The black objects are neurons the thin lines are axons and (more.) This drawing depicts a section through a small part of a mammalian brain-the olfactory bulb of a dog, stained by the Golgi technique. The complex organization of nerve cell connections. The precision required is not so great as in a man-made computer, for the brain performs its computations in a different way and is more tolerant of vagaries in individual components but the brain nevertheless outstrips all other biological structures in its organized complexity. The problem is formidable: the human brain contains more than 10 11 neurons, each of which, on average, has to make connections with a thousand others, according to a regular and predictable wiring plan. The central challenge of neural development is to explain how the axons and dendrites grow out, find their right partners, and synapse with them selectively to create a functional network ( Figure 21-89). A neuron is extraordinary above all for its enormously extended shape, with a long axon and branching dendrites connecting it through synapses to other cells ( Figure 21-88). Their structure is like that of no other class of cells, and the development of the nervous system poses problems that have no real parallel in other tissues.
Nerve cells, or neurons, are among the most ancient of all specialized animal cell types.
