Threshold Potential

Introduction

• Threshold Potential: Critical level of membrane depolarization needed to trigger an action potential.
• Essential for signaling in the Central Nervous System (CNS) and Peripheral Nervous System (PNS).

Action Potential and Threshold

• Schematic and actual recordings of action potentials differ due to recording techniques.
• Typical threshold potential ranges between -50 to -55 mV.
• Threshold potential can be influenced by sodium (Na+) and potassium (K+) ions.
  • Influx of Na+ depolarizes the membrane.
  • Efflux of K+ or influx of chloride (Cl-) hyperpolarizes the cell.

Historical Discoveries

• Walther Nernst and the concept of ion concentration affecting nervous excitability.
• Hodgkin and Huxley discovered the mechanism behind the threshold for excitation using voltage clamp techniques.
  • Equal and opposite currents of Na+ and K+ at threshold.
  • Instability leads to activation of more Na+ channels.

Physiological Function

• Threshold value: Determines if stimuli generate action potentials based on excitatory/inhibitory balance.
• Synaptic inputs are summed to reach threshold.
• Ion conductances depend on membrane potential and timing.

Resting Membrane Potential

• High impermeability of the phospholipid bilayer to ions.
• Leak channels and sodium-potassium ATPase maintain membrane potential.
• Na+ influx causes depolarization, potentially reaching threshold.

Factors Influencing Threshold

• Ion conductance changes, axon diameter, sodium channel properties.
• Threshold adapts to slow input changes.

Threshold Tracking and Electrotonus

• Threshold Tracking: Tests nerve excitability and is sensitive to membrane potential.
  • Measures control vs. changed environment thresholds.
• Threshold Electrotonus: Produces long-lasting subthreshold currents.
  • Changes in excitability observed through depolarizing or hyperpolarizing currents.

Clinical Significance

• Febrile Seizures: Associated with reduced threshold potential.
  • GABAB receptor inhibition and excessive heat.
• Amyotrophic Lateral Sclerosis and Diabetes: Abnormal threshold potentials.
• Cardiovascular Implications:
  • Diastolic depolarization and arrhythmias.
  • Drugs prolonging QT interval affect threshold potential.
  • Dietary influences like polyunsaturated fatty acids impact threshold potential.

Conclusion

• Threshold potential is crucial for normal neuron and muscle function.
• Clinical applications in neural and cardiac health illustrate its importance.