Electrophysiology of Neurons

Introduction to Neuronal Excitability

Neurons are specialized cells capable of responding to various stimuli and conducting electrical signals. Their ability to generate and propagate electrical changes is fundamental to nervous system function.

• Excitable (Responsive): Neurons respond to chemical signals, local electrical signals, and mechanical deformation.

• Conductivity: Neurons rapidly conduct electrical changes along the length of their plasma membrane.

• Two main forms of electrical changes in neurons:

  • Local Potentials: Travel short distances within the neuron.

  • Action Potentials: Travel the entire length of the axon.

Basic Principles of Electricity in Neurons

Membrane Potentials and Their Changes

Neuronal membranes maintain a voltage difference across their surface, known as the membrane potential. This is essential for the generation and propagation of electrical signals.

• Resting Membrane Potential: The baseline electrical charge across the neural membrane, typically about -70 mV. This is maintained by the sodium-potassium pump, which keeps more Na+ outside and K+ inside the cell.

• Depolarization: A reduction in membrane potential, making the inside of the cell more positive. This change can trigger an action potential.

• Hyperpolarization: The membrane potential becomes more negative than the resting potential, often due to the sluggish closing of Na+ gates and opening of K+ and Cl- gates. This inhibits the generation of action potentials.

Example: During an action potential, the membrane rapidly depolarizes and then repolarizes, followed by a brief hyperpolarization before returning to resting potential.

Principles of Electrophysiology

Ion Channels and Gradients

Ions move across the neuronal plasma membrane through specialized protein channels, which are crucial for generating electrical signals.

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

• Gated Channels: Open or close in response to specific stimuli.

• Types of Gated Channels:

  • Ligand-Gated Channels: Open in response to the binding of a chemical (ligand) to the channel or its associated receptor.

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

  • Mechanically Gated Channels: Open or close in response to mechanical stimulation such as stretch, pressure, or vibration.

Types of Ion Channels in Neurons and Other Excitable Cells

Ion channels are classified based on their structure, function, and the stimulus required for opening or closing.

Ion Movements and Local Potentials

Local potentials are small, graded changes in membrane potential that occur in response to stimulation. These changes can be depolarizing or hyperpolarizing, depending on the direction of ion movement.

• Depolarization: Positive ions enter the cell, making the membrane potential less negative.

• Hyperpolarization: Positive ions exit or negative ions enter, making the membrane potential more negative.

• Local potentials are reversible and decremental (they decrease in strength as they spread).

Example: A ligand-gated channel opens in response to neurotransmitter binding, allowing Na+ to enter and depolarize the membrane locally.

Summary Table: Ion Channel Types and Their Stimuli

Key Equations

• Nernst Equation: Used to calculate the equilibrium potential for a particular ion.

• Resting Membrane Potential: Determined by the relative permeability and concentration gradients of Na+, K+, and Cl- ions.

Additional info: The notes above are expanded with academic context to clarify definitions, mechanisms, and examples for college-level Anatomy & Physiology students. The tables are reconstructed from slide content and standard textbook knowledge.