Action Potential

Overview of Action Potential

  • Definition: An action potential is a rapid, transient change in the membrane potential of a neuron that allows for long-distance signaling between neurons.

Graphing the Action Potential

  • Axes:

    • Vertical Axis: Membrane Potential (measured in millivolts, mV)

    • 0 mV (neutral point)

    • Positive Values: 15 mV, 30 mV (maximum positive)

    • Negative Values: -15 mV, -30 mV, -45 mV, -60 mV, -75 mV, -90 mV (cell is resting at -70 mV)

    • Horizontal Axis: Time (measured in milliseconds)

Neuron Anatomy

  • Description of neuron: Consists of a soma (cell body) and its axon.

  • Purpose: The axon communicates signals between neurons, allowing for the propagation of action potentials.

Membrane Structure at the Axon Hillock

  • Components:

    • Phospholipid Bilayer

    • Voltage-Gated Sodium Channels:

    • Activation Gate: Closed at resting potential.

    • Inactivation Gate: Open at resting potential.

    • Leak Channels:

    • Potassium Leak Channel

    • Chloride Leak Channel

    • Sodium Leak Channel

Resting Membrane Potential

  • At rest, the neuron is at approximately -70 mV.

  • Determined by the efflux of potassium ions (K+) and the negative charge maintained within the cell.

  • Chloride ions (Cl-) can influx, but are limited due to the existing negative charge inside the cell.

  • Sodium ions (Na+) are attracted to the negative charge inside, but are few in number and also bound by both chemical and electrical gradients.

Initiation of Action Potential

  1. Graded Potential: Excitation

    • When a stimulus excites the neuron, it leads to depolarization (influx of positive charge).

    • This does not yet constitute an action potential but prepares the membrane for one.

  2. Threshold Level

    • The threshold for triggering an action potential is -55 mV.

    • Achieved through:

      • Temporal Summation: Rapid succession of incoming graded potentials.

      • Spatial Summation: Simultaneous activation of multiple channels.

  3. Opening of Sodium Channels

    • At -55 mV, the inactivation gate of the voltage-gated sodium channel closes, while the activation gate opens.

    • Sodium then floods into the neuron, leading to rapid depolarization.

    • Membrane potential shifts from -70 mV to +30 mV (a change of 100 mV).

Phases of Action Potential

  1. Resting State: -70 mV

  2. Depolarization:

    • Action potential reaches +30 mV.

    • Inactivation gate closes to prevent further sodium influx.

  3. Repolarization:

    • At +30 mV, potassium channels open, allowing K+ to exit and restore negative charge inside the neuron.

    • This outward flow causes the membrane potential to become more negative.

  4. Hyperpolarization:

    • Membrane potential temporarily exceeds -70 mV, dropping to -90 mV before stabilizing back to resting potential.

  5. Return to Resting State:

    • Sodium and potassium channels reset to their original positions, maintaining separation of charges.

Key Characteristics of Action Potentials

  • All or Nothing: An action potential either fires fully or not at all when the threshold is reached.

  • Non-decaying: Action potentials do not weaken over distance; they are regenerated at each node of Ranvier due to the presence of voltage-gated sodium channels.

  • Speed of Conductance:

    • Factors that Affect Conduction Velocity:

    • Degree of Myelination: Increased myelination leads to faster conduction.

    • Axon Diameter: Larger diameter axons conduct signals more quickly.

    • Types of Fibers:

    • Type A: Conduct at approximately 300 mph (heavily myelinated, large diameter).

    • Type B: Conduct at approximately 30 mph (lightly myelinated).

    • Type C: Conduct at approximately 3 mph (unmyelinated, thin fibers).

Conclusion of Action Potential Mechanics

  • At the axon terminal, the action potential prompts the release of neurotransmitters to the postsynaptic neuron, facilitating continued signaling.

  • These neurotransmitters can be excitatory or inhibitory, further influencing the subsequent neuron’s activity.

Final Remarks

  • The action potential is essential for long-distance signaling in the nervous system, allowing rapid communication between neurons. Further details on ion channels will be addressed in subsequent discussions.