Professor Long's Lecture on Nervous System: Lecture 5

Disclaimer

• Lecture conducted during coronavirus shutdown.
• Recorded at home with possible background noises (kids, dogs).
• Videos are approximately 15–25 minutes long.

Overview

• This lecture is part of a series on the nervous system for Human Anatomy and Physiology (Biology 2401).
• Topics covered so far:
  • Overview of the nervous system and its subdivisions (CNS, PNS).
  • Anatomy and morphology of neurons.
  • Ion channels and resting membrane potential.
  • Action potentials.

Multipolar Neurons and Action Potentials

• Multipolar Neuron Anatomy:

  • Axon with synaptic knobs containing synaptic vesicles.
  • Neurotransmitters are released upon action potential arrival.

• Mechanism of Action Potential:

  • Action potential opens voltage-gated calcium channels.
  • Calcium influx leads to neurotransmitter release.
  • Neurotransmitters cross synaptic cleft and bind to receptors on the next neuron.
  • This can lead to depolarization or hyperpolarization.

Synaptic Transmission

• The synapse allows communication between neurons using neurotransmitters.
• Enzymes in the synaptic cleft can break down neurotransmitters to end the signal.

Action Potential Propagation

• Voltage-Gated Channels:

  • Sodium and potassium channels essential for action potentials.
  • Propagation occurs as the action potential stimulates successive patches of membrane.

• Summation:

  • Neurons receive mixed excitatory and inhibitory signals.
  • Action potentials fire when excitatory signals reach threshold.

Temporal and Spatial Summation

• Temporal Summation:

  • A single synapse fires rapidly over time.
  • Continuous neurotransmitter presence holds sodium channels open.

• Spatial Summation:

  • Multiple synapses fire simultaneously in a small area.
  • Enough sodium influx to reach threshold.

Facilitation

• Continuous stimulation makes neurons more likely to fire.
• Substances like caffeine can lower threshold, leading to increased excitability.

Action Potential Velocity

• Two types of action potential propagation:

  • Continuous Conduction:

    • Unmyelinated axons, each patch of membrane must be stimulated.
    • Slower propagation.

  • Saltatory Conduction:

    • Myelinated axons, action potentials jump between nodes of Ranvier.
    • Faster propagation.

Myelin and Nodes of Ranvier

• Myelin:

  • Fatty substance produced by Schwann cells.
  • Forms myelin sheath around axons.

• Nodes of Ranvier:

  • Gaps in myelin where action potentials regenerate.

Enhancing Action Potential Velocity

• Larger axon diameter results in faster propagation.
• Myelination increases velocity.

Upcoming Topics

• Synaptic activity and glial cells in subsequent lectures.