Action Potentials and Electrical Signaling in Neurons

Action Potentials and Electrical Signaling in Neurons

Introduction to Neuronal Excitability

Neurons are specialized cells capable of generating and transmitting electrical signals known as action potentials. These signals are essential for communication within the nervous system and are triggered by changes in membrane potential.

• Excitable cells: Cells such as neurons and muscle cells that can generate action potentials.

• Membrane potential: The voltage difference across a cell's plasma membrane, typically measured in millivolts (mV).

Summation and Threshold

The initiation of an action potential depends on the summation of multiple graded potentials at the axon hillock. If the combined signals reach a critical value called the threshold, an action potential is triggered.

• Graded potentials: Small changes in membrane potential that can be excitatory or inhibitory.

• Summation: The process by which multiple graded potentials combine to influence the likelihood of reaching threshold.

• Threshold: The membrane potential at which voltage-gated sodium channels open, typically around -55 mV.

• Example: If signals W, X, and Y arrive at the axon hillock, their combined effect may reach threshold and trigger an action potential.

Phases of the Action Potential

An action potential consists of three main phases: depolarization, repolarization, and hyperpolarization. Each phase is characterized by specific ion movements across the membrane.

• Depolarization: Rapid influx of Na+ ions causes the membrane potential to become more positive.

• Repolarization: Efflux of K+ ions restores the membrane potential toward its resting value.

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

Equation:

$\text{Resting membrane potential} \approx -70\ \text{mV}$ $\text{Threshold potential} \approx -55\ \text{mV}$ $\text{Peak of action potential} \approx +30\ \text{mV}$

Generation of an Action Potential

The generation of an action potential involves the sequential opening and closing of voltage-gated Na+ and K+ channels.

1. Na+ channels open: Na+ rushes into the cell, causing depolarization.

2. Na+ channels close, K+ channels open: K+ leaks out, leading to repolarization.

3. Restoration: The Na+/K+ ATPase pump restores ion gradients after the action potential.

Equation:

$\text{Na}^+/\text{K}^+\ \text{ATPase}:\ 3\ \text{Na}^+\ \text{out},\ 2\ \text{K}^+\ \text{in}$

Refractory Periods

After an action potential, the neuron enters a refractory period during which it is less responsive to stimuli. This ensures unidirectional propagation and limits the frequency of action potentials.

• Absolute refractory period: No new action potential can be generated, regardless of stimulus strength.

• Relative refractory period: A stronger-than-normal stimulus can initiate another action potential.

• Unidirectional propagation: Action potentials travel in one direction along the axon due to refractory periods.

All-or-None Principle and Frequency Coding

Action potentials follow the all-or-none principle: once threshold is reached, an action potential is always generated with the same amplitude. The frequency of action potentials encodes the intensity of a stimulus.

• All-or-none principle: Action potentials are not graded; they either occur fully or not at all.

• Frequency coding: Stronger stimuli result in higher frequency of action potentials.

Propagation of Action Potentials

Action potentials propagate along axons by local current flow. The speed of propagation depends on axon diameter and myelination.

• Unmyelinated axons: Slower conduction due to continuous depolarization along the axon.

• Myelinated axons: Faster conduction via saltatory conduction, where action potentials jump between nodes of Ranvier.

• Axon diameter: Larger diameter axons conduct action potentials more rapidly due to lower resistance.

Conduction Velocities in Different Nerve Fiber Types

The velocity of action potential conduction varies among different types of nerve fibers, depending on their diameter and myelination.

Function	Fiber Diameter (μm)	Conduction Velocity
Stimulation of skeletal muscle contraction	22	Fast
Touch, pressure sensation	12	Fast
Pain, temperature sensation	2.5	Moderate
Autonomic postganglionic fibers	0.3–1.3	Slow

Clinical Application: Local Anesthetics

Local anesthetics such as lidocaine inhibit voltage-gated Na+ channels, preventing the propagation of action potentials and resulting in loss of sensation.

• Lidocaine: Commonly used by dentists to block nerve conduction during procedures.

• Mechanism: Inhibits Na+ channels, making post-synaptic neurons unable to propagate action potentials.

Additional info: The notes above expand on the original slides by providing definitions, equations, and clinical context for action potentials and their propagation in neurons.