Question: I wondered if you could tell me anything about Aplysia [sea slug] and it's impact on our understanding of learning and memory?

Carolyn McGettigan answered on 25 Mar 2010:

Some of the earliest learning experiments were done by Pavlov. He had dogs chained up in his lab and he measured how much saliva they produced when given food. He was able to show that if he paired a certain sound with the presentation of food, over enough experience the dogs would eventually salivate just to the sound in the absence of the food. This is now referred to as the classical conditioning response, and the Aplysia sea slug also shows this. A light tactile stimulus that would normally not produce any defensive reflex in the slug is repeatedly paired with a strong noxious stimulus like an electric shock. After repeated pairings of these two events, the slug shows classic condition by exhibiting the defensive reflex to the light tactile stimulus. The reason why scientists find the slug so appealing to work with, even though it only shows this very simple type of conditioning, is that it has very large neurons (with cell bodies up to 1mm in diameter) and only about 20,000 of them in total. So, it’s a closed system that physiologists can really get stuck in to fully understanding.

A similar example is the use of squid giant axons back in the first half of the 20th Century, when Hodgkin and Huxley took advantage of these cells’ large diameter to work out the ionic mechanisms of action potentials (i.e. cellular excitation). They eventually won the Nobel Prize for this work.