OF THE MANY CHANGES that mark the conversion of ape to human, the most conspicuous and far-reaching is the increase in brain size, which has tripled in a few million years. This increase is responsible for our greatly enhanced intellectual abilities.1 It is time to take a look at these most remarkable of all eukaryotic cells, the neurons, and at the rules that govern their assembly.
A neuron is essentially a miniature receiver-transmitter device. It has a cell body, which possesses a nucleus, a cytomembrane network, cytoskeletal and cytomotor elements, organelles, and all the other characteristic attributes of animal cells. The cell body takes care of all the housekeeping functions necessary for cellular life; it is the combined power, maintenance, and repair unit. Thin filamentous extensions form the receiver and transmitter parts. These extensions can be exceedingly long, up to three feet in humans, thirty feet in a whale. The transmitter consists of a single fibril, the axon, which usually branches into terminal ramifications only in the neighborhood of its target. The receiver part is most commonly made of bushy arborescences called dendrites (from the Greek dendron, tree). A neuron acts as a one-way relay, from dendrite to axon. If a dendrite is disturbed, physically or chemically, the axon fires, in most instances by discharging some specific chemical, called a neurotransmitter. This chemical, in turn, sets off a response in any cell displaying the appropriate receptors and connected to the discharging axon tip by a special junction called a synapse. Depending on the nature of the target cell, the response may be contraction (muscle cell), secretion (gland cell), or stimulation or inhibition of firing (another neuron).
Sending out extensions is a general property of eukaryotic cells. Such protrusions,