Neurons

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

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