We shall now survey the anatomy and broad functions of the major structures of the nervous system; future chapters will describe these functions in more detail. First, we must deal with some potentially confusing terminology. Recall that a long extension from a single neuron is called an axon or a nerve fiber and that the term "nerve" refers to a group of many nerve fibers that are traveling together to the same general location in the peripheral nervous system. There are no nerves in the central nervous system. Rather, a group of nerve fibers traveling together in the central nervous system is called a pathway, a tract, or, when it links the right and left halves of the central nervous system, a commissure.
Information can pass through the central nervous system along two types of pathways: (1) long neural pathways, in which neurons with long axons carry information directly between the brain and spinal cord or between large regions of the brain, and (2) multineuronal or multisynaptic pathways (Figure 8-35). As their name suggests, the multineuronal pathways are made up of many neurons and many synaptic connections. Since synapses are the sites where new information can be integrated into neural messages, there are many opportunities for neural processing along the multineuronal pathways. The long pathways, on the other hand, consist of chains of only a few sequentially connected neurons. Because the long pathways contain few synapses, there is little opportunity for alteration in the information they transmit.
The cell bodies of neurons having similar functions are often clustered together. Groups of neuron cell bodies in the peripheral nervous system are called ganglia (singular, ganglion), and in the central nervous system they are called nuclei (singular, nucleus), not to be confused with cell nuclei.
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