tent elevation of glutamate neurotransmitter release onto the motor
neuron controlling gill withdrawal. And other genes transcribed
via this process facilitate growth of new synaptic connections to
the gill-withdrawal motor neuron, presumably via the action of
neurotrophins.
These studies of a simple behavior in Aplysia provide examples of
how precise molecular changes may be related to short-term and
long-term memory. Scaling this up to neuroplastic processes among
millions and billions of synaptic connections across multiple regions
of the human brain provides a rough sketch of an account that may
begin to explain the profound capacities of human memory.
As described in Chapter 18, sensory signals enter the cortex and
impact global patterns of activity throughout the cerebral neuropil.
The networks of neurons that are activated depend upon what has
been strengthened by previous activation—Hebbian networks. Thus,
all past experience plays an essential role in determining the patterns
of activation generated by the incoming sensory signals, and the pat-
terns of motor actions generated as a result.
Not only in the brain, but throughout the entire body, neural con-
nections are strengthened by repeated activation. In the autonomic
nervous system, the neuromuscular system, and the neural connec-
tions with the endocrine and immune systems, past experience plays
an essential role in determining present and future patterns of activ-
ity. This provides a substrate for knowledge and meaning; meaning is
constructed from the multitudes of linked associations that have been
acquired through the entirety of one’s past experiences. Meaning isa
whole-body experience, and its foundation is memory.
Many neuroscientists believe that scenarios similar to this will
somehow be sufficient to eventually account for all the amazing prop-