The development of the nervous system poses problems that have little parallel in other tissues. A typical nerve cell, or neuron, has a structure unlike that of any other class of cells, with a long axon and branching dendrites, both of which make many synaptic connections to other cells (Figure 21–66). 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 neural network—an electrical signaling system—that functions correctly to guide behavior (Figure 21–67). The problem is formidable: the human brain contains more than 1011 neurons, each of which, on average, has to make connections with a thousand others, according to a regular and predictable wiring plan. The precision required is not so great as in a man-made computer, because the brain performs its computations in a different way and is more tolerant of vagaries in individual components. But the human brain nevertheless outstrips all other biological structures in its organized complexity.
The components of a typical nervous system—the various classes of neurons, glial cells, sensory cells, and muscles—originate in a number of widely separate locations in the embryo. Thus, in the first phase of neural development, the different parts of the nervous system develop according to their own local programs: neurons are born and assigned specific characters according to the place and time of their birth, under the control of inductive signals and transcription regulators, by mechanisms of the types we have already discussed. In the next phase, newborn neurons extend axons and dendrites along specific routes toward their target cells, guided by extracellular signals that attract or repel them. In the third phase, neurons form synapses with other neurons or muscle cells, setting up a provisional but orderly network of connections. In the final phase, which continues into adult life, the synaptic connections are adjusted and refined through mechanisms that usually depend on synaptic signaling between the cells involved terminal branches of axon make 25 µm synapses on target cells
Figure 21–66 A typical neuron of a vertebrate. the arrows indicate the direction in which signals are conveyed. the neuron shown is a basket cell, a type of neuron in the cerebellum. (adapted from
S. ramón y cajal, histologie du Système Nerveux de l’homme et des Vertébrés, 1909–1911. paris: Maloine; reprinted, Madrid: c.S.I.c., 1972.)
Figure 21–67 The complex organization of nerve cell connections. 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 black objects are neurons; the thin lines are axons and dendrites, through which the various sets of neurons are interconnected according to precise rules. (From c. Golgi, Riv. sper. freniat. Reggio-Emilia 1:405–425, 1875.) (Figure 21–68). At all stages, neurons are in intimate contact with various types of non-neuronal supporting cells—the glial cells.
Neurons are assigned Different characters according to the time and place of their Birth
We start our account here with the first phase of neural development: the generation of neural progenitors and their differentiation into hundreds of different neuronal subtypes, along with a much smaller number of glial types. Although the nervous system is exceptional in the extent of cell diversity, the process depends on the same principles that generate different cell types in other organs. We have already discussed some of the underlying machinery in the developing Drosophila nervous system. We turn now to vertebrates.
The vertebrate spinal cord, the brain, and the retina of the eye together constitute the central nervous system (CNS). They all originate as parts of the neural tube, whose formation was described earlier (see Figure 21–56). The brain and eyes develop from the anterior neural tube and the spinal cord from the posterior.