CELLULAR ORGANIZATION OF THE BRAIN: A PHARMACOLOGICAL VIEW OF NEURONS
Neurons are classified according to function (sensory, motor, or interneuron), location, and identity of the transmitter(s) that they synthesize and release. They exhibit the cytological characteristics of highly active secretory cells with large nuclei: large amounts of smooth and rough endoplasmic reticulum; and frequent clusters of specialized smooth endoplasmic reticulum (Golgi complex), in which secretory products of the cell are packaged into membrane-bound organelles for transport from the cell body proper to the axon or dendrites (Figure 12-1). Neurons are rich in microtubules, which support the complex cellular structure and assist in the reciprocal transport of essential macromolecules and organelles between the cell body and the distant axon or dendrites. The sites of interneuronal communication in the CNS are termed synapses. Although synapses are functionally analogous to "junctions" in the somatic motor and autonomic nervous systems, the central junctions contain an array of specific proteins presumed to be the active zone for transmitter release and response. Like peripheral "junctions," central synapses also are denoted by accumulations of tiny (500-1500 A) organelles, termed synaptic vesicles. The proteins of these vesicles have been shown to have specific roles in transmitter storage, vesicle docking onto presynaptic membranes, voltage- and Ca2+-dependent secretion (see Chapter 6), and recycling and restorage of released transmitter.
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