Electrical Properties of Neurons

Resting Potential

Steady difference between charge inside and outside of the cell

Relies on leak channels

Reflects both electrochemical driving forces and channel permeability

Depends on two factors:

• Chemical

• Electrical

Concentration Gradient

Maintaining the resting potential requires maintaining the concentration differences within and outside the cell

Maintained by the sodium-potassium pump

Positive Ions

Leave neuron: Charge becomes negative

Enter neuron: Charge becomes positive

Negative Ions

Leave neuron: Charge becomes positive

Enter neuron: Charge becomes negative

Ionotropic Receptors

Ligand gated channels open directly when neurotransmitters bind

Effects on the cell are fast but only last for a short amount of time

Metabotropic Receptor

Use G-proteins to trigger internal signaling cascades

Effects on the cell are slower but have longer ranging effects

Depolarization

Cell becomes more positive (excitatory)

Hyperpolarization

Cell becomes more negative (inhibitory)

Passive Flow

Electricity flows both toward and away from the axon

Signals decay over time and space due to:

• Resistance to diffusion within the neuron

• Resting ion channels counteracting changes in charge

Effects are stronger when closer to the axon

Action Potential

Mediated by voltage-gated channels

Generated where voltage-gated channels are highly concentrated

Three Phases:

• Rapid Depolarization (Sodium channels open quickly)

• Delayed hyperpolarization (Potassium channels open slowly)

• Return to resting potential (channels reset)

Massive Depolarization

Happens when the neuron reaches about -45 mV which causes the sodium channels to open

Absolute Refractory Period

Sodium channels begin to inactivate (close) as the neuron is driven closer to the resting potential

Temporary Hyperpolarization

Potassium channels start to close near the resting potential of the neuron

Role of Myelin

Increases membrane thickness —> Reduces capacitance —> speeds up current flow

Without this, axons would need to be wider to maintain similar speeds

Nodes of Ranvier

Unmyelinated gaps where the action potential slows down and regenerates via voltage-gated channels (Saltatory Conduction)