Lecture Notes: Action Potential in Muscle Physiology

Introduction to Action Potential

• Action potential is a key concept in muscle physiology.
• It is a brief and large reversal of membrane potential.
  • Example: Change from -70 mV to +30 mV.
• Involves opening of specific voltage-gated channels.
• Occurs only at the axon.
• It is an all-or-none phenomenon, meaning:
  • It does not decay like local or graded potentials.
• Threshold must be reached for action potential to occur.

Connection to Previous Lecture

• Local potentials occur inside dendrites or soma.
• Strong currents/stimuli in local potentials can travel some distance.
• Importance of Axon Hillock:
  • Critical part of neuron for deciding whether sufficient signal exists for action potential.
  • Requires enough local change in dendrites or soma.
  • Sufficient sodium ion influx leading to depolarization is key.
• Threshold is cell-specific:
  • Typically 15 to 20 mV above resting membrane potential.
  • Example: Resting at -70 mV, threshold at -55 mV.

Generation of Action Potential

• Starts with local or graded potential reaching axon hillock.
• Microelectrode measurements focus on axon hillock.

Steps of Action Potential

1. Rest

   • All channels closed; no significant ion flow.
   • Potassium leaks, sodium-potassium pumps work but no net change.
   • Membrane potential remains at -70 mV.

2. Depolarization

   • Voltage-gated sodium channels open, sodium rushes in.
   • Large upward change in membrane potential.

3. Repolarization

   • Sodium channels close, potassium channels open.
   • Potassium exits cell, reducing internal positivity.
   • Membrane potential decreases.

4. Hyperpolarization

   • Sodium channels fully closed, potassium channels remain open.
   • Excessive potassium exit causes overshoot of resting potential.
   • Returns to rest after overshoot.