Neuron Structure

Cell membrane: hydrophobic phospholipid bilayer

• Semi-permeable

  • Charged ions, polar molecules, and large molecules cannot pass freely

  • Ions require channels or pumps to pass into/out of cell

Resting state of neurons:

• Excess of Na+ outside the cell

• Excess of K+ inside the cell

• Inside is more negative than the outside (greater excess of Na+ than the excess of K+)

• Na+/K+ pump (sodium-potassium pump) → 3 Na+ for 2 K+

• Resting membrane potential = -70 mV (net negative charge)

• In resting state, both Na+ and K+ channels are closed

Neuron Signaling

Action potential: momentary reversal of membrane potential in response to a stimulus

1. Resting membrane potential: Na+ outside, K+ inside

2. Depolarization: Na+ flowing in, Na+ gates opened, inside becomes positive

3. Repolarization: K+ flowing out, K+ gates opened, inside becomes negative

4. Refractory period: after action potential, neuron is unresponsive to signal

• Ions are in the “wrong” place → Na+/K+ pump will restore resting membrane potential

Nerve impulse/neuron firing: series of action potentials propagated down the axon from dendrites to terminal branches (wave of action potentials)

• Neuron firing is all or none

  • Firing threshold: minimum stimulation for action potential to begin

  • Once firing threshold is reached, neuron will fire → action potential will trigger action potentials down the neuron

1. Resting membrane potential

2. Enough stimulation (until threshold)

3. Na+ and K+ flow → action potential

4. Action potentials “travel” down the axon = nerve signal

5. Refractory period, no action potential until sodium-potassium pump restores resting membrane potential

6. Repeat

To distinguish between strong and weak stimuli:

• More neurons fire for strong stimulus

• Increased rate of firing for strong stimulus

Saltatory Conduction: in myelinated axons, action potentials jump between nodes of Ranvier → results in faster signaling