Cellular Electricity and Electrophysiology

Introduction to Electrophysiology

Electrophysiology is the study of electrical charges across plasma membranes and the physiological processes that accompany these charges. Understanding how cells generate and propagate electrical signals is fundamental to the function of the nervous and muscular systems.

• Electrophysiology: The study of electrical properties of biological cells and tissues.

• Cellular electricity is essential for nervous system conduction (in nerve cells) and muscle contraction (in muscle cells).

• Electrical signals are produced by the movement of ions across the cell membrane.

Ion Channels

Types and Functions of Ion Channels

Ion channels are specialized protein structures embedded in the cell membrane that allow the passage of hydrophilic charged molecules (ions) which cannot cross the phospholipid bilayer directly. These channels are crucial for maintaining and changing the membrane potential.

• Ions: Hydrophilic charged molecules (e.g., Na+, K+, Ca2+, Cl-).

• Ion channels: Integral membrane proteins that facilitate ion diffusion across the plasma membrane.

• Types of ion channels:

  • Non-gated (leak) channels: Always open, allowing ions to move down their concentration gradient.

  • Gated channels: Open and close in response to specific stimuli. Three main types:

    • Voltage-gated channels: Respond to changes in membrane potential.

    • Ligand-gated channels: Respond to binding of specific chemicals (ligands), such as neurotransmitters.

    • Mechanically-gated channels: Respond to mechanical stimulation (e.g., pressure, stretch).

Resting Membrane Potential (RMP)

Definition and Mechanism

The resting membrane potential (RMP) is the difference in electrical potential across the cell membrane when the cell is not actively sending signals. It is typically measured as -70 mV in neurons, indicating that the inside of the cell is more negative than the outside.

• Resting Membrane Potential (RMP): The voltage difference across the cell membrane at rest, usually around -70 mV in neurons.

• Created by unequal distribution of ions:

  • Excess Na+ ions in the extracellular fluid (ECF).

  • Excess K+ ions in the intracellular fluid (ICF).

• Maintained by:

  • Passive diffusion through leak channels (K+ out, Na+ in).

  • Active transport via the Na+/K+ ATPase pump (3 Na+ out, 2 K+ in).

Electrochemical Gradient

The movement of ions is driven by the electrochemical gradient, which combines the chemical (concentration) gradient and the electrical gradient.

• Chemical gradient: Ions move from areas of high concentration to low concentration.

• Electrical gradient: Ions are attracted to areas of opposite charge.

• At equilibrium, no net movement of ions occurs.

Equation for Resting Membrane Potential

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

Where: Eion = equilibrium potential for the ion R = universal gas constant T = temperature in Kelvin z = charge of the ion F = Faraday's constant [ion]outside = concentration of ion outside the cell [ion]inside = concentration of ion inside the cell

Graded (Local) Potentials

Definition and Characteristics

Graded potentials are short-range changes in membrane potential that occur in response to stimulation, typically at dendrites or the cell body of a neuron. They are local, brief, and reversible.

• Graded (local) potentials: Small, local changes in membrane potential due to the opening of ligand-gated ion channels.

• Can be depolarizing (less negative, excitatory) or hyperpolarizing (more negative, inhibitory).

• Magnitude varies with the strength of the stimulus and the number of channels opened.

• Decremental: The potential decreases with distance from the site of stimulation.

• Reversible: Membrane potential returns to RMP when the stimulus is removed.

Mechanism of Graded Potentials

• Opening of ligand-gated Na+ channels causes Na+ to enter the cell, leading to depolarization.

• Opening of ligand-gated K+ channels causes K+ to leave the cell, leading to hyperpolarization.

• Depolarization is excitatory; hyperpolarization is inhibitory.

Key Terms

• Depolarization: Movement of membrane potential toward 0 mV (less negative).

• Repolarization: Return of membrane potential to RMP after depolarization.

• Hyperpolarization: Movement of membrane potential to a more negative value than RMP.

Effects of Graded Potentials

Role in Neuronal and Muscular Activity

Graded potentials play a critical role in initiating neuronal events and muscle contraction. They are responsible for the transmission of electrical signals and nerve impulses.

• At synapses between neurons: Neurotransmitters bind to ligand-gated channels, causing graded potentials in the postsynaptic neuron.

• At neuromuscular junctions: Neurotransmitters (e.g., acetylcholine) bind to ligand-gated channels on muscle fibers, leading to muscle contraction.

Summary Table: Types of Ion Channels

Summary Table: Key Ions in Membrane Potential

Summary

• Cell membranes produce electrical signals via ion movement through channels.

• Resting membrane potential is maintained by leak channels and the Na+/K+ ATPase pump.

• Graded potentials are local, brief, and reversible changes in membrane potential, crucial for neuronal signaling.

• Types of ion channels include non-gated, voltage-gated, ligand-gated, and mechanically-gated channels.

Additional info: Action potentials (AP) are mentioned in the learning outcomes but not covered in detail in the provided materials. For completeness, students should study the mechanism and phases of action potentials in further readings.