Nervous Tissue: Membrane Potential, Action Potential, and Neurotransmission

Introduction

This study guide covers the foundational concepts of nervous tissue physiology, focusing on the origin and significance of membrane potentials, the mechanisms underlying resting and action potentials, and the propagation of nerve impulses. Understanding these principles is essential for comprehending how neurons communicate and how the nervous system functions as a whole.

Membrane Potential

Definition and Significance

• Membrane potential refers to the difference in electrical charge across the plasma membrane of a cell.

• This potential is essential for the function of excitable cells such as neurons and muscle cells.

• The resting membrane potential is the voltage difference across the membrane of an unstimulated cell, typically ranging from -10 mV to -100 mV (negative inside relative to outside).

• Typical values: fat cells (–40 mV), neurons (–70 mV), skeletal muscle cells (–85 mV), cardiac muscle cells (–90 mV).

Key Point: The membrane potential is a form of potential energy resulting from the separation of positive and negative charges across the cell membrane.

Establishment of Resting Membrane Potential

• The extracellular fluid (ECF) is rich in Na+ and Cl–, while the intracellular fluid (cytosol) contains high concentrations of K+ and negatively charged proteins.

• Ions cannot freely cross the lipid bilayer; they move through ion channels (membrane proteins specific for certain ions).

• Types of ion channels include sodium (Na+), potassium (K+), calcium (Ca2+), and chloride (Cl–) channels.

• The sodium-potassium pump (Na+/K+ ATPase) actively transports 3 Na+ out and 2 K+ into the cell, maintaining the concentration gradients and contributing to the negative resting potential.

Equation:

Ion Channels and Membrane Permeability

• Leak channels: Always open, allowing passive movement of ions (especially K+ and Na+).

• Gated channels: Open or close in response to specific stimuli. Types include:

  • Chemically (ligand)-gated channels: Open when a specific chemical binds (e.g., neurotransmitter at synapse).

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

  • Mechanically gated channels: Open in response to physical deformation (e.g., touch, pressure).

Current, Resistance, and Membrane Potential Changes

• Current: Movement of charges (ions) to eliminate a potential difference.

• Resistance: Measure of how difficult it is for ions to cross the membrane. High resistance = low current; low resistance = high current.

• Opening ion channels decreases resistance and increases current, allowing membrane potential to change.

Graded (Local) Potentials

Definition and Properties

• Graded potentials are small, localized changes in membrane potential that occur in response to a stimulus.

• They decrease in intensity with distance from the site of stimulation ("local").

• Opening of Na+ channels causes depolarization (membrane potential becomes less negative).

• Opening of K+ channels causes hyperpolarization (membrane potential becomes more negative).

• When the stimulus is removed, the membrane returns to resting potential (repolarization).

Example: A neurotransmitter binds to a ligand-gated Na+ channel, causing Na+ influx and a local depolarization.

Action Potentials

Definition and All-or-None Principle

• An action potential is a rapid, large change in membrane potential that propagates along the axon of a neuron.

• It is triggered when a graded potential depolarizes the membrane to a threshold value (typically around –60 mV).

• All-or-none principle: If threshold is reached, an action potential of fixed size occurs; if not, no action potential is generated.

Phases of the Action Potential

1. Resting state: Membrane at –70 mV; voltage-gated channels closed.

2. Depolarization: Voltage-gated Na+ channels open; Na+ influx causes membrane potential to rise to +30 mV.

3. Repolarization: Na+ channels inactivate; voltage-gated K+ channels open; K+ efflux returns membrane toward negative potential.

4. Hyperpolarization: K+ channels close slowly, causing membrane potential to become more negative than resting (e.g., –90 mV).

5. Return to resting potential: Na+/K+ pump and leak channels restore resting state.

Equation (Nernst Equation for equilibrium potential):

Additional info: The Nernst equation calculates the equilibrium potential for a specific ion based on its concentration gradient.

The Refractory Period

• The refractory period is the time during which a neuron cannot fire another action potential or requires a stronger stimulus to do so.

• Ensures unidirectional propagation of the action potential.

• Analogy: Like a toilet that cannot be flushed again until the tank refills.

Propagation of Action Potentials

Continuous vs. Saltatory Propagation

• Continuous propagation: Occurs in unmyelinated axons; action potential moves along every segment of the membrane (slow, ~1 m/s).

• Saltatory propagation: Occurs in myelinated axons; action potential "jumps" from node to node (nodes of Ranvier), greatly increasing speed (up to 100 m/s).

• Myelin acts as an insulator, preventing ion flow across the membrane except at the nodes.

Myelination and Axon Diameter

• Myelinated axons conduct impulses faster than unmyelinated axons.

• Larger diameter axons conduct impulses faster due to lower internal resistance.

• Classification of axons:

  • Type A fibers: Largest, myelinated, 4–20 μm diameter, 120 m/s (sensory/motor to skeletal muscle).

  • Type B fibers: Smaller, myelinated, 2–4 μm diameter, 18 m/s (visceral sensory/motor).

  • Type C fibers: Smallest, unmyelinated, <2 μm diameter, 1 m/s (pain, temperature, autonomic).

Additional info: Only about one-third of all axons carrying sensory information are myelinated; most sensory information arrives over slender Type C fibers.

Summary Table: Key Differences

Practice Questions

• What is the membrane potential?

• What would happen to the membrane potential if the plasma membrane were freely permeable to sodium ions?

• What is the resting membrane potential?

• What effect would blocking voltage-gated sodium or potassium channels have on a neuron?

• How does myelination and axon diameter affect propagation speed?

• Which axon is myelinated: one propagating at 50 m/s or one at 1 m/s?