hysiology of action potential. (IAS 2019/10 Marks)

Introduction

Action potential is a crucial physiological process that plays a key role in the communication between neurons and muscle cells. It is a rapid and transient change in the membrane potential of a cell, which allows for the transmission of electrical signals along the length of the cell. 

Physiology of Action Potential

1. Resting Membrane Potential

• The membrane potential when a neuron is not conducting an impulse, typically around -70 mV (millivolts).
• Cause: Maintained by the sodium-potassium pump (Na+/K+ pump), which actively transports 3 Na+ ions out of the cell for every 2 K+ ions it pumps into the cell.
• Ionic Distribution:
  
  • High Na+ concentration outside the cell and high K+ concentration inside the cell.
  • Presence of negatively charged proteins inside the cell contributes to the negative resting potential.

2. Depolarization

• Triggering Event: A stimulus (such as a neurotransmitter) causes sodium channels to open.
• Ion Movement:
  
  • Sodium ions (Na+) rush into the cell due to both the concentration gradient and the electrical gradient.
  • This influx of positively charged Na+ ions reduces the negative membrane potential, leading to depolarization.
• Threshold Potential: If depolarization reaches a critical value (typically -55 mV), it triggers the action potential.

3. Action Potential

• All-or-Nothing Response: Once the threshold is crossed, a full action potential is generated. There is no partial action potential.
• Phases:
  
  • Depolarization Phase: Rapid rise in membrane potential (from negative to positive), primarily due to the influx of Na+ ions.
  • Peak: The membrane potential peaks at around +30 mV.

4. Repolarization

• Ion Movement:
  
  • Voltage-gated potassium (K+) channels open, allowing K+ ions to flow out of the cell.
  • This movement of K+ out of the cell restores the membrane potential to a negative value (repolarization).
• Sodium Channels: At the same time, sodium channels close to stop further influx of Na+.

5. Hyperpolarization

• Cause: Sometimes the membrane potential becomes more negative than the resting potential, usually around -80 mV to -90 mV.
• Reason: This occurs because potassium channels are slow to close, leading to excessive efflux of K+ ions.
• Significance: This period prevents the immediate generation of another action potential, thus ensuring proper signal propagation.

6. Refractory Period

• Absolute Refractory Period: During this time, no new action potential can be generated, no matter how strong the stimulus is. It coincides with the depolarization and repolarization phases.
• Relative Refractory Period: A new action potential can be initiated, but only with a stronger-than-usual stimulus. It occurs during the hyperpolarization phase.

7. Propagation of Action Potential

• Wave of Depolarization: The action potential travels along the axon due to the local currents caused by Na+ entering the cell and depolarizing adjacent regions.
• Saltatory Conduction: In myelinated fibers, the action potential jumps from one node of Ranvier to the next, significantly increasing the speed of transmission.

8. Restoration of Ionic Balance

• After the action potential, the sodium-potassium pump restores the original ionic concentrations by pumping Na+ out and K+ in. This process requires energy in the form of ATP.

Conclusion

The physiology of action potential is a complex process that is essential for the functioning of the nervous system and muscle cells. By studying the physiology of action potential, zoologists can further our understanding of how organisms respond to their environment and coordinate their movements.