Synaptic varicosities are bulges or swellings along the axon of a neuron. These varicosities contain neurotransmitter vesicles, which are small sacs that store and release neurotransmitters. In the context of Aplysia sensitization, synaptic varicosities play a crucial role in the strengthening of the synaptic connection between sensory neurons and motor neurons.
Here's how it works:
1. Sensitization: When an Aplysia experiences a noxious stimulus (like a shock), it undergoes a sensitization response. This involves a long-lasting increase in the strength of the gill withdrawal reflex.
2. Signal transduction: The noxious stimulus triggers a cascade of intracellular events, including the release of serotonin from interneurons onto the sensory neuron.
3. Increased neurotransmitter release: Serotonin binds to G-protein coupled receptors on the sensory neuron, activating a signaling pathway that leads to the phosphorylation of CREB (cAMP response element-binding protein).
4. Synaptic plasticity: Phosphorylated CREB promotes the synthesis of new proteins, including synapsin and actin. These proteins contribute to the growth of new synaptic varicosities along the sensory neuron's axon.
5. Enhanced transmission: The increased number of varicosities results in more neurotransmitter release from the sensory neuron onto the motor neuron, strengthening the synapse and causing a more robust gill withdrawal reflex.
In summary, the growth of new synaptic varicosities in Aplysia sensitization is a key mechanism underlying the long-term potentiation of the gill withdrawal reflex. This process provides a clear example of how changes in synaptic structure can lead to changes in neuronal function and behavior.
Note: The formation of new synaptic varicosities is just one aspect of the complex molecular and cellular changes occurring during Aplysia sensitization. Other factors, such as changes in presynaptic calcium influx and postsynaptic receptor density, also contribute to the increased synaptic strength.
