Why is Aplysia such a good model system?
- TheAplysianervous system is relatively simple (105 to 106 neurons as compared to ~1012 neurons in the human brain).
- Many of the nerve cell bodiesof the Aplysiaare relatively large (up to 1 mm for the largest neurons in the abdominal ganglion) facilitating many types of experiments.
- The large neurons are found in the same locations in the brains of all individuals, enabling scientists to find them easily during experiments that involve behavior, learning, and electrophysiology or molecular biology of the individual neurons.
- Aplysia is capable of a number of behaviors and of simple forms of learning.
Kandel and his co-workers (Kandel, 1979) were the first to show that simple behaviors such as habituation can be produced by defined changes in neurons over time. Habituation, perhaps the simplest form of nonassociative learning, is a progressive decrease in the strength of a behavioral response to a relatively weak natural stimulus of constant intensity when that stimulus is repeatedly presented. Habituation is not the result of sensory adaptation or motor fatigue, but represents a learned change in the animal's responsiveness to the stimulus. Sensitization, the mirror image process of habituation, is a slightly more complex form of learning; it is the enhancement of a reflex response to one stimulus as a result of the presentation of another, noxious, stimulus. Distinct forms of these learned responses, such as short and long term, are recognized in Aplysia (and many other animals).
The best characterized pathways for habituation and sensitization in Aplysia are those involving the siphon and gill withdrawal reflexes. Briefly, these reflexes involve sensory nerves, motor nerves and interneurons, all of which have cell bodies located in the abdominal ganglion. During habituation, the synaptic connections between the sensory nerves that sense stimulation of the tail and the motor neurons that contract the tail undergo a depression in activity due to a decrease in neurotransmitter release by the sensory neurons. In contrast, a sensitizing stimulus produces rapid facilitation (enhancement) of the same connections due to an increase in the amount of transmitter released per impulse. A simplified version of this pathway, involving just two cells, can be reconstituted in cell culture, permitting detailed analysis of the molecular changes involved in these forms of learning.
