Action Potentials

Action Potentials in Neurons

Introduction

In this lecture, we explore the fundamentals of action potentials, focusing specifically on how they occur within neurons. This is part of Biology 111, Anatomy and Physiology, First Semester, following the 11th edition of the textbook authored by Maria Ben Hohen and contributors, published by Pearson.

Excitable Cells and Membrane Potential

  • Excitable Cells: These are specialized cells that respond to stimuli. Key characteristics include:

    • Ability to generate action potentials.

    • Measureable membrane potential even at rest.

    • Neurons (a primary example) must have a resting membrane potential.

  • Resting Membrane Potential: This is the electrical charge difference across the cell membrane while the neuron is at rest. Typical value:

    • Negative 70 millivolts (mV), indicating more positive charges outside the cell compared to inside.

Importance of Membrane Potential

  • Membrane potential is crucial for enabling neurons to respond to stimuli. Any significant change in this potential can lead to generating an action potential, allowing communication between neurons and other cells, such as muscle contractions.

Measurement of Membrane Potential

  • A voltmeter measures the voltage inside a neuron's axon:

    • Example: Voltage measurement shows an initial difference of -70 mV indicating polarized state.

    • Changes in this measurement will determine the response of the cell to a stimulus.

The Process of Generating an Action Potential

Stages of Action Potential

  1. Resting State: Starts at -70 mV (polarized).

  2. Depolarization:

    • Occurs when a stimulus causes membrane potential to reach a specific threshold (approximately -55 mV).

    • Sodium ions (Na⁺) enter the neuron, increasing the membrane potential to about +30 mV (positively charged condition).

    • Influx of sodium is termed sodium influx.

    • The neuron transitions from polarized to depolarized state.

  3. Repolarization:

    • Sodium channels inactivate (stop allowing Na⁺ in), leading to a decrease in membrane potential from +30 mV back down toward -70 mV.

    • Potassium channels (K⁺) open, allowing potassium ions to exit, which facilitates the return of membrane potential to a more negative charge (negative outflux).

  4. Hyperpolarization:

    • This is characterized by potential dropping below -70 mV, reaching approximately -90 mV.

    • Conditions are even more polarized than resting due to excess K⁺ leaving.

    • The sodium-potassium pump helps re-establish resting potential by moving 3 Na⁺ ions out and 2 K⁺ ions back into the cell.

Graphing the Action Potential

  • X-axis: Represents time.

  • Y-axis: Represents membrane potential (in mV).

  • Graph shows resting potential (-70 mV), peak depolarization (+30 mV), repolarization back down, and hyperpolarization (-90 mV).

  • Key regions on the graph:

    • Depolarization: transition up to +30 mV.

    • Repolarization: return back towards resting potential.

    • Hyperpolarization: drop below resting potential.

Characteristics of Action Potentials

  • All-or-Nothing Principle: An action potential either occurs fully (when threshold is met) or not at all if the threshold is not met.

  • Action potentials are self-regenerating along the axon, maintaining the same magnitude and shape as they travel.

  • They use voltage-gated ion channels (specifically Na⁺ and K⁺ channels).

Voltage-Gated Channels

  1. Voltage-Gated Sodium Channels (VGNa):

    • Three states: closed, open, inactive.

    • Open when membrane reaches approximately -55 mV (threshold potential).

    • Inactive at +30 mV, preventing further Na⁺ influx until returning to resting potential.

  2. Voltage-Gated Potassium Channels (VGK):

    • Two states: open or closed.

    • Open at around +20 to +30 mV, allowing K⁺ to leave the cell, helping to repolarize the membrane.

    • Close when membrane potential returns to around -90 mV.

Graded Potentials vs. Action Potentials

  • Graded Potentials:

    • Result from different stimuli causing localized changes in membrane potential.

    • Decay rapidly and are not all-or-nothing; they vary in size and duration.

    • Typically caused by ligand-gated (chemically gated) channels.

  • Action Potentials:

    • Generated when graded potentials reach the threshold level.

    • Are uniform in size and do not diminish over long distances.

Critical Comparisons

  • Graded potentials can build to reach threshold for an action potential.

  • Graded potentials have decremental amplitude, whereas action potentials maintain consistent amplitude regardless of distance traveled.

Summary of Key Points

  • Membrane potential changes are fundamental for neuron communication.

  • Action potentials enable long-distance signaling between neurons, with specific ionic movements characterizing each stage.

  • Understanding the mechanisms of membrane potential changes, graded potentials, and the all-or-nothing response is crucial for grasping neuronal function and synaptic communication.