of growth-promoting and growth-inhibiting signals provides the growing tip of the axon with a map of connectivity to get to the right location and connect with the right target cell (Chilton, 2006; Tessier-Lavigne and Goodman, 1996).

Dendritic growth and branching begins early in development, initially proceeding slowly but then accelerating rapidly starting in the third trimester (de Graaf-Peters and Hadders-Algra, 2006), producing a thickening of the cortex (the complex, multilayered collection of cells composing the entire outer surface of the brain) (Huisman, Martin, et al., 2002). The timing of dendritic growth differs by brain region and by layer of the cortex. For example, dendritic elaboration is slower in frontal than in visual cortex, and it begins in the deeper layers earlier than in more superficial ones (Becker, Armstrong, et al., 1984; de Graaf-Peters and Hadders-Algra, 2006; Huttenlocher, 1990; Michel and Garey, 1984; Mrzljak, Uylings, et al., 1992). Overall, dendritic development is highly active from the third trimester of gestation through the first postnatal year, continuing at lower rates through age 5 years (de Graaf-Peters and Hadders-Algra, 2006).

Differing neuronal cell types have diverse shapes and sizes. Some have relatively simple shapes. Others have many axonal branches, allowing them to innervate and influence more target cells. Some have complex dendritic trees that provide a greater range of input from other cells. This diversity of form and structure provides for a range of computational functions across different kinds of neurons, from a limited signal input and response to a complex integration of multiple signals. Connections are established with cells that are nearby and cells that are much more distant, eventually linking and integrating information from different regions of the brain.

The basis of this communication between neurons is their physical connection, called a synapse. The formation of synapses requires the development of specialized cellular machinery on both the presynaptic side of the synapse (where neurotransmitters are prepared and released from the terminals of axons) and at the postsynaptic target (where receptors for those neurotransmitters receive and process the signal) (Waites, Craig, and Garner, 2005). The rate of synapse formation increases rapidly after about weeks 24-28 and peaks, at the rate of almost 40,000 new synapses per second, between 3 and 15 months after birth (in the primary sensory and prefrontal cortices, respectively) (de Graaf-Peters and Hadders-Algra, 2006; Levitt, 2003).

The synapse is the primary site of information transfer in the nervous system, and it is also likely to be the primary site of learning and memory. Several disorders that begin early in life and are associated with profound intellectual and emotional disability can be considered disturbances of learning and memory. These include fragile X and other causes of mental



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