm. Depolarization and action potential. When a stimulus of any kind is applied to the nerve, it disturbs the set up. There is marked change in the potential and it is called action potential. The polarity of membrane gets reversed after excitation because the Na⁺ ions move inward and K⁺ ions move outward. Thus the membrane is said to be depolarised. It is termed active phase, during which the inner surface is positively charged and outer surface is negatively charged. Conduction of impulse. The electrochemical changes are conducted up to synapses as the electric wave of change of potential progresses forward along the fibre. Repolarization. In the recovery phase, the axon again restores the original +ve concentration by the outside of movement of Na⁺ ions from the inner side of membrane. The process of change requires something during which nerve cannot be stimulated again. It is of the duration 1/ 1000 second and called refractory period. (d) Transmission of nerve impulses across a chemical synapseSynapse. Synapse is the close proximity of terminal branch of the axon of one neuron with dendrites of the next neuron in a chain without actual contact. There are two types of synapses i.e. electrical and chemical depending upon the nature of transfer of information across the synapse. In transmission of nerve impulse across a synapse the impulse reaches the end knob, a neurotransmitter called acetylcholine is released from the synaptic vesicles present in the end knob of axon. It results in depolarization of dendrite membrane initiating a new impulse (action potential- passes along the next neuron). The neurotransmitter is released by synaptic membrane and received by post – synaptic membrane and received by post-synaptic membrane of dendrite of next neuron. To check continued stimulation of dendrite membrane an enzyme acetylcholinestrase inactivates the acetylcholine. Some sympathetic fibres norepinephrine as neurotransmitter. It is inactivated by enzyme monoamino oxidase to prevent continued stimulation of muscle. It involves two processes neurosecretion by axon endings and chemoreception by dendrites and get into state of excitation.