Explain how action potential is generated and propagated in the nerve fibre. (IFS 2019, 10 Marks)

Introduction

Action potential is a crucial process in the transmission of signals along nerve fibres. It is a rapid and transient change in the membrane potential of a nerve cell, which allows for the propagation of electrical signals along the length of the nerve fibre. 

Generation of Action Potential

• Resting Potential:
  
  • A neuron at rest has a resting membrane potential of approximately -70 mV.
  • This is maintained by the sodium-potassium pump (Na+/K+ ATPase), which pumps 3 sodium ions (Na⁺) out and 2 potassium ions (K⁺) into the cell, creating an electrochemical gradient.
  • The inside of the membrane is negatively charged compared to the outside, which is positively charged.
• Stimulus and Depolarization:
  
  • A stimulus causes a depolarization of the membrane, leading to the opening of voltage-gated sodium channels.
  • Sodium ions (Na⁺) rush into the cell due to the electrochemical gradient, causing the inside of the cell to become less negative (depolarized).
  • If the depolarization reaches the threshold potential (usually around -55 mV), an action potential is triggered.
• Threshold and All-or-None Law:
  
  • The action potential follows the "all-or-nothing" principle, meaning that once the threshold is reached, the action potential is generated fully, without any partial responses.
  • If the threshold is not reached, no action potential occurs.
• Rapid Depolarization:
  
  • Once the threshold is crossed, a large number of sodium channels open quickly, leading to rapid depolarization.
  • The inside of the cell becomes positively charged (around +30 mV).
• Repolarization:
  
  • After a brief period, the sodium channels close and potassium channels open.
  • Potassium ions (K⁺) flow out of the cell, which restores the negative charge inside the cell (repolarization).
  • The efflux of K⁺ ions continues until the membrane potential returns to its resting state.
• Hyperpolarization:
  
  • The potassium channels remain open longer than needed, causing the membrane potential to become more negative than the resting potential (hyperpolarization).
  • The sodium-potassium pump restores the resting potential by actively pumping Na⁺ out and K⁺ back into the cell.

Propagation of Action Potential

• Local Current Flow:
  
  • When the action potential is generated, it causes the adjacent membrane to depolarize.
  • The influx of sodium ions into one region of the axon creates a local current, which depolarizes the neighboring segment of the membrane.
• Saltatory Conduction:
  
  • In myelinated fibers, the action potential "jumps" from one node of Ranvier to the next.
  • The myelin sheath insulates the axon, preventing ion flow along the length of the axon except at the nodes, where ion channels are concentrated.
  • This type of conduction speeds up the transmission of the action potential compared to unmyelinated fibers.
• Continuous Conduction:
  
  • In unmyelinated fibers, the action potential propagates by continuous conduction.
  • The depolarization at one segment of the axon causes the next segment to reach the threshold and generate an action potential, continuing down the length of the axon.
• Refractory Period:
  
  • After an action potential is generated, the neuron enters a refractory period.
  • During the absolute refractory period, no new action potential can be initiated.
  • During the relative refractory period, a new action potential can be triggered, but only by a stronger-than-normal stimulus.

Conclusion

The generation and propagation of action potential in a nerve fibre is a crucial process that allows for the transmission of signals in the nervous system. The mechanisms underlying this process is essential for gaining insights into the functioning of the nervous system and its role in various physiological processes.