A neuron's key function is to communicate with other neurons or with target cells (gland or muscle). Communication is mediated either electrically or chemically. Electrical synapses are rather stereotyped. In contrast, chemical transmission, which involves the slow to rapid release of one or more of over 100 possible transmitter substances, is quite complex. This complexity, which also includes the time and amount of transmitter released, is the basis of synaptic plasticity. Synaptic plasticity, in turn, is the basis of higher mental function, for example, learning and memory. Electrical transmission essentially is instantaneous because the necessary cytoplasmic connection between the neuron and its target cell is continuous. In contrast, in chemical transmission, the target cell is situated at some distance from the signaling neuron, causing synaptic delay. For a transmitter released from the signaling neuron to act successfully as a chemical messenger, it must bind to an appropriate receptor in the target cell. Typically the separation is not much greater than the distance between two adjacent

Encyclopedia of the Human Brain Volume 3

Copyright 2002, Elsevier Science (USA).

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neurons (the synaptic cleft is about 200 mm across), but occasionally the separation can be much greater.

Chemical transmission is a specialized form of secretion. In simple organisms and during embryogenesis, communication between cells is mediated only by peptide growth factors. These are hormones that are secreted by one cell and diffuse to relatively distant targets. Neurotransmitters can be categorized into two groups: neuropeptides and small molecules (classical transmitters). Each group uses a distinct set of molecular mechanisms that are familiar from general cell biology: neuropeptides are synthesized, processed, and packaged within vesicles of the secretory pathway. In contrast, small-molecule neurotransmitters are made in the cytoplasm and pumped into vesicles derived from the lysosomal pathway.

It is important to realize that neurons are constructed according to the general polarized plan of epithelial cells. Toward the end of the nineteenth century, Santiago Ramon y Cajal discussed the building plan of neurons in terms of functional polarity. He pointed out that a typical nerve cell has a receiving end (dendrites), a cell body (soma), and a transmitting end (axon). This corresponds to the basalto-apical blueprint of a typical epithelial cell. In neurons, most of the Golgi apparatus is situated between the cell's nucleus and the axon hillock. Membranous vesicles filled with newly synthesized secretory products are transported from the trans face of the Golgi apparatus to the axon and from there to nerve terminals for release.

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