dendrites and cell body and receives signals from many synapses simultaneously, both excitatory and inhibitory. These signals often amount to a rough balance; it is only when the net potential of the membrane in one region shifts significantly up or down from the resting level that a particular neurotransmitter can be said to be exerting an effect. Interestingly, in the membrane's overall balance sheet, the importance of a particular synapse varies with its proximity to where the axon leaves the nerve cell body, so that numerous excitatory potentials out at the ends of the dendrites may be overruled by several inhibitory potentials closer to the soma. Other kinds of synapse regulate the release of neurotransmitters into the synaptic cleft, where they go on to affect the postsynaptic channels as described above.
The list of known neurotransmitters, once thought to be quite short, continues to grow as more substances are found to be synthesized by neurons, contained in presynaptic boutons, and bound on the postsynaptic membrane by specific receptors. Despite stringent requirements for identifying a substance as a neurotransmitter (see Chapter 5 ), well over two dozen have been so named, and another several dozen strong candidates are under review.
The most cursory look at the human brain can excite awe at its complex functions, the intricacy of its structure, and the innumerable connections all maintained on microscopic fibers a few millionths of a meter in diameter. But a slightly more intimate acquaintance with this 3-pound organ inside our heads, an acquaintance that builds on observation of the brain in action and discovery of the principles by which it works, can yield something more satisfying than awe: the sense of mastery and of rewarded curiosity that comes with understanding. With the rewarding of curiosity as our goal, let us take a closer look at a few aspects of the functioning brain.