Self And The Phenomenon Of Life: A Biologist Examines Life From Molecules To Humanity

Self and Memory 221

“9x6” b2726 Self and the Phenomenon of Life: A Biologist Examines Life from Molecules to Humanity

Fig. 10.4. Drawing of Aplysia showing the gill, the siphon, the head, and the tail.

Pairing of touching and electric shock (touch preceding shock) results
in classical conditioning (Pavlovian conditioning), wherein touch alone
without shock causes a strong gill withdrawal response. These behaviors
can be demonstrated not only in the intact animal, but also in isolated
neurons in tissue culture. Unlike habituation and sensitization, classical
conditioning is an associative learning, where two stimuli are closely cor-
related to achieve the behavioral outcome. The three types of learning
in Aplysia are summarized in Figure 10.5.
Kandel and his group found that habituation is correlated with a
decrease, whereas sensitization with an increase, in the release of neuro-
transmitters across the synapse. In the gill withdrawal reflex (Fig. 10.5),
the transmitter released by the sensory (touch) neuron “A” acting on the
motor neuron “B” is glutamate, whereas the neuro-modulatory neuron
“C” that conveys tail shock message releases serotonin on the pre-synap-
tic portion of the sensory neuron “A”. Note that the connection between
neuron C and A is “axon-axonic,”serving to regulate the activity of the