ABSTRACT
The marine snail, Aplysia, has served as a model invertebrate for studying the biochemistry that underpins procedural learning. Ten repetitions of the tactile stimulus causes the withdrawal reflex to be progressively attenuated and this habituation lasts for several minutes. Because sensitization involves strengthening of the synapse between the sensory and motor neuron by activity in a separate, modulatory synapse it is called heterosynaptic facilitation. If sensitization is done after administering drugs which inhibit the synthesis of messenger ribonucleic acid or proteins it does not persist beyond about 3 hours. Transcription of other proteins responsible for sprouting and growth of the sensory neuron axon, and the formation of new synapses, are seen in long-term sensitization. As well as habituation and sensitization, Aplysia is capable of the more complicated associative learning. The cellular mechanism relies on the tail shock setting up the usual sensitization cascade that results in elevated neurotransmitter release from the sensory terminals.
