The Action Potential and Membrane Physiology

12.4 The Action Potential

Introduction to Action Potentials

The action potential is a fundamental electrical signal used by neurons and muscle cells to communicate. Understanding the mechanisms underlying the action potential is essential for comprehending how the nervous system transmits, integrates, and responds to information.

• Action Potential: A rapid, temporary change in a cell's membrane potential, allowing the transmission of electrical signals along neurons or muscle fibers.

• Resting Membrane Potential: The baseline electrical charge difference across the cell membrane when the cell is not actively sending a signal.

• Key Functions: Enables communication between cells, underlies muscle contraction, and is essential for nervous system function.

Electrically Active Cell Membranes

Ion Movement and Membrane Potential

Cell membranes are composed of a phospholipid bilayer with embedded proteins, including ion channels and pumps. The movement of ions across the membrane creates electrical signals.

• Ion Channels: Proteins that allow specific ions to move across the membrane, contributing to changes in membrane potential.

• Types of Ions: Sodium (Na+), potassium (K+), calcium (Ca2+), and chloride (Cl-) are the main ions involved.

• Concentration Gradient: Ions move from areas of high concentration to low concentration, often through channels or pumps.

• Membrane Potential: The voltage difference across the cell membrane, typically measured in millivolts (mV).

Example: The sodium-potassium pump (Na+/K+ ATPase) actively transports 3 Na+ ions out of the cell and 2 K+ ions into the cell, helping maintain the resting membrane potential.

Types of Ion Channels

Classification and Function

Ion channels are classified based on how they open and close (gating mechanisms):

• Ligand-Gated Channels: Open in response to the binding of a chemical messenger (ligand), such as a neurotransmitter.

• Mechanically Gated Channels: Open in response to physical deformation of the membrane (e.g., touch, pressure).

• Voltage-Gated Channels: Open or close in response to changes in membrane potential.

• Leakage Channels: Always open, allowing ions to move according to their concentration gradients.

Example: Acetylcholine binding to its receptor on a muscle cell opens ligand-gated sodium channels, initiating muscle contraction.

Table: Types of Ion Channels and Their Properties

Channel Type	Stimulus for Opening	Example
Ligand-Gated	Binding of a chemical (ligand)	Acetylcholine receptor channel
Mechanically Gated	Physical deformation (stretch, pressure)	Touch receptors in skin
Voltage-Gated	Change in membrane potential	Voltage-gated Na+ channel in neurons
Leakage Channel	Always open	K+ leak channel

The Membrane Potential

Definition and Measurement

The membrane potential is the electrical potential difference across the cell membrane, resulting from the unequal distribution of ions. It is measured using a voltmeter, with one electrode inside the cell and one outside.

• Resting Membrane Potential: Typically ranges from -70 mV to -90 mV in neurons, with the inside of the cell being negative relative to the outside.

• Establishment: Maintained by ion gradients (mainly Na+ and K+) and selective permeability of the membrane.

• Measurement: A voltmeter can be used to compare the charge inside and outside the cell membrane.

Equation: The Nernst equation can be used to calculate the equilibrium potential for a particular ion:

$E_{ion} = \frac{RT}{zF} \ln \left( \frac{[ion]_{outside}}{[ion]_{inside}} \right)$

Where:

• Eion: Equilibrium potential for the ion

• R: Universal gas constant

• T: Temperature in Kelvin

• z: Valence of the ion

• F: Faraday's constant

• [ion]outside: Ion concentration outside the cell

• [ion]inside: Ion concentration inside the cell

Example: The resting membrane potential of a neuron is primarily determined by the K+ gradient, as the membrane is more permeable to K+ than to Na+ at rest.

Summary Table: Key Features of Membrane Potentials

Feature	Description
Resting Membrane Potential	Baseline voltage across the membrane; typically -70 mV in neurons
Depolarization	Membrane potential becomes less negative (moves toward zero)
Hyperpolarization	Membrane potential becomes more negative than resting potential
Repolarization	Return of membrane potential to resting value after depolarization

Additional info:

• These concepts are foundational for understanding nerve impulses, muscle contraction, and many physiological processes relevant to personal health.

• Disruptions in ion channel function can lead to neurological and muscular disorders.