Neuronal Communication and Action Potentials

Overview

• Neurons communicate via dendrites and axons.
• Incoming signals: received at dendrites.
• Outgoing signals: travel along the axon to nerve terminal.
• Rapid communication via electrical signals (action potentials).

Action Potentials

• Definition: brief reversal of electric polarity across the cell membrane.
• Resting membrane potential: ~-70 millivolts (negative inside the cell).
• Maintained by sodium-potassium pump (Na+ out, K+ in).
• Sodium (Na+): higher outside; Potassium (K+): higher inside.

Generation of Action Potentials

• Stimulus at dendrites opens ligand-gated sodium channels.
• Sodium influx reduces negative charge (depolarization).
• Current travels to axon hillock (trigger zone).
• Axon hillock: high concentration of voltage-gated ion channels.
• Threshold: ~-55 millivolts to generate action potential.

Mechanism

• At threshold:
  • Na+ channels open quickly.
  • K+ channels open slowly.
• Rising phase: Na+ influx makes inside more positive.
• Peak: Na+ channels close, K+ channels fully open.
• Falling phase: K+ efflux returns voltage to resting potential.
• Hyperpolarization: K+ overshoots due to slow channel closure.
• Resting potential restored by diffusion and sodium-potassium pump.

Refractory Period

• Absolute refractory period:
  • From start of action potential to first return to resting voltage.
  • Na+ channels inactivated and cannot respond to new stimuli.
• Relative refractory period:
  • Until end of hyperpolarization.
  • K+ channels still open, requiring stronger signal for new action potential.

Propagation of Action Potentials

• Na+ influx at one point spreads and depolarizes adjacent membrane.
• Unidirectional propagation due to refractory properties.
• Typically travels from axon hillock to nerve terminal.
• Higher concentration of ion channels in axon compared to cell body.