Habituation vs. Sensitization

Definition: Habituation is a learning behavior characterized by the weakening of a response to a stimulus with repeated presentations of that stimulus.
- Conceptual Understanding: Over time, individuals become less responsive or "ignorant" to stimuli due to chronic exposure.

Decreased Calcium Influx:
- During habituation, there is a reduction in the influx of calcium ions ( $Ca^{2+}$ ) in response to an action potential.
Reduced Neurotransmitter Release:
- This decrease leads to less neurotransmitter being released at the presynaptic membrane, reducing the signaling.
Lowered Postsynaptic Response:
- Consequently, there is less depolarization of the postsynaptic membrane, leading to diminished neural response.

Conclusion of Habituation: The withdrawal response weakens with repeated presentations of stimuli, such as a water jet. This is due to decreased calcium influx and less neurotransmitter release from the presynaptic axon terminal.

Question: What happens to the gill response after repeated stimulation?
- Procedure:
1. A siphon sensory neuron is stimulated by a water jet, causing a gill withdrawal response.
2. Upon repeated exposure to the water jet, the gill no longer withdraws, demonstrating habituation.
Findings: Less neurotransmitter is released from a habituated neuron, indicated by:
- Reduced sensitivity of $Ca^{2+}$ channels.
- Decreased neurotransmitter (nt) release with continued stimulation.

Definition: Sensitization is a learning behavior wherein the response to a stimulus strengthens with repeated presentations due to the novelty of the stimulus or stress.

Sensitization is considered the opposite of habituation.\
- When an animal is stressed, responses to stimuli become heightened.

Procedure:
1. The gill withdrawal is tested again with the application of a water jet, followed by a shock to the tail.
Results: A single shock to the tail enhances the gill-withdrawal response (indicative of sensitization).
- An interneuron, which receives input from a shocked sensory neuron in the tail, releases serotonin onto the axon of a siphon sensory neuron.

Serotonin's Role:
- Serotonin's actions lead to the inactivation of non-gated potassium channels ( $K^{+}$ channels).
- This modification changes the resting potential of the neuron.
Prolonged Action Potential:
- The inactivation results in more $Ca^{2+}$ influx, causing an increase in neurotransmitter release.
  - As a result, there is greater depolarization of the postsynaptic membrane after sensitization.

Conclusion of Sensitization: The enhancement of the withdrawal response after a shock is due to increased $Ca^{2+}$ influx and the subsequent increase in neurotransmitter release from the presynaptic axon terminal.

Types of Learning:
- Distinction between Short Term Learning and Long Term Learning.
- Repeated stimulation can produce both habituation and sensitization that may persist for extended periods (e.g., months).
Changes in Synapse Number:
- The number of sensory synapses within habituated and sensitized states of the organism (Aplysia) shows the following:
- Habituated: Fewer synapses present.
- Sensitized: Increased synapses present due to enhanced responsiveness.

Observation on Motor Neuron Response:

The motor neuron response remains consistent in both short term and long term memory concerning total neurotransmitter amounts.

Casual remarks noted in the transcript:
- Interaction about a screen protector indicating transitions to informal discussion, noting the effort taken in writing or using the new screen without specific scientific relevance.