Aplysia Research

Habituation and sensitization have been studied in a variety of contexts and
in a number of organisms, from simple protozoans (such as Stentor) to human
subjects. Such studies have focused on the adaptive significance of
these simple learning processes, their neurological control, and the range
of behavioral responses that result from interaction between these two
forms of learning.
One particular organism in which the neurological basis of habituation
and sensitization has been extensively studied is the marine slug Aplysia. Eric
Kandel and his associates at Columbia University showed that when the
mantle of this organism is prodded, the slug quickly withdraws its gills into a
central cavity. After repeated prodding, it learns to ignore the stimulus; that
is, it becomes habituated. Conversely, when the slug is stimulated with an
electric shock, its sensitivity to prodding increases greatly, and it withdraws
its gills in response to even the slightest tactile stimulation (that is, it becomes
sensitized).
Because Aplysia possesses only a few, large neurons, it is an excellent organism
in which to study the physiological basis of learning. Capitalizing on
this unique system, Kandel and his colleagues have been able to establish
the neurological changes that accompany simple forms of learning. In the
case of habituation, they have shown that repeated stimulation interferes
with calcium ion channels in the nerve which, under normal circumstances,
causes synaptic vesicles to release neurotransmitters, which in turn relay a
nervous impulse between two neurons. Thus, habituation results in a blocking of the chemical signals between nerves and thereby prevents gill withdrawal.
When Aplysia is stimulated (or sensitized) by an electric shock, an interneuron
(a closed nerve circuit contained within one part of the nervous system)
stimulates the sensory neuron by opening calcium ion channels,
increasing neurotransmitter production, and promoting gill withdrawal.
Thus, the proximate neurological changes that take place during sensitization
and habituation are nearly opposite, but they are achieved by very different
neurological circuits. 375