Excitable Cells

Excitable Cells

Overview

  • Excitable Cells: Cells capable of generating and transmitting electrical signals in response to a stimulus.

  • Major types:

    • Neurons (nerve cells)

    • Myocytes (muscle cells)

  • Excitability (Electrophysiology): Ability of a cell to receive (detect a stimulus) and send (transduce) that stimulus into a chemical electrical signal, resulting in rapid changes in membrane potential due to voltage-gated ion channels.

Learning Objectives

8.1 - Characterization of Excitable Cells

  • Describe cell types, including their structures and functions.

    • Neuron Anatomy:

    • Dendrites: Projections from the cell body for signal input, contain ligand-gated channels.

    • Soma (Cell body): Contains nucleus, organelles, and majority of cytoplasm.

    • Axon: Long process for the action potential to travel, contains voltage-gated channels.

    • Axon Hillock: Trigger zone for action potential generation.

    • Axon Terminal: Site of signal output.

  • Myocyte Types:

    • Skeletal Myocytes: Contraction via neuromuscular response, assessed by Electromyography (EMG).

    • Cardiac Myocytes: Monitored by Electrocardiogram (ECG), resting heart rate ranges from 60–100 beats per minute.

8.2 - Basis of Membrane Potential

  • Membrane Potential: Form of electrical potential energy created by the uneven distribution of ions across a cell's plasma membrane.

    • Ions: Cations (positively charged) and anions (negatively charged).

    • Major ions include:

      • Sodium (Na⁺), Potassium (K⁺), Chloride (Cl⁻), Calcium (Ca²⁺).

    • Changes in Membrane Permeability: Affect the movement of ions (efflux and influx), leading to physiological changes in the cell.

  • Concentration Gradient: Difference in ion concentration across cellular compartments, calculated as: extGradient=[extIon]<em>extoutside−[extIon]</em>extinsideext{Gradient} = [ ext{Ion}]<em>{ ext{outside}} - [ ext{Ion}]</em>{ ext{inside}}

    • Typical Ion Concentrations:

    • K⁺: 5mM (outside) vs. 140mM (inside)

    • Na⁺: 150mM (outside) vs. 15mM (inside)

    • Cl⁻: 120mM (outside) vs. 10mM (inside)

    • Ca²⁺: 0.008mM (outside) vs. 5mM (inside)

8.3 - Factors Contributing to Resting Membrane Potential

  • Active transport mechanisms (e.g., Na⁺/K⁺ ATPase) maintain concentration gradients, requiring ATP.

  • Leakage Channels: Passive ion channels that help maintain ion homeostasis and resting state of the cell.

    • At resting membrane potential, net ion flux is zero.

8.4 - Graded Potentials vs. Action Potentials

  • Graded Potentials:

    • Short/localized changes in membrane potential that are subthreshold and degrade rapidly.

    • Important for initiating action potentials, typically occur at dendrites.

  • Action Potentials:

    • Long-distance signals characterized by significant changes in membrane potential that can reach depolarization threshold.

    • Self-propagating and do not diminish in strength over distance.

Action Potential Phases

  • 1. Resting Membrane Phase: Maintained by Na+/K+ pumps.

  • 2. Stimulus Phase: Release of neurotransmitters (e.g., Acetylcholine) leads to local depolarization.

  • 3. Depolarization Phase: If threshold is reached, voltage-gated Na⁺ channels open.

  • 4. Repolarization Phase: Potassium efflux begins and voltage-gated K⁺ channels open.

  • 5. Hyperpolarization Phase: Continued opening of K⁺ channels leads to increased negativity.

  • 6. After-Hyperpolarization Phase: Restoration of the resting membrane potential.

Clinical Considerations: Neurotoxins

  • Tetrodotoxin: Produced by pufferfish, blocks Na⁺ channels, inhibiting action potentials.

  • Dendrotoxin: From black mamba, blocks K⁺ channels, prolonging action potentials and increasing neurotransmitter release.

  • Botulinum Toxin: Inhibits Acetylcholine release, leading to flaccid muscle paralysis.

  • Tetanus Toxin: Blocks GABA release, causing continual muscle contractions.

  • Batrachotoxin: Opens Na⁺ channels irreversibly, leading to uncontrolled muscle contractions and paralysis.

8.6 - Action Potential Propagation

  • Propagation involves sodium channel activation across segments of the axon:

    • Factors Affecting Propagation:

    • Axon Diameter: Larger diameters increase speed.

    • Myelination: Myelin sheaths provide insulation, speeding up conduction.

8.7 - Chemical and Electrical Synapses

  • Chemical Synapses: Involves neurotransmitter release into synaptic cleft.

    • Requires time for release and binding, modulated responses.

  • Electrical Synapses: Direct ion flow through gap junctions, facilitating rapid signaling with no latency.

Additional Equations

  • Nernst Equation: E<em>ion=rac61.5mVzimesextlog</em>10rac[extion]<em>extoutside[extion]</em>extinsideE<em>{ion} = rac{61.5 mV}{z} imes ext{log}</em>{10} rac{[ ext{ion}]<em>{ ext{outside}}}{[ ext{ion}]</em>{ ext{inside}}}

    • Used to calculate the equilibrium potential for an ion based on its concentrations.

  • Parallel Conductance Equation and Goldman-Hodgkin-Katz Equation: Used for calculating membrane permeability based on multiple ions.

Important Notes

  • Equilibrium potential differs from membrane potential, as the latter also depends on ion permeability.

  • Membrane potential behavior reflects dynamic interactions between ion movements and channel activities.