Electrical properties

Types of Neuronal Electrical Signals

Part 1: Receptor Potential

  • Example: Touch on skin

  • Typical Recording:

    • Initial potential: -50mV

    • Recording potentials fluctuate toward -55mV, -60mV.

Part 2: Synaptic Potential

  • Activation of Synapse

    • Stimulation decreases potential to -65mV.

    • Recording fluctuates around -70mV.

Part 3: Action Potential

  • Motor Neuron Activation

    • Rapid depolarization to +40mV found upon stimulation.

    • Recording displays fluctuations transitioning from -60mV.

Recording Passive and Active Electrical Signals

  • Equipment Used:

    • Ground electrode, amplifier, oscilloscope for digital display.

    • Microelectrode setup for current injection and membrane potential measurement.

Passive Membrane Properties

  • Small current injections yield small changes in membrane potential.

  • Once current injection stops, membrane potential returns to resting level, indicating passive recovery.

Passive Current Flow in Axon

  • Key Observation:

    • Passive conduction shows decay over distance.

  • Recordings of potential changes across 1mm show decreasing values down the axon, indicating loss of potential over distance.

Graded Potentials

  • Characteristics:

    • Result from both hyperpolarizing and depolarizing stimuli.

    • Hyperpolarization increases negative charge; depolarization can occur without generating action potentials.

Excitable Membranes

  • Composition:

    • Extracellular fluid, ion pumps, channel proteins, phospholipid bilayer.

Movement of Ions - Diffusion

  • Definition:

    • Ions distribute evenly through solution, flowing down their concentration gradient when specific channels are permeable.

Electrochemical Equilibrium

  • Equilibrium Potential (Eion):

    • Achieved when no net ion movement occurs, typically due to balance by ionic concentration gradients and electrical potential differences.

  • Key Ion Channel Example: K+ channels play a critical role in stabilizing equilibrium.

Membrane Potential Influences on Ion Fluxes

  • Battery Analogy:

    • Various conditions yield different net fluxes of K+ based on voltmeter readings, influencing potential across the membrane.

Nernst Equation - Equilibrium Potential

  • Function:

    • Predicts membrane potential at electrochemical equilibrium for one specific permeant ion.

  • Variables:

    • Includes gas constant, absolute temperature, z (valence), Faraday constant, ion concentration on both sides.

Applying the Nernst Equation

  • Example using K+ ion with discrepancies in concentrations illustrates net flux versus no net flux conditions when equilibrium is reached.

Role of Active Transporters and Ion Channels

  • Ionic Movement:

    • Active transporters work against concentration gradients, forming ion gradients necessary for neuron function.

    • Ion channels allow selective ion passage, contributing to membrane potential stability.

Summary of Nernst Equilibrium Potential

  • Total membrane potential results from combined effects of multiple ions - K+, Na+, Cl-, etc. - affected by relative permeabilities.

Resting Membrane Potential - Goldman Equation

  • Concept:

    • The Goldman equation incorporates the relative permeability of different ionic species when calculating resting potential.

Ion Concentrations Across Membrane

  • Key Findings:

    • [Na+] is much higher outside than inside.

    • [K+] is much higher inside than outside, contributing to the resting potential.

Molecular Basis of Resting Membrane Potential

  • Na+/K+ Transporter Mechanism:

    • Moves 2 K+ ions into the cell and 3 Na+ ions out, resulting in an inside-negative charge.

    • K+ leak channels participate in maintaining resting potential by allowing K+ to diffuse outwards.

Summary of Passive Properties of Membrane

  • The resting membrane potential is primarily influenced by K+ permeability due to selective channels, leading to a significant K+ concentration gradient maintained by transporters.

Key Concepts

Resting and Action Potentials

  • Resting Potential: Determined by K+ concentration gradient across the membrane.

  • Action Potential: Initiated by Na+ influx and maintained by K+ efflux.

Ion Channel Dynamics

  • Voltage-Gated Channels: Responsible for ion permeability changes during an action potential.

  • Na+ Channels: Open rapidly upon depolarization; responsible for the early phase of the action potential.

  • K+ Channels: Open more slowly; involved in repolarization.

Saltatory Conduction

  • Myelinated axons use Nodes of Ranvier for faster conduction.

  • Myelin: Increases action potential speed by reducing capacitance and increasing resistance.

Experimental Methods

  • Voltage Clamp: Measures ionic currents by controlling membrane potential.

  • Patch Clamp: Records ionic currents through single channels.

Ion Channel Disorders (Channelopathies)

  1. Epilepsy: Mutations in SCN1A affecting Na+ channels.

  2. Myotonia: Impaired muscle relaxation due to Cl⁻ channel mutations.

  3. Episodic Ataxia: Dysfunction in K+ or Ca²⁺ channels causing motor coordination issues.

  4. Congenital Stationary Night Blindness: Genetic defects affecting photoreceptor signaling.

Ion Pumps and Transporters

  • Na+/K+ Pump: Maintains ion gradients by active transport.

  • Ca²⁺ Pump: Regulates intracellular calcium levels.

Active Recall Questions

Resting and Action Potentials

  1. What determines the resting membrane potential in nerve cells?

    • Answer: The K+ concentration gradient across the membrane.

  2. What ions are primarily involved in generating an action potential?

    • Answer: Na+ (influx) and K+ (efflux).

Ion Channel Dynamics

  1. What is the role of voltage-gated Na+ channels in an action potential?

    • Answer: They open rapidly upon depolarization, initiating the early phase.

  2. How do K+ channels contribute to action potentials?

    • Answer: They open slowly to facilitate repolarization.

Saltatory Conduction

  1. How does myelin increase action potential conduction speed?

    • Answer: By reducing capacitance and increasing membrane resistance.

  2. What are Nodes of Ranvier?

    • Answer: Gaps in myelin where action potentials are regenerated.

Experimental Methods

  1. What does the voltage clamp method measure?

    • Answer: Ionic currents by controlling the membrane potential.

  2. What does the patch clamp technique record?

    • Answer: Ionic currents through single ion channels.

Channelopathies

  1. What mutation is commonly associated with epilepsy?

    • Answer: Mutations in the SCN1A gene (affecting Na+ channels).

  2. What is the primary defect in myotonia?

    • Answer: Impaired muscle relaxation due to Cl⁻ channel mutations.

Ion Pumps and Transporters

  1. What is the primary function of the Na+/K+ pump?

    • Answer: To maintain ion gradients by actively transporting Na+ out and K+ into the cell.

  2. How does the Ca²⁺ pump contribute to cellular homeostasis?

    • Answer: By regulating intracellular calcium levels.