Action Potential in the Neuron

Neuron Overview

• A neuron consists of four main parts:

  • Dendrites: Receive incoming signals.

  • Cell Body: Processes and integrates signals.

  • Axon: Transmits information along long distances.

  • Axon Terminal: Communicates signals to the next cell in the sequence.

• A bundle of axons traveling together is termed a nerve, which can be quite long to send messages over extended distances.

Signal Transmission in Neurons

• Dendrites collect signals, and based on the strength of stimulation, the cell determines whether to transmit the signal.

• If stimulation is sufficient, an action potential occurs, causing the neuron to "fire."

Ion Movement and Neuron Resting State

• Neuronal signaling relies on the movement of ions (charged particles).

  • Key ions include sodium (Na+), potassium (K+), and chloride (Cl-).

  • At rest:

    • Sodium Ions: Higher concentration outside the cell.

    • Potassium Ions: Higher concentration inside the cell.

• Electrochemical Gradient:

  • The charge difference across the membrane; at rest, the inside of the neuron is approximately -70 mV compared to the outside, which is more positive.

• The resting membrane potential is the state of electrochemical equilibrium, requiring ion channels for ion movement.

Membrane Potential Dynamics

• Ion Channels:

  • Allow ions to cross the membrane, enabling signal transmission.

  • Includes:

    • Voltage-Gated Channels: Open at specific membrane potentials.

    • Ligand-Gated Channels: Activated by binding of molecules.

    • Mechanically-Gated Channels: Open upon physical changes (e.g., pressure).

• Ion channels are selectively permeable, permitting specific ions to pass.

Graded Potentials vs. Action Potentials

• When ions move through channels, they can change the membrane potential:

  • A small change is termed a graded potential.

  • If the membrane potential reaches threshold voltage (-55 mV), an action potential is triggered at the axon hillock.

• Sodium-Potassium Pump: Resets ion concentrations using ATP to restore resting potential by moving 3 Na+ out and 2 K+ in.

Action Potential Mechanics

• Initiation and Propagation:

  • Begins when a threshold potential is reached; sodium channels open allowing Na+ influx, leading to depolarization (potential rises above 0).

  • Potassium channels later open, resulting in repolarization (K+ leaves the cell), sometimes overshooting the resting potential, causing hyperpolarization.

• Refractory Periods:

  • Absolute Refractory Period: No new action potential can be generated, regardless of stimulation strength.

  • Relative Refractory Period: A stronger stimulation is required to generate a new action potential because the neuron is still hyperpolarized.

Action Potential Characteristics

• The amplitude of action potentials is invariant (all-or-nothing response).

• Neuronal firing frequency can increase in response to stronger stimuli (e.g., intense pain).

Conduction Velocity

• Myelin sheaths increase the speed of action potential propagation through saltatory conduction, jumping across nodes (Nodes of Ranvier).

• Myelin in the peripheral nervous system is formed by Schwann cells; in the central nervous system, by oligodendrocytes.

Summary of Neuronal Action

• Initially at resting potential, a small stimulus generates a graded potential.

• A strong enough stimulus triggers an action potential, resulting in neuron firing.