PS352 Lecture Notes: Learning in Invertebrates (2)

Assignments

• MCQ: 20% of module, 30 minutes, date TBA (likely live in class). Questions based on lecture material and tutorials.
• Exam: 80% of the grade. Both components (MCQ and Exam) must be completed.

Tutorials

• Sign up and select tutorial time.
• International students only need to attend three tutorials.
• International students must complete both the MCQ and Exam.

Learning in Invertebrates (2)

Last Lecture Review

• Different types of learning:
  • Habituation
  • Sensitization
  • Classical conditioning
• Introduced the Aplysia (sea slug).
• Aplysia can habituate, and its neural mechanism was discussed.
• This lecture focuses on sensitization and classical conditioning.

Sensitization

• Definition: Increase of a response due to the presentation of a novel, often noxious (harmful) stimulus.
• Example: Walking along a well-lit street at night, hearing dogs barking, cars. Suddenly, there is a complete blackout. A dog barks, a car passes, you become more edgy and jumpy; you become more sensitive.

Sensitization in Aplysia

• Scientifically: Squirt water on the siphon, the gill withdraws. Then, shock the tail. Now, continue to squirt water. The gill reflex is a lot stronger.

Cellular Mechanism of Sensitization

The process is divided into three phases:

• Normal reaction (Phase 1)
• During the shock (Phase 2)
• Post-shock (Phase 3)

Phase 1: Normal Reaction

• Sensory neuron connected to motor neuron.

Phase 2: At Shock

• Shock applied to the tail (L29 sensory neuron).
• Tail shock leads to the release of serotonin.
• Serotonin binds to 5HT receptors on the sensory neuron.
• Activation of G-protein and a second messenger.
• Serotonin closes K+ (potassium) channels.
  • Normal State: -65mV, Open Na^+ (Sodium channels), + ions go into the cell.
  • Now K+ (potassium channels) Open, + ions go out of the cell.
• If K^+ channels remain closed, it prolongs any action potential (AP).

Phase 3: Post-Shock

• Return to squirting water after the shock.
• Increased Ca++ (calcium) influx.
• K^+ channels remain closed, prolonging the action potential (AP).

Summary of Cellular Mechanisms

1. Serotonin is released.
2. Serotonin attaches to 5HT receptors on the sensory neuron.
3. K^+ channels close.
4. Action potential is prolonged.
5. More calcium comes into the pre-synaptic terminal.
6. More neurotransmitter is released.
7. Greater depolarization.
8. After a while, the K^+ channels open again.

Classical Conditioning

• Involves associating a stimulus that evokes a measurable response with a second stimulus that normally doesn’t evoke this response.
• Timing is critical.

Classical Conditioning in Aplysia

• Behavior: If you squirt water on the siphon, the gill withdraws. If you shock the tail and at the same time squirt water on the siphon, the gill reflex is a lot stronger.

Cellular Mechanism During Shock + Stimulation

• Calcium comes into the pre-synaptic terminal from the tail shock (L29).
• Serotonin is released.
• Activation of G-protein and a second messenger.
• K^+ channels close.

Post-Shock

• Even more K^+ channels are closed.
• Prolonged action potential (AP).

Summary of Cellular Events

At Shock

1. Serotonin released.
2. Attaches to 5HT receptors on the sensory neuron.
3. Closes K^+ channels.
4. Prolongs action potential (AP).
5. Calcium comes into the pre-synaptic terminal at the same time as the shock.
6. Causes more activation of the second messenger.
7. Causes more K^+ channels to close.
8. Prolongs the action potential even more.
9. Even more neurotransmitter released.
10. Even greater depolarization.

• After a while, the K^+ channels open again, but this takes a longer period of time.

References

• Pinel (2000) Biopsychology 4th Ed. Chapter 15, Pages 412-417.
• Kolb and Wishaw (2001) An introduction to brain and behaviour. Chapter 3. Pages 176-180.
• Kandel & Hawkins (1992). The biological basis of learning and individuality. Scientific American, Sept. 78-56.- Need to look up back issues in Library
• Own Chapters – Chapters 5 & 6