The nervous system is one of the two main control mechanisms of the body, the other being the endocrine system. Nervous tissue is among the most sensitive in the body to toxins. Because nervous tissue has a very high metabolic rate, its role is so critical that even small amounts of damage can have significant effects, and some toxins (i.e., the insecticides) are actually designed to be neurotoxic in order to be effective against insects at low dosages.
The primary functional components of the nervous system are the nerve cells, or neurons. They can have many shapes; Figure 9.2 shows a common type. The main parts are the cell body, or soma, the highly branched dendrites, and the axon. The soma contains the nucleus and cytoplasm, with its typical cell machinery, such as mitochondria and ribo-somes. The neuron lacks a centriole, which makes it impossible for it to divide. Thus, neurons cannot regenerate once damaged. This also means that neurons cannot become cancerous. Brain cancer in adults occurs in other nervous system cells, called glial cells. The dendrites provide most of the sites for the reception of nerve signals. The axon is a long extension of the cell that serves to transmit the nerve signal over a great distance. It may have branches, and it is often sheathed in a lipid coating called myelin that serves as an insulator for the signal transmission. Neurons transmit signals from one cell to another at a specialized point of contact called a synapse. A nerve is a bundle of neuron axons.
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