The segregation of inputs from the two eyes is only the first of a series of activity-dependent adjustments of visual connections, whose maintenance is extraordinarily sensitive to experience early in life. If, during a certain sensitive period (ending at about 5 years of age in humans), one eye is kept covered for a time so as to deprive it of visual stimulation, while the other eye is allowed normal stimulation, the deprived eye loses its synaptic connections to the cortex and becomes almost entirely, and irreversibly, blind. In accordance with what the firing rule would predict, a competition has occurred in which synapses in the visual cortex made by inactive axons are eliminated while synapses made by active axons are consolidated. In this way, cortical territory is allocated to axons that carry information and is not wasted on those that are silent.
Activity-dependent synaptic changes are not confined to early life. They also occur in the adult brain, where many synapses show both functional and morphological alterations with use. This synaptic plasticity is thought to have a fundamental role in learning and memory. Clearly, for the nervous system as for other parts of the body, developmental processes do not end at birth, as we discuss in the next chapter.
The development of the nervous system proceeds in four phases. First, neurons and glial cells are generated from dividing neural progenitor cells. Then, the newborn neurons send out axons and dendrites toward their targets. Next, they make synaptic connections with appropriate target cells so that communication can begin. Finally, excessive neurons are eliminated by normal neuronal cell death, after which the system of synaptic connections is refined and remodeled according to the pattern of electrical and synaptic activity in the neural network.
Neurons born at different times and places are specialized to express different sets of genes, and they have a cell memory that plays a major role in determining the connections they will form. Their specialization depends not only on spatial patterning by morphogens but also on intrinsic developmental programs that unfold as the neural progenitors proliferate. Axons and dendrites grow out from the neurons by means of growth cones, which follow specific pathways delineated by attractive and repellant signals along the way, including cell-surface and extracellular matrix molecules and soluble signal proteins to which growth cones from different classes of neurons respond differently. In many parts of the nervous system, neural maps are set up—orderly projections of one array of neurons onto another. In the retinotopic system, the map is based on the matching of complementary systems of position-specific cell-surface markers—ephrins and Eph receptors—possessed by the two sets of cells. Other cell-surface molecules such as DSCAM proteins in Drosophila and protocadherins in vertebrates mediate self-avoidance between the branches arising from a single neuron, helping the cell spread out its processes.
The formation of synapses involves back-and-forth signaling between target cells and the growth cone. After the growth cones have reached their targets and initial connections have formed, individual synapses are eliminated in some places and reinforced in others by mechanisms that depend on synaptic and electrical activity. These mechanisms adjust the architecture of the neural network according to the way in which it is used.
Which statements are true? Explain why or why not.
What regulates the pace of development? Why does a mouse embryo develop faster than a human embryo, for example?
What are the mechanisms that allow cell memory to be stored during development, explaining how each cell’s history determines its future behavior?
how do signals move through tissues? What are the roles of the extracellular matrix and of elongated cell projections?
how does a cell know exactly where it is in a multicellular organism? how does it know that its neighbors are the correct ones and that, if not, it should move or kill itself?
how do cells respond to tiny gradients of molecules in their environment, as required for knowing their positions? how are morphogen gradients reliably interpreted?
What are the genetic changes that allow the repurposing of existing body parts during evolution? For example, how did bat wings evolve from arms?
how do cells use genetic instructions to form the shape of something as complex as the human nose?