Action Potentials & Ion Movement Review Guide

Review Guide for Action Potentials & Ion Movement

Introduction

• Study material focuses on the fundamental concepts of action potentials and ion movement within the context of neuronal function.

Key Terms and Definitions

1. Types of Body Fluids:

   • Extracellular fluid (ECF):

     • Also known as interstitial fluid and plasma.

   • Intracellular fluid (ICF):

   • Intravascular fluid: Contains blood plasma and is a component of extracellular fluid.

   • Illustrate and label a diagram showing these fluids in the human body.

Body Fluid Composition

2. Proportion of Total Body Weight (TBW):

   • Intracellular fluid (ICF): Approximately 60% of TBW

   • Extracellular fluid (ECF): Approximately 40% of TBW

   • Intravascular fluid: A subset of ECF, constituting about 5% of TBW.

Ion Distribution and Function

3. Predominant Cations:

   • ICF: Potassium ions (K+) are the predominant cation.

   • ECF: Sodium ions (Na+) are the predominant cation.

Neuron Structure and Function

4. Major Structures of a Neuron:

   • Cell Body (Soma): Contains the nucleus and organelles; responsible for metabolic activities.

   • Dendrites: Receive incoming signals and transfer them to the cell body.

   • Axon: Transmits electrical impulses (action potentials) away from the cell body.

   • Axon Terminals: Release neurotransmitters into the synaptic cleft to communicate with neighboring neurons.

Axons vs. Dendrites

5. Differences Between Axons and Dendrites:

   • Axons:

     • Single, long processes that conduct impulses away from the cell body.

     • Often covered in myelin, which speeds up transmission.

   • Dendrites:

     • Usually multiple, short processes that receive signals.

     • Unmyelinated, increasing surface area for connections.

Myelin and its Function

6. Myelin:

   • A fatty substance that insulates axons.

   • Primary Function: To increase the speed of action potential propagation through saltatory conduction.

Membrane Properties

7. Polarity of the Neuron Cell Membrane:

   • Refers to the difference in charge distribution across the membrane, resulting in a negative internal environment relative to the outside.

   • Important for generating action potentials.

8. Propagation in Axons:

   • The process by which an action potential is transmitted along the length of the axon.

   • Involves depolarization and repolarization phases that allow signals to travel rapidly.

Channel Types

9. Passive vs. Active Channels:

   • Passive channels:

     • Allow ions to move according to concentration gradients without energy input — also known as leak channels.

   • Active channels:

     • Require energy (ATP) to operate (e.g., sodium-potassium pump) and can open/close in response to stimuli.

10. Gated Channels Classification:

    • Voltage-gated channels: Open/close in response to changes in membrane potential (important for action potentials).

    • Ligand-gated channels: Open/close in response to neurotransmitter binding.

    • Mechanically-gated channels: Open/close in response to mechanical deformation (e.g., ion channels in the sensory neurons).

11. Sodium and Potassium Channels in Action Potential:

    • Found in the voltage-gated channel category.

    • Why: They are activated by changes in membrane potential during depolarization and repolarization.

Action Potential Mechanics

12. Key Terms around Action Potential (with ion movement direction):

    • Membrane Potential: The electrical potential difference across the membrane, influenced by various ions.

    • Action Potential: A rapid change in membrane potential that occurs when depolarization reaches the threshold level; characterized by a spike in voltage.

    • Resting Period (Resting Membrane Potential): Typically around -70 mV, maintained by ion distribution.*

    • Threshold: The critical level that must be reached for an action potential to occur, typically around -55 mV.

    • Depolarization (Rising phase): An influx of Na+ ions into the cell causes the membrane potential to become more positive.

    • Repolarization (Falling phase): An efflux of K+ ions out of the cell brings the membrane potential back down toward the resting state.

    • Hyperpolarization: Membrane potential becomes more negative than resting potential due to excess K+ outflow.

    • Refractory Period: There are two types:

      • Absolute Refractory Period: A period during which no new action potential can be generated regardless of stimulus strength.

      • Relative Refractory Period: A period following the absolute refractory period where a stronger-than-normal stimulus is required to elicit a new action potential.

13. All-or-None Phenomenon:

    • Describes how an action potential either occurs fully or not at all, meaning there is no partial action potential despite the strength of the stimulus.

Sodium-Potassium Pump

14. Sodium-Potassium Pump:

    • A critical active transport mechanism that moves sodium (Na+) out and potassium (K+) into the cell against their concentration gradients.

    • Importance: Maintains resting membrane potential and is essential for nerve impulse transmission.

15. Ion Movement by the Na+/K+ Pump:

    • Moves 3 Na+ ions out of the cell for every 2 K+ ions moved into the cell.

    • This results in a net loss of positive charge, contributing to the negative resting potential of the cell.

Electrolyte Imbalance and Action Potentials

16. Impact of Electrolyte Imbalance (Loss of Homeostasis):

    • An imbalance in Na+ and K+ can disrupt the generation and propagation of action potentials, potentially leading to neurological issues, muscle weakness, or cardiac disturbances.

    • Specific examples could include hypotonic or hypertonic solutions causing cellular swelling or shrinking, respectively; both can affect excitability and conductivity of neurons and muscle cells.