Action Potentials: Mechanisms, Phases, and Conduction in Neurons

Action Potentials

Introduction to Action Potentials

An action potential (AP) is a rapid, temporary change in a cell's membrane potential that allows neurons to transmit electrical signals. This process is fundamental to neural communication and underlies all nervous system activity.

Phases of the Action Potential

Depolarization, Repolarization, and Hyperpolarization

The action potential consists of distinct phases, each characterized by specific changes in membrane potential:

• Depolarization: The membrane potential becomes more positive, moving away from the resting state. This is typically due to the influx of sodium ions (Na+) into the cell.

• Repolarization: The membrane potential returns toward the resting membrane potential, usually as potassium ions (K+) exit the cell.

• Hyperpolarization: The membrane potential temporarily becomes more negative than the resting potential, often due to continued K+ efflux.

Voltage-Gated Sodium Channels

Structure and Function

Voltage-gated Na+ channels are essential for initiating and propagating action potentials. Their operation involves several key components:

• Activation Gate: Opens when the cell reaches a threshold voltage, allowing Na+ to enter and depolarize the membrane.

• Inactivation Gate: Functions like a 'ball and chain' that blocks the channel after depolarization peaks (around +30 mV), preventing further Na+ influx.

• Threshold Potential: The critical level of depolarization required to open voltage-gated Na+ channels and initiate an action potential.

Example: When the membrane potential reaches approximately -55 mV, Na+ channels open, leading to rapid depolarization.

Mechanically Gated Channels and Hyperpolarization

Channel Dynamics and Membrane Potential

• When repolarization occurs, the membrane returns to its normal (resting) charge, allowing the inactivation gate to release and reset.

• Both Na+ and K+ channels close simultaneously after the action potential.

• Hyperpolarization can occur if K+ channels remain open longer than necessary, making the membrane potential more negative than the resting state.

Ion Channel States and Ion Flow During Action Potential

Channel States and Ion Movement

At each phase of the action potential, specific ion channels are open or closed, dictating the flow of ions:

• Depolarization: Na+ channels open, Na+ influx.

• Repolarization: Na+ channels inactivate, K+ channels open, K+ efflux.

• Hyperpolarization: K+ channels remain open briefly, excess K+ leaves the cell.

Initiation of Action Potential

Axon Hillock and Propagation

• The axon hillock is the region where the cell body meets the axon and is the site where action potentials are typically initiated.

• Once triggered, the action potential propagates down the axon toward the synaptic terminals.

Threshold and the All-or-None Principle

Triggering an Action Potential

• A specific threshold potential must be reached to trigger an action potential (usually around -55 mV).

• All-or-None Principle: Once the threshold is reached, an action potential will always occur with the same magnitude; subthreshold stimuli do not produce an action potential.

Refractory Periods

Types and Significance

• Absolute Refractory Period: No new action potential can be generated, regardless of stimulus strength, until the cell has repolarized.

• Relative Refractory Period: A new action potential can be generated only if the stimulus is stronger than usual (supra-threshold), as the cell is hyperpolarized but recovering.

Conduction Velocity

Factors Affecting Speed of Action Potential

• Conduction velocity refers to the speed at which an action potential travels down a neuron.

• Axon Diameter: Larger axons conduct action potentials faster due to lower resistance to ion flow.

• Myelination: Myelinated axons conduct action potentials more rapidly by reducing ion leakage and allowing saltatory conduction (action potentials 'jump' between nodes of Ranvier).

Summary Table: Phases and Channel States

Phase	Na+ Channel State	K+ Channel State	Membrane Potential
Resting	Closed	Closed	-70 mV
Depolarization	Open	Closed	Rising toward +30 mV
Repolarization	Inactivated	Open	Falling toward -70 mV
Hyperpolarization	Closed	Open (briefly)	Below -70 mV

Key Equations

Nernst Equation (for equilibrium potential)

The equilibrium potential for an ion can be calculated using the Nernst equation:

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

Ohm's Law (for membrane current)

$I = g(V - E)$

Where I is the current, g is the conductance, V is the membrane potential, and E is the equilibrium potential.

Additional info:

• Saltatory conduction in myelinated axons allows action potentials to 'jump' between nodes, greatly increasing conduction speed.

• Action potentials are essential for rapid communication in the nervous system, including muscle contraction and sensory perception.