Neuroscience Chapter on Action Potentials

Module 3 - Neural Physiology

• Module 3.1: Intercellular Communication Focus on Neural Communication
  • Covers the principles of neural communication between cells.
  • Discusses membrane potential, ion permeability, and concentration differences.
  • Explains graded potentials, action potentials, signal propagation along nerve fibers, synapses, and neuronal integration.
  • Key factors regarding membrane potential: separation of charge across a membrane; difference in the relative number/concentration of cations (+ ions) and anions (- ions) in the intracellular fluid (ICF) and extracellular fluid (ECF); difference in permeability of key ions.
• Module 3.2: The Peripheral Nervous System Focus on the Somatic System
  • Focuses on the somatic system in the Peripheral Nervous System.
  • Textbook references: Human Physiology, Nelson 4th edition, Chapter 2, pages 54-83, Chapter 3, page 92, Chapter 4, pages 141-142 and 187-203 (Specific page numbers may vary depending on the editions.)
  • Nelson 5th edition reference: Chapters 2-6, find the relevant sections to lecture content.

Module 3.1 - Learning Objectives

• Principles of neural communication between cells, including the concept of membrane potential, with separation of charges, ion permeability, and ion concentrations.
• Graded potentials, action potentials, signal propagation along nerve fibers, synapses and neuronal integration.

Module 3.1 - Membrane Resting Potential

• The plasma membranes of all living cells are electrically polarized.
• This separation of charge creates a membrane potential, which is the potential for ion movement across the membrane.
• Factors affecting membrane potential: difference in permeability of key ions and differences in concentrations of cations and anions inside and outside of the cell.

Module 3.1 - Ion Movement

• The movement of ions across a membrane is determined by concentration gradients (ions move from high to low concentration) and electrical gradients (opposites attract, similar charges repel).
• The combined effects of these gradients are termed the electrochemical gradient.
• Membrane permeability (restrictions to what passes across the membrane) determines the flow of ions across the cell membrane.

Module 3.1 - Example 1

• Membrane (maximal permeability), equal charges on both sides of membrane, no potential.

Module 3.1 - Example 2

• Membrane (maximal permeability), unequal charges on both sides of the membrane, a potential is generated.

Module 3.1 - Key Ions

• Sodium (Na+), potassium (K+), and negatively charged intracellular proteins (A-) are key ions regulating membrane potential in nerve and muscle cells.

Module 3.1 - Ion Concentrations in Resting Nerve Cells

• Important Considerations
  • The concentrations of extracellular and intracellular fluids of these ions.
  • Differences of the ions' relative permeability.
  • Sodium (Na+), potassium (K+), and intracellular proteins (A-).

Module 3.1 - How do K+ and Na+ cross?

• Ions like Na+ and K+ cannot cross the cell membrane, since they're water-soluble, therefore they must pass through protein channels.
• These channels are always open and very specific.

Module 3.1 - Action Potentials

• Very Different from Graded Potential
  • The membrane potential reverses during an action potential.
  • The inside becomes more positive than the outside, which is different from graded potential.
  • Action potential signaling is not decremental: signal is propagated throughout the entire membrane.

Module 3.1 - Action Potential Timeline

• Triggering Event: Initiates depolarization.
• Depolarization: Membrane potential becomes less negative, approaching 0.
• Threshold Potential: Reached when enough depolarization occurs to trigger an action potential.
• Rapid Depolarization: Inside of the cell becomes more positive (+30mV at peak).
• Repolarization: Inside of the cell returns to resting potential (less positive).
• Hyperpolarization: Inside of the cell becomes more negatively charged than resting potential .

Module 3.1 - Action potential generation

• Generating an action potential requires a sufficient magnitude of a graded potential that is large enough to reach the threshold potential.
• The magnitude and duration of the graded potential depend on the strength and duration of the triggering event such as a stimulus

Module 3.1 - What if initial depolarizing is lower than threshold

• Subthreshold: The positive feedback cycle does not start, so no action potential will occur.

Module 3.1 - What if initial depolarizing reaches the threshold

• Action potential: A rapid change in the membrane potential that results in an action potential.

Module 3.1 - All or None Law

• All action potentials have similar amplitude regardless of how long it took to reach the threshold.
• Once the threshold is met, it fires completely.

Module 3.1 - How do we integrate the magnitude of stimulus

• The strength of a stimulus is encoded by the frequency at which action potentials occur. A stronger stimulus produces action potentials at a higher frequency

Module 3.1 - Refractory Period and Action Potential

• Absolute refractory period: There cannot be another action potential generated during this time. During this period, the Na+ channels are inactive.
• Relative refractory period: There can be a second action potential generated, but only if the stimulus is larger than usual.

Module 3.1 - Restoration of Concentration Gradients

• After an action potential is completed, the resting membrane potential is restored through leak channels.
• However, the concentration of Na+ and K+ is altered.
• Restoring the original concentration of the ions occurs through the Na+/K+ ATPase pump.

Module 3.1 - The Sodium-Potassium Pump

• The pump counterbalances the rate of passive leakage of ions against the concentration gradient.
• When 2K+ are pumped back into the cell, 3Na+ are pumped out.
• This is a primary active transport process that requires ATP.
• This active transport of ions results in the transfer of 1 positive charge out of the cell for each ATP molecule that's hydrolyzed.

Module 3.1 - Synaptic Integration

• thousands of inputs contribute.

Module 3.1 - Two Key Events in Synaptic Integration

• Temporal summation: Multiple EPSPs occurring close together in time sum together.
• Spatial summation: Postsynaptic potentials occurring at different locations on the membrane sum together.
• Spatiotemporal summation: A combination of temporal and spatial summation.

Module 3.1 - Importance of Postsynaptic Integration

• The level of activity between presynaptic axons and post synaptic neurons can be influenced by other neurons and circuits.

Module 3.2 - The Central and Peripheral Nervous System

• The central nervous system (CNS) consists of the brain and spinal cord.
• The peripheral nervous system (PNS) connects the CNS to the rest of the body.

Module 3.2 - Nervous System Divisions

• Afferent division: Carries information from sensory receptors to the CNS.
• Efferent division: Carries information from the CNS to muscles/glands (effectors).
• somatic nervous system (voluntary)
• autonomic nervous system (involuntary; sympathetic; parasympathetic)

Module 3.2 - Autonomic Nervous System; Key Structure; Purpose

• Ganglion: A cluster of neuronal cell bodies outside the CNS.
• Purpose: Coordinates and relays signals from various neurons.

Module 3.2 - Pathways

• Each autonomic neural pathway consists of a two-neuron chain.

Module 3.2 - Varicosity

• In autonomic pathways, postganglionic autonomic fibers end in terminal branches with numerous swellings called varicosities.
• These release neurotransmitters over large areas of target organs rather than single cells.

Module 3.2 - Sympathetic and Parasympathetic Nervous Systems

• Sympathetic system (thoracic and lumbar regions): short preganglionic fibers; long postganglionic fibers.
• Parasympathetic system (brain and sacral/lower spinal cord): long preganglionic fibers, short postganglionic fibers; ganglia in or near effector organs.

Module 3.2 - The Adrenal Medulla

• Modified sympathetic ganglion; no axons.
• Releases secretions (epinephrine and norepinephrine) into the blood.

Module 3.2 - Cholinergic Receptors

• Found on postganglionic cell bodies in all autonomic ganglia.
• Nicotinic receptors: ion channels that cause depolarization.
• Muscarinic receptors: metabotropic receptors with diverse effects.

Module 3.2 - Adrenergic Receptors

• Coupled to G-proteins, have second messengers such as cyclic AMP and Ca2+.
• Beta receptors: usually have excitatory effects (e.g., heart stimulation, smooth muscle relaxation).
• Alpha receptors: can either have excitatory or inhibitory effects.

Module 3.2 - Drugs as Agonists or Antagonists

• Drugs can be agonists or antagonists to affect autonomic responses.
• Agonists mimic neurotransmitters; antagonists block neurotransmitters.

Module 3.2 - Comparison between Sympathetic and Parasympathetic Systems

• Similarities: Both use acetylcholine (ACh) at preganglionic synapses and have nicotinic receptors on the postganglionic cell bodies.
• Differences: Sympathetic neurons are generally thoracic and lumbar; parasympathetic originate in brain and sacral.
• Sympathetic has short preganglionic and long postganglionic fibers; parasympathetic has long preganglionic, short postganglionic
• Sympathetic has ganglia closer to the spinal cord; parasympathetic has ganglia near or on the effector organs..
• Neurotransmitters: Sympathetic postganglionic: norepinephrine or epinephrine; Parasympathetic postganglionic: acetylcholine

Module 3.2 - Somatic Nervous System

• Consists of motor neurons that innervate skeletal muscles.
• Signaling is continuous, one neuron from CNS directly to target organ; the neurotransmitter released is always acetylcholine (ACh).
• Neuromuscular junction: The axon terminal is enlarged (terminal button) and contacts a specialized portion on skeletal muscle (Motor end plate.)

Test your understanding of action potentials and graded potentials in this neuroscience quiz. Explore key concepts such as the refractory period, propagation, and the conditions required for generating action potentials in neurons.