Nervous system hard-wiring

First, computer circuits are hard-wired at the factory. A computer s circuitry never changes. In contrast, as we noted previously, brain circuitry is constantly adapting through a process of pruning and reestablishing connections. This pruning affords the nervous system a plasticity that enables it to adapt continually to the demands of an ever-changing environment. 

Classically, the central nervous system has been envisioned as a series of hard-wired synaptic connections between neurons, not unlike millions of telephone wires within thousands upon thousands of cables (Fig. 1—4). This idea has been referred to as the anatomically addressed nervous system. The anatomically addressed brain is thus a complex wiring diagram, ferrying electrical impulses to wherever the wire is plugged in (i.e., at a synapse). There are an estimated 100 billion neurons, which make over 100 trillion synapses, in a single human brain. 

The reader should now appreciate that chemical neurotransmission is the foundation of psychopharmacology. It has three dimensions, namely, space, time, and function. The spatial dimension is both that of hard wiring as the anatomically addressed nervous system and that of a chemical soup as the chemically addressed nervous system. The time dimension reveals that neurotransmission can be fast (milliseconds) or slow (up to several seconds) in onset, depending on the neurotransmitter or neuromodulator, of which there are dozens. Neurotransmission can also cause actions... 

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