Question

Long answer question.

Explain the process of conduction of nerve impulses up to the development of action potential.

Answer 1

Polarisation and Depolarisation along a nerve

i. The nerve impulse is a wave of bioelectrical or electrochemical disturbances passing along a neuron.

ii. Neurons have a charged cellular membrane with a voltage that is different on the outer and inner side of the membrane. The plasma membrane separates the outer and inner solutions of different chemical compounds but having approximately the same total number of ions.

iii. The external tissue fluid has both Na⁺ and K⁺ but there is a predominance of Na⁺ and Cl^-, while K⁺ is predominant within the fibre or in the intracellular fluid. This condition of a resting nerve is also called a polarised state.

iv. The polarized state of a neuron is established by maintaining an excess of Na⁺ on the outer side. On the inside, there is an excess of K⁺ along with large negatively charged protein molecules and nucleic acid.

v. Some amount of Na⁺ and K⁺ always leaks across the membrane. The Na⁺/K⁺ pump actively maintains the ionic gradient across the resting membrane. The sodium pump or Na-K allows the entry of K⁺ inside the membrane and exit of Na⁺.

vi. The difference in the distribution of Na⁺ and K⁺ on the two sides of the membrane produces a potential difference of – 50 to –100 millivolts (average is – 70 millivolts).

vii. The potential difference seen in a resting nerve is thus called resting potential (–70 millivolts) and it is mainly due to differential permeability of the resting membrane, which is much more permeable to K⁺ than to Na⁺. This results in slightly more K⁺ diffusing out than Na⁺ moving inside and causing a slight difference in polarity.

viii. Also, ions like negatively charged proteins and nucleic acids inside the cell make the overall charge negative on the inside and positive charge on the outside. The nerve membrane not only has leakage channels but also has many gated channels for Na⁺ /K⁺. These are also called voltage-gated channels. These channels enable the neuron to change its membrane potential to active potential in response to stimuli. The Na⁺ /K⁺ gated channels are separate so the transport of both these ions is separately done. However, during resting potential, both these gates are closed and the membrane resting potential is maintained.

ix. The resting potential of the membrane is maintained unless the stimulus reaches the neuron. Any change or disturbance to the membrane will cause Na⁺ to enter into the membrane and lower the potential difference (lesser than –70 millivolts). Thus, the membrane becomes more permeable to Na⁺

x. During resting potential, both gates are closed, and resting potential is maintained. However, during depolarization, the Na⁺ gates open and the K⁺ gates remain closed. This causes Na⁺ to rush into the axon and bring about depolarisation (opposite of polarity).

xi. The Extra Cellular Fluid (ECF) becomes electronegative with respect to the inner membrane which becomes electropositive. The value of action potential reaches +30 millivolts to +60 millivolts. This triggers depolarization in the next part while it itself starts going to repolarisation.