Lecture on Neuron Communication

Introduction

• Final discussion on neuron communication: how neurons communicate and how target neurons process information.
• Terminology:
  • Synapse: Structure where communication between neurons occurs.
  • Presynaptic Neuron: Neuron sending information.
  • Postsynaptic Neuron: Neuron receiving information.
  • Terms are relative; a neuron can be both presynaptic and postsynaptic.

Types of Synapses

• Electrical Synapse:

  • Direct flow of action potential from presynaptic to postsynaptic neuron.
  • Very fast and high fidelity.
  • Useful for rapid and coordinated response (e.g., crayfish tail flip).
  • Relatively rare; found in certain reflexes.

• Chemical Synapse:

  • Involves release of neurotransmitters by presynaptic neuron.
  • Vastly more common than electrical synapses.
  • Neurotransmitters bind to receptors on postsynaptic neuron.

Chemical Synapse Mechanism

• At Rest:

  • Synaptic vesicles with neurotransmitters wait in the presynaptic terminal.

• Action Potential:

  • Travels down the axon to axon terminal.
  • Opens voltage-gated calcium channels.
  • Calcium influx triggers vesicle docking and neurotransmitter release.

• Neurotransmitter Release:

  • Neurotransmitters released into synaptic cleft.
  • Bind to postsynaptic receptors, causing changes in postsynaptic cell.
  • Neurotransmitters are cleared from the cleft to stop signaling.

Types of Receptors and Transmission

• Fast Transmission:

  • Involves ionotropic receptors (ligand-gated ion channels).
  • Excitatory Postsynaptic Potential (EPSP): Brings neuron closer to action potential threshold.
  • Inhibitory Postsynaptic Potential (IPSP): Moves neuron away from action potential threshold.

• Slow Transmission:

  • Involves metabotropic receptors.
  • Activate secondary messengers, affecting cell properties indirectly.

Example: Neuromuscular Junction

• Mechanism:
  • Motor neurons release acetylcholine.
  • Acetylcholine binds to nicotinic receptors on muscle fibers, causing contraction.
• Key Terms:
  • End Plate Potential: Postsynaptic potential in muscle fiber.
  • Motor Unit: A motor neuron and the muscle fibers it innervates.

Central Nervous System Synapses

• Excitatory Synapses:

  • Use glutamate neurotransmitter.
  • AMPA and Kainate Receptors: Similar to acetylcholine receptors, permeable to Na+ and K+.
  • NMDA Receptors: More complex, involve calcium permeability and voltage dependency.

• NMDA Receptor Dynamics:

  • Requires cofactor glycine and is blocked by magnesium at low voltages.
  • Allows Ca2+ entry at higher depolarizations, triggering more complex cellular processes.
  • Involvement in memory formation.

Summary

• Neuron communication involves complex interactions at synapses.
• Electrical synapses offer rapid, direct transmission, while chemical synapses offer modulated, versatile communication.
• Understanding synaptic processes is crucial for insights into nervous system function and reflexes.