Common Features

As cells, neurons share some features in common with cells in all other organ systems (see Figure 3). They have a plasma membrane acting as an external cell wall to form a distinct boundary between the environment inside (intracellular) and outside (extracellular) the cells. The intracellular material enclosed by the plasma membrane is termed the cytoplasm. Like all other cells (except red blood cells), neurons have numerous specialized intracel-lular organelles, which permit them to maintain their vitality while performing their specialized functions.

Thus, neurons have mitochondria (singular, mitochondrion), by which they convert sugar and oxygen into intracellular energy molecules, which then fuel other metabolic reactions. Neurons have abundant microtubules, thin intracellular tubular struts, by which they form and maintain their often highly irregular cell structure. Neurons are also rich in a network of intracellular membranous channels, the endoplasmic reticulum, through which they distribute the energy molecules, membrane components, and other synthesized products required for functioning. Like other cells that must secrete some of their synthesized products for functioning, as neurons do with their neurotransmitters, some parts of the endoplasmic reticulum, the smooth endoplasmic reticulum, are specialized for the packaging of secretion products into storage particles, which in neurons are termed synaptic vesicles. At the center of the pool of cell material, the cytoplasm, neurons possess a nucleus, which, as in other nucleated cells, contains the full array of the genetic information characteristic of the individual organism. From this nucleus, selected subsets of genetic information are expressed to provide for the general shared and the specific unshared features of the cell. The nucleus of the neuron cell is enclosed within a membranous envelope that, as in

Figure 3

Features of the Neuron

Figure 3

Features of the Neuron many other types of cells, exhibits multiple nuclear pores through which information can be conveyed to and from the nucleus.

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