Membrane Potential and Action Potential Review

Membrane Potential Basics

• Membrane Potential: The electrical potential difference across a cell's membrane, primarily determined by the distribution of ions.

• Key Terms:

  • Resting Membrane Potential: The baseline electrical charge of a neuron when it is not actively transmitting signals.

  • Depolarization: A change that makes the inside of the cell more positive, stimulating the cell.

  • Hyperpolarization: A change that makes the inside of the cell more negative, inhibiting the cell.

  • Repolarization: The return to the resting membrane potential after depolarization.

Changes in Membrane Potential

• Causes of Change:

  • Primarily through the transport of ions (e.g., sodium, potassium, calcium, chloride) across the membrane.

  • These changes can be small or large, and their frequency carries information about the state of the cell.

• Frequency and Magnitude:

  • The magnitude (size) of the change and how often it occurs provide important information regarding cellular activity.

  • Constant membrane potential changes are critical for survival (e.g., thought processes, heartbeat).

Action Potentials

• Definition: A rapid and large depolarization followed by repolarization, primarily occurring in specialized cells such as neurons and muscle cells.

• Key Features:

  • Threshold: The level of depolarization that must be reached for an action potential to occur.

  • All-or-None Law: An action potential occurs fully or not at all, depending on whether the threshold is reached.

• Example of Threshold:

  • Analogous to a light switch—only when a specific threshold is reached does the light turn on.

Dynamics of Action Potentials

• Ion Channels Involved:

  • Voltage-Gated Sodium Channels: Open when threshold is reached, allowing sodium ions to enter, causing rapid depolarization.

  • Voltage-Gated Potassium Channels: Open after the action potential peak, allowing potassium ions to exit, leading to repolarization.

• Phases of Action Potential:

  1. Resting State: All channels are closed.

  2. Depolarization: Sodium channels open (influx of sodium), membrane potential becomes positive.

  3. Repolarization: Sodium channels close, potassium channels open (efflux of potassium), membrane potential returns to resting.

Conduction of Action Potentials

• Conduction Types:

  • Continuous Conduction: Occurs in unmyelinated fibers (e.g., some muscle cells), where action potentials continue along the entire axon length.

  • Saltatory Conduction: Occurs in myelinated fibers, where action potentials jump between nodes of Ranvier, increasing conduction speed significantly (3 to 20 meters per second).

• Communication through Action Potentials:

  • Action potentials enable communication between neurons and other cells, allowing processing of sensory information (e.g., sound, pain).

  • Frequency of action potentials correlates with the intensity of the stimulus (e.g., louder sounds result in more frequent action potentials).

Synapses and Communication

• Synapse Definition: A junction between two cells that allows for communication.

  • Neurons communicate with other neurons, muscle cells, glial cells, and gland cells.

• Mechanism: When action potentials reach the presynaptic terminals, they trigger neurotransmitter release, enabling signals to be transmitted to adjacent cells.

• Examples of Neuronal Communication:

  • Communication with glial cells for support and maintenance.

  • Communication with muscle cells to trigger contractions via action potentials.

  • Communication with gland cells (e.g., adrenal gland) to regulate hormones (e.g., adrenaline/epinephrine).

Importance of Action Potentials

• Action potentials are crucial for bodily functions, facilitating processes such as movement, sensory perception, and cognition.

• Without action potentials, essential biological functions would cease to exist, highlighting their critical role in physiology.