Synaptic Development

1. Increases in Dendritic Arborization and Axon Density

One major process in brain development is the establishment of synaptic interconnection between individual neurons or between a neuron and its target cell. After the neuroblasts have divided and been transformed into neurons, the neurons, in order to serve as functional units of the nervous tissue, develop by increasing in size. This increase in cell size manifests itself in an increase in the size of the main cell body as well as in the size and length of the cellular processes—the axons and the dendrites. This in turn results in increases in dendritic arborizaion and axon density. The axons continue to grow in length until they find and form synapses with their target membranes. A target membrane may be the cell soma, dendrite, or the axon of another neuron (the axosomatic, axodendritic, and axoaxonal synapses) within the CNS, or it may be the plasma membrane of an excitable cell such as muscle cell (neuromuscular junction) in the peripheral nervous system. Thus, an increase in axon density is immediately followed by an increase in synaptic number (Fig. 9).

Synaptic activity begins when the presynapic neuron acquires the ability to secrete its neurotransmitter upon stimulation and the neurotransmitter thus released produces an effect, excitatory or inhibitory, on the postsynaptic membrane. The brain and other nervous regions become increasingly active when the number of synapses and their activities increase during development. Concomitant with this increase in sy-naptic number and synaptic activity is an increase in the content of the neurotransmitters and in the activities of the enzymes involved in their metabolism. In the simple nervous system of the leech, it has been shown that by the time of body closure, the neurons show characteristic action potentials and are therefore connected to functioning synapses. At this time, the synthetic activity for acetylcholine and other neuro-transmitters is achieved, but at very low levels. The

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