NRS 250 Final - The Neuron

What is axoplasmic transport and why is it needed? 

• Axoplasmic transport is the movement of materials (organelles, proteins, vesicles) within the axon. 

• It moves materials from the soma to the axon terminal (anterograde) and back (retrograde). 

• This transport is vital because proteins and organelles are made in the soma and need to reach distant axon terminals. 

• Damaged materials are also returned to the soma for recycling. 

• Microtubules act as tracks, and motor proteins like kinesin (anterograde) and dynein (retrograde) use ATP to move cargo along them

What drives ions across the cell membrane? 

• Concentration gradients: Ions move from high to low concentration (diffusion). 

• Electrical gradients: Ions move toward areas of opposite electrical charge. 

• These two combined forces form the electrochemical gradient, which determines the net direction of ion movement. 

• Ions can only move when their specific ion channels are open

What does the Nernst potential calculate? 

• It calculates the equilibrium potential (voltage) for a specific ion. 

• This is the voltage where the electrical gradient exactly balances the concentration gradient, so there is no net ion movement. 

• For example, for Na⁺, it's the point where diffusion inward is exactly offset by the electrical push outward. 

How do the Nernst potentials (for different ions) relate to the membrane potential? 

• Each ion has a different Nernst potential due to its unique intra- and extracellular concentrations. 

• The membrane potential is not equal to any single ion's Nernst potential unless only that ion is permeable. 

• In real neurons (with multiple permeable ions), the resting membrane potential is a weighted average—closer to ions with higher permeability (e.g., usually K⁺). 

What is the Na⁺/K⁺ ATPase? What is its role? 

• The Na⁺/K⁺ ATPase (or pump) is a membrane protein that uses ATP to transport 3 Na⁺ out and 2 K⁺ into the cell. 

• It maintains the Na⁺ and K⁺ concentration gradients required for resting potential and action potentials. 

• It’s electrogenic: it moves more positive charges out than in, contributing directly to the membrane potential. 

• It compensates for passive ion leaks and uses 25–50% of the nervous system’s energy.