(1) Learning = Synaptic Growth

Genetic and developmental processes specify the connections among neurons—that is, which neurons form synaptic connections with which other neurons and when. But they do not specify the strength of those connections…Learning selects among a large repertoire of preexisting connections and alters the strength of a subset of those connections…The anatomy of the neural circuit is a simple example of Kantian a priori knowledge, while changes in the strength of particular connections in the neural circuit reflect the influence of experience.

The growth and maintenance of new synaptic terminals makes memory persist. Thus, if you remember anything of this book, it will be because your brain is slightly different after you have finished reading it.

Habituation weakens the synapse, whereas sensitization or classical conditioning strengthens it…The strength can be modified in two ways, depending on which of two neural circuits is activated by learning—a mediating circuit or a modulatory circuit…
The [modulatory circuit] activated by a shock to Aplysia’s tail release a neurotransmitter called serotonin…In fact, we could simulate the enhancement of synaptic strength, and the strengthening of [behavioral reflexes] simply by applying serotonin to the connections between the sensory and motor neurons.

The stage was set for a biochemical analysis of memory storage…But even a single Aplysia neuron contains thousands of different proteins and other molecules. Which of these molecules are responsible for short-term memory?...The sequence of biochemical reactions that I sought would have to…translate the brief action of serotonin into molecules whose signals would last for minutes within the sensory neuron. Second, those molecules would have to broadcast signals from the cell membrane, where serotonin acts…to the specialized regions of the axon terminal involved in the release of glutamate. We speculated on the possibility that a specific molecule known as cyclic AMP might be involved….