Understanding Neurophysiology and Action Potentials

Apr 21, 2025

Lecture Notes on Neurophysiology and Action Potentials

Introduction and Announcements

  • Weather: Nice weather around 68°F.
  • Upcoming Schedule:
    • Activities in class on Monday and Wednesday next week.
    • Exam review session style for the exam in a week from Monday.
    • Study guide will be posted this week.
    • Sunday session moved to Saturday, noon to 1 PM in Bio 256, also available on Zoom.
    • Quiz 7 due next week, recommended to complete it by this Friday.

Lecture Overview

  • Second of three lectures on neurophysiology.
  • Third lecture will be available on YouTube.

Neurophysiology Key Concepts

Action Potentials

  • Definition: A rapid electrical signal that travels along the axon when a neuron fires.
  • **Characteristics: **
    • All or nothing response.
    • Non-decremental (does not fade).
    • Not reversible once threshold is reached.

Important Terms

  • ECF (Extracellular Fluid): Higher sodium concentration.
  • ICF (Intracellular Fluid): Higher potassium concentration.
  • Axolemma: Membrane of a neuron.
  • Voltage-Gated Sodium Channel: Allows sodium to enter the cell via simple diffusion.
  • Potassium Channel: Allows potassium to exit the cell.
  • Sodium-Potassium Pump: Maintains resting membrane potential by pumping sodium out and potassium in.

Resting Membrane Potential

  • Concept: Due to open potassium leak channels.
  • Ion Distribution:
    • Sodium high outside, low inside.
    • Potassium high inside, low outside.

Nerve Impulse Propagation

  • Continuous Conduction: Unmyelinated axons, signal moves continuously.
  • Saltatory Conduction: Myelinated axons, signal jumps from node to node.

Factors Affecting Conduction Speed

  • Myelination increases speed.
  • Larger diameter increases speed.

Refractory Periods

  • Absolute Refractory Period: No new action potential can be initiated.
  • Relative Refractory Period: A stronger stimulus is needed to initiate another action potential.

Synaptic Transmission

  • Synapse Structure: Facilitates control of signal transmission.
  • Neurotransmitters:
    • Can be excitatory or inhibitory.
    • Examples: Acetylcholine (Cholinergic), Norepinephrine (Adinergic), Dopamine, Serotonin, etc.

Neurotransmitter Mechanism

  • Ionotropic: Directly alter membrane potential.
  • Metabotropic: Use second messenger systems like cyclic AMP.

Visualizing Action Potentials

  • Phases:
    1. Sodium ions depolarize the axon hillock.
    2. Depolarization reaches threshold, sodium channels open.
    3. Sodium rushes in, depolarizing the cell.
    4. Sodium channels close, potassium channels open.
    5. Potassium exits, repolarizing the cell.
    6. Slight hyperpolarization as potassium gates close slowly.
    7. Sodium-potassium pump restores resting potential.

Disorders Related to Myelination

  • Multiple Sclerosis (MS):
    • Autoimmune disease attacking myelin.
    • Leads to impaired muscle function and potential blindness.

Homework and Study Tips

  • Draw a somatic cell: Indicate sodium, potassium, and chloride concentrations.
  • Discuss ion movement: Voltage gates, lian gates, mechanical gates.
  • Study Action Potential Diagram: Understand each step thoroughly.
  • Revisit Lecture Video: For concept reinforcement.

Note: Ensure to attend SI and TA sessions for additional support.

Full transcript