Detailed Notes on Membrane Potentials and Action Potentials in Neurons

Basic Principles of Electricity

  • Opposite charges attract each other, requiring energy to separate them across a membrane.
  • Energy is released when charges move toward one another.
  • Separated charges represent potential energy, measured as voltage/charge.

Membrane Potentials

  • Neurons have a resting membrane potential (RMP) that can rapidly change.
  • Neurons are highly excitable and can modify their resting membrane potential.

Current

  • Current is the flow of electrical charge (ions) and is used to perform work.
  • The flow of current depends on voltage and resistance.
  • Ions flow across the plasma membrane through various means:
    • Osmosis
    • Active transport
    • Facilitated transport
    • Diffusion

Ion Channels

  • Large proteins that serve as selective ion channels in the membrane:
    • Leakage Channels (Nongated): Always open.
    • Gated Channels: Change shape to open/close.
      1. Chemically Gated: Open with a specific chemical binding (e.g., neurotransmitter).
      2. Voltage-Gated: Open in response to changes in membrane potential.
      3. Mechanically Gated: Open in response to physical deformation of receptors.

Measuring Membrane Potential

  • The resting membrane potential of a typical neuron is approximately -70 mV.
  • The inside of the neuron is negatively charged relative to the outside.
  • Actual resting potential varies from -40 mV to -90 mV.
  • Determined by:
    • Ionic composition differences between intracellular fluid (ICF) and extracellular fluid (ECF).
    • Plasma membrane permeability to different ions.

Generating the Resting Membrane Potential

  • RMP is influenced by concentrations of K+ and Na+.
    • Higher Na+ outside the cell (140 mM) and higher K+ inside (140 mM).
    • Na+-K+ Pump maintains concentration gradients across the membrane.
  • K+ predominantly influences the membrane potential due to its permeability.
Resting Membrane Potential Summary
  1. K+ channels leakage: K+ leaks out, creating a negative interior.
  2. Na+ influx slightly raises potential, stabilizing around -70 mV.
  3. Na+-K+ pumps compensate for K+ and Na+ leakage.

Alterations of Resting Membrane Potential

  • Depolarization: Decrease in membrane potential, moving toward zero, increasing impulse probability.
  • Hyperpolarization: Increase in membrane potential, becoming more negative, decreasing impulse probability.
  • Changes can result from ion concentration variations and membrane permeability changes.

Graded Potentials

  • Localized, short-lived changes in membrane potential proportional to stimulus intensity.
  • Occurs at dendrites and cell body.
  • Types:
    • Receptor Potential: In sensory receptors.
    • Postsynaptic Potential: In synapses.
  • Graded potentials are critical for initiating action potentials.

Action Potentials

  • Principal method of long-distance neural communication.
  • Brief reversal of membrane potential with a change of about 100 mV.
  • Action potentials do not decay like graded potentials.
  • Involves specific voltage-gated channels.
Stages of Action Potential Generation
  1. Resting State: All channels closed, maintaining RMP.
  2. Depolarization: Na+ channels open and Na+ rushes in, causing rapid depolarization to around +30 mV.
  3. Repolarization: Na+ channels inactivate, K+ channels open, leading to K+ exiting the cell and returning to RMP.
  4. Hyperpolarization: Some K+ channels remain open, making the inside more negative than at rest.

Propagation of Action Potentials

  • Propagation effects differ in myelinated (fast; saltatory conduction) and nonmyelinated axons (slow; continuous conduction).

Coding for Stimulus Intensity

  • Action potentials are identical in magnitude; frequency conveys strength.
  • Higher frequencies indicate stronger stimuli.

Refractory Periods

  • Absolute Refractory Period: No new action potential can occur, ensuring unidirectionality.
  • Relative Refractory Period: A stronger-than-normal stimulus is required to trigger an action potential.

Synapses

  • Synapses mediate information transfer between neurons.
  • Key Terms:
    • Presynaptic Neuron: Sends signals toward synapse.
    • Postsynaptic Neuron: Receives signals away from synapse.

Chemical Synapses

  • Most common, utilizing neurotransmitters across synaptic cleft.
  • Action potential causes release of neurotransmitters from presynaptic neuron, leading to graded potentials in postsynaptic neuron.

Neurotransmitters Overview

  • >50 neurotransmitters identified, affecting nervous system signaling.
  • Categories:
    • Acetylcholine (ACh): Role in muscle activation and memory.
    • Biogenic Amines: Includes dopamine, norepinephrine, and serotonin for various functions.
    • Amino Acids: Glutamate (excitatory), GABA (inhibitory).
    • Neuropeptides: Chains of amino acids with diverse physiological roles.

Summation and Integration

  • Neurons integrate inputs from multiple sources; can have excitatory or inhibitory effects.
  • EPSP (excitatory) vs. IPSP (inhibitory).

Neural Processing Patterns

  • Serial Processing: Input follows a single pathway (e.g., reflexes).
  • Parallel Processing: Input splits into multiple pathways, allowing for various processing outputs.

Developmental Aspects of Neurons

  • Neurons develop from the neural tube and interact with their environment via growth cones during development.
  • Axons find targets to form synapses, with many neurons undergoing apoptosis if they do not connect.
  • Lifetime synaptic changes are crucial for learning and memory formation.