Answer: Aplysia is a species of sea slug used as a model organism for the exploration of molecular mechanisms in neuroscience research.
Article written by Khayla Black
Sea slugs have been used extensively in the field of neuroscience to explore molecular mechanisms of processes such as learning and memory. Aplysias have characteristically large neurons, and their simple nervous systems contain approximately 20,000 neurons as opposed to the 100 billion in the human nervous system. These characteristics make them the ideal model organism for research.
Molecular mechanisms are generally studied utilizing the siphon-gill withdrawal reflex in Aplysia. Sensory neurons from the siphon skin synapse with motor neurons of the gill muscle. Additionally, facilitator neurons (or interneurons), from the sensitizing stimulus synapse with the sensory neuron just before its synapse with the motor neuron.
Utilizing the siphon-gill withdrawal reflex, scientists have gained tremendous insight to two short term learning processes, habituation and sensitization.
Habituation occurs when when, upon repeated stimulation, intensity of reaction in the muscle decreases, though the stimulus remains the same. For example, repeated touch of the siphon has shown to lead to a decrease in response to action potentials in the sensory neuron. This is a consequence of a decrease in the number of voltage gated calcium channels which open upon the arrival of the action potential. Thus, less glutamate will be released at the synapse, leading to a weaker stimulation of the motor neuron.
Sensitization, however, leads to an increased response after stimulation. This is where the interneuron (facilitator neuron) that was discussed earlier comes into play. Upon activation, the facilitator neuron will release serotonin, which sets off the molecular cascade characterizing this learning process. Serotonin leads to the production of cAMP, which activates Protein Kinase A (PKA), which phosphorylates K+, leading to the closing of these K+ channels. When these channels close, the action potential is prolongues, more calcium is released, and more glutamate becomes available. This leads to an increased response, precisely what is expected for a sensitizing stimulus.
A great deal is known about the molecular basis of these two properties, especially in comparison to other aspects of learning and memory. Habituation and sensitization are just two examples of the ways in which Aplysia have transformed the field of neuroscience; many scientists are still utilizing Aplysia as their model organism in hopes of making similar contributions in the future.