FoundationalConceptsNeuroscience

Hebbian network.
We now know these neuroplastic processes are taking place all the
time in the brain. Several scenarios for the strengthening of chemical
synapses are discussed in Chapter 10: increasing the amount of neu-
rotransmitter released from the presynaptic axon terminal, decreas-
ing the efficacy of neurotransmitter reuptake by reducing the number
of reuptake transporters, and increasing the number of postsynaptic
neurotransmitter receptors—these are all ways by which the signal
strength between two neurons is enhanced. Doing the opposite of any
of these things would decrease the strength of the synapse.
Detailed investigations of cellular and molecular changes that may
underlie memory formation have been carried out by Eric Kandel (b.
1929) and colleagues, using an animal with a very simple nervous sys-
tem and repertoire of behaviors: Aplysia californica, the sea hare or sea
slug. Years of study of this marine mollusk have led to a description
of molecular mechanisms related to the short-term and long-term
strengthening of synapses regulating the reflexive behavior of gill
withdrawal.
Aplysia possesses a respiratory gill attached to a structure called a
siphon (Fig. 19.3). If the siphon is touched, sensory neurons send sig-
nals that rapidly retract the gill into the mantle cavity, protecting the
delicate gill from potential injury.