M04.04.03 EQUILIBRIUM POTENTIAL

Action potentials are fundamental to the functioning of neurons and muscle cells. They are rapid, transient changes in the membrane potential that propagate along the cell membrane, allowing for communication between cells. This guide provides a detailed overview of action potentials, suitable for medical students.

Introduction An action potential is a brief reversal of membrane potential, typically occurring in excitable cells such as neurons and muscle cells. It is the basic mechanism for the transmission of information in the nervous system and the activation of muscle contraction.

Phases of Action Potential The action potential consists of several distinct phases, each associated with specific changes in membrane potential and ion channel activity.

Phases

1. Resting Potential
2. Depolarization
3. Repolarization
4. Hyperpolarization

Ion Channels and Ionic Basis Action potentials depend on the coordinated activity of various ion channels.

Voltage-Gated Ion Channels

• Sodium Channels (Na+): Open rapidly during depolarization, allowing Na+ influx.
• Potassium Channels (K+): Open more slowly, allowing K+ efflux during repolarization and hyperpolarization.

Ionic Movements

1. Resting Potential: High K+ inside, high Na+ outside. Maintained by Na+/K+ pump.
2. Depolarization: Na+ channels open, and Na+ enters the cell, making the inside more positive.
3. Repolarization: Na+ channels close, K+ channels open, and K+ exits the cell, restoring negative potential.
4. Hyperpolarization: K+ channels stay open longer, causing an overshoot.

Propagation of Action Potentials Action potentials propagates along the axon without decrement, ensuring the signal reaches its target with full intensity.

Mechanism

• Continuous Conduction: In unmyelinated axons, action potentials propagate by sequential depolarization along the axon.
• Saltatory Conduction: In myelinated axons, action potentials jump from one Node of Ranvier to the next, increasing conduction velocity.

Clinical Relevance Understanding action potentials is crucial for diagnosing and treating various neurological and muscular disorders.

Examples

• Epilepsy: Abnormal, excessive action potentials in the brain.
• Multiple Sclerosis: Demyelination leads to slowed action potential propagation.
• Cardiac Arrhythmias: Abnormal action potentials in cardiac muscle cells.

Points to Remember

• Resting potential is maintained by Na+/K+ pumps.
• Depolarization is due to Na+ influx.
• Repolarization is due to K+ efflux.
• Action potentials are all-or-none phenomena.
• Propagation can be continuous (unmyelinated) or saltatory (myelinated).

Bibliography

1. Kandel, E.R., Schwartz, J.H., & Jessell, T.M. (2013). Principles of Neural Science. McGraw-Hill.
2. Purves, D., Augustine, G.J., Fitzpatrick, D., et al. (2018). Neuroscience. Sinauer Associates.
3. Marieb, E.N., & Hoehn, K. (2019). Human Anatomy & Physiology. Pearson.
4. Hille, B. (2001). Ion Channels of Excitable Membranes. Sinauer Associates.