action potential

Chapter 12: Physiology of Action Potential

Introduction to Action Potentials

  • Understanding action potentials requires a grasp of the resting membrane potential.

  • Resting membrane potential is crucial for initiating an action potential.

Membrane Charge and Action Potential

  • Stimulus triggers a change in membrane charge, leading to an action potential.

  • The initial change in membrane charge due to a stimulus is called a graded potential.

    • Can involve both positive and negative movements in membrane potential.

Sequence of Events Before Action Potential

  • Resting Membrane Potential:

    • Primarily occurs in dendrites and cell body.

  • Graded Potential:

    • Found mainly in the trigger zone, leading into action potential.

  • Upon reaching the action potential, it propagates down the axon to the synapse.

  • Neurotransmitters (e.g., acetylcholine) are released at the synapse to interact with target tissues, including muscle tissues.

Neurotransmitter Interaction

  • Different types of neurotransmitters interact with various tissues.

  • The postsynaptic cell interprets released neurotransmitters to understand received signals.

Resting Membrane Potential

  • Established using the sodium-potassium pump:

    • Moves 3 sodium ions out and 2 potassium ions into the cell.

    • Creates a potential where:

    • Outside: slightly positive

    • Inside: slightly negative

  • Measured resting potential: −70 millivolts.

Ion Movement and Action Potential

  • Ion channels play a critical role in action potentials.

    • Sodium channels: high concentration outside; via concentration gradient, sodium flows into the cell during action potentials.

    • Potassium: high concentration inside; generally, potassium flows out of the cell.

Graded and Action Potentials

  • Graded Potentials Characteristics:

    • Graded: Response magnitude varies with stimulus strength.

    • Decremental: Weaker potential with distance from the stimulus.

    • Reversible: Stops if stimulation halts.

    • Excitatory/Inhibitory: Can promote or inhibit action potentials.

  • Action potentials require reaching a threshold of −55 millivolts to occur:

    • Depolarization (moving positive) approaches threshold; hyperpolarization (moving negative) moves away.

The Action Potential: Phases and Characteristics

Phases of Action Potential:

  1. Threshold Reached: Sodium channels at trigger zone open at −55mV.

  2. Depolarization: Sodium floods into the cell, driving the potential up.

    • Peaks at +35mV.

  3. Repolarization: Sodium channels close; potassium channels open, potassium exits.

  4. Hyperpolarization: Potential dips below −70mV due to prolonged potassium outflow.

  5. Return to Resting Potential:

    • Sodium-potassium pumps restore original ion concentrations.

Action Potential Characteristics

  • All-or-None: Must hit threshold to fire; no partial action potentials.

  • Non-decremental: Strength remains consistent down the axon.

  • Irreversible: Once started, continues to completion.

  • Action potentials propagate unidirectionally towards the synapse due to refractory periods.

Refractory Periods

  • Absolute Refractory Period: Impossible to initiate another action potential due to inactive sodium channels.

  • Relative Refractory Period: Harder to initiate action potential when the membrane is hyperpolarized; requires stronger stimulus.

Summary of Action Potential Steps

  • Start: Begin at resting membrane potential (−70mV).

  • Excitatory local potential increases toward threshold (−55mV).

  • Threshold Achieved: Sodium channels open—rapid influx of sodium.

  • Peak: Membrane potential reaches +35mV, sodium channels close.

  • Repolarization: Potassium exits; cell potential drops back to −70mV.

  • Hyperpolarization: Overshoots resting potential due to slow potassium channel closure.

  • Return to Resting Potential: Achieved via sodium-potassium pumps.