Self and Memory 223“9x6” b2726 Self and the Phenomenon of Life: A Biologist Examines Life from Molecules to Humanitya person who just experienced bombing. In the latter, the first stimulus
(light touch) must precede the second (shock) by a very short interval;
once established, light touch alone (not any other mild stimulus) is capa-
ble of eliciting a strong gill withdrawal (now a conditioned response).^11
As shown in Figure 10.6, classical conditioning incurs changes not
only in the nerve terminal but also in the cell nucleus. First, synaptic activ-
ity triggers a series of events called signal transduction, which transmits
the message to the nucleus to activate the gene, resulting in the syntheses
of new messenger RNA and proteins, the latter being transported back to
the same nerve terminal to strengthen the existing synapse and to make
more synapses as well.^11 At the same time, changes also occur in the post-
synaptic side in the receiving neuron, involving the glutamate receptors
Fig. 10.6. Diagram showing gene activation in the nerve terminal (pre-synaptic com-
ponent of a synapse) in Aplysia in the formation of long-term memory. During training,
cyclic AMP in the cytoplasm activates PKA (protein kinase A, a signal transducer). The
latter is then translocated to the nucleus to activate the transcription factor CREB-1,
which induces the immediate early genes. The products of these genes in turn induce
the late genes to produce proteins responsible for the permanent structural changes of
the synapses, including growth of new synaptic connections. In the meantime, PKA also
activates MAPK, another signal transducer, which inhibits the action of CREB-2, a sup-
pressor of CREB-1. The net effect is the activation of CREB-1. These changes are uni-
versal across species, both invertebrates and vertebrates. The abbreviations are: CREB,
cyclic AMP response element binding protein; MAPK, mitogen-activated protein kinase
(also known as MAP kinase). [See Note 11: Squire & Kandel.]