12.4 The Action Potential

Neurons communicate through electrical signals called action potentials. These rapid changes in membrane voltage allow information to zip along nerve cells. Understanding how neurons generate and transmit these signals is key to grasping how our nervous system functions.

Action potentials rely on the movement of ions across cell membranes. By exploring ion channels, resting potentials, and the steps of an action potential, we can see how neurons create and propagate these crucial electrical messages throughout the body.

Membrane Potential and Action Potentials

Ion channels and resting potential

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• Resting membrane potential is the difference in electrical charge across a neuron's membrane when not conducting an impulse, typically around -70 mV with the inside more negative than the outside
• Ion concentrations inside and outside the cell contribute to the resting potential, with high K+ concentration inside and high Na+ concentration outside
• Ion channels allow specific ions to move across the membrane
  • K+ leak channels enable K+ to diffuse out of the cell along its concentration gradient
  • Na+ leak channels permit some Na+ to enter the cell
• Na+/K+ pump actively transports ions to maintain concentration gradients, pumping 3 Na+ out and 2 K+ into the cell for each ATP consumed, helping maintain the resting potential

Sequence of action potential generation

1. Depolarization occurs when a stimulus causes the membrane potential to become less negative, triggering an action potential if it reaches the threshold potential (around -55 mV)
2. Rising phase: Voltage-gated Na+ channels open, allowing rapid Na+ influx, causing the membrane potential to peak around +30 mV
3. Falling phase: Voltage-gated Na+ channels inactivate and close, while voltage-gated K+ channels open, allowing K+ efflux and membrane potential to return towards resting level
4. Afterhyperpolarization: Voltage-gated K+ channels remain open, briefly causing the membrane potential to become more negative than the resting potential
5. Refractory periods:
   • Absolute refractory period: Na+ channels are inactivated, preventing another action potential
   • Relative refractory period: Na+ channels have partially recovered, requiring a stronger stimulus to trigger an action potential

Continuous vs saltatory conduction

• Continuous conduction in unmyelinated axons:
  • Action potential propagates continuously along the axon membrane, with depolarization of one segment causing depolarization of the adjacent segment
  • Slower conduction velocity compared to saltatory conduction
• Saltatory conduction in myelinated axons:
  • Myelin sheath insulates the axon, preventing ion flow across the membrane
  • Action potentials occur only at nodes of Ranvier (gaps in myelin sheath)
  • Depolarization at one node triggers depolarization at the next, causing the action potential to "jump" from node to node (action potential propagation)
  • Faster conduction velocity due to reduced membrane capacitance and increased membrane resistance
• Advantages of saltatory conduction:
  • Faster velocity allows rapid transmission over long distances (spinal cord)
  • Reduces energy requirements for action potential propagation

Synaptic Transmission and Neurotransmitter Release

• Action potentials arriving at the axon terminal trigger neurotransmitter release
• Voltage-gated Ca2+ channels open, allowing Ca2+ influx
• Increased intracellular Ca2+ causes synaptic vesicles to fuse with the presynaptic membrane
• Neurotransmitters are released into the synaptic cleft
• Neurotransmitters bind to receptors on the postsynaptic membrane, potentially initiating a new action potential
• Membrane excitability of the postsynaptic neuron determines its response to neurotransmitter binding