Nervous Tissues and CellsNervous tissue is composed mainly of cells called neurons, which are highly specialized. They sense certain aspects of their surroundings and respond by transmitting electrical impulses. Nervous tissue also contains neuroglial (or glial) cells that play several roles, including support, insulation, and transport of nutrients to neurons. The functioning of neurons will be explored in detail in the next chapter, but let us now observe that all neurons follow a generalized structural plan. They typically consist of a centralized cell body, which contains the nucleus; several cytoplasmic extensions called dendrites, which receive information from other cells; and an axon (also called a nerve fiber), a long extension through which impulses travel away from the cell body, bound for another cell (see Figure 13.1). The axons of many neurons are surrounded by specialized neuroglial cells called Schwann cells, each of which wraps itself around a portion of the axon in such a way that little cytoplasm is present, and multiple layers of cell membrane encircle the axon. Schwann cell membranes are largely composed of a protein called myelin, which is a lipoprotein and thus very hydrophobic. These layers of cell membranes form a myelin sheath around an axon. Narrow gaps occur in the myelin sheath between each Schwann cell and are referred to as nodes of Ranvier. Axons surrounded by a myelin sheath are referred to as myelinated nerve fibers. The sheath functions to electrically insulate the axon and allow impulses to travel at much greater speeds than would be possible in unmyelinated fibers.
A nerve is simply a collection or bundle of neurons. Nervous tissue is found in the brain and spinal cord, which together make up the central nervous system, and in the peripheral nerves, which originate from the central nervous system and make up the peripheral nervous system. Nervous pathways, in general, go in two directions. Sensory nerves bring information (a stimulus) from the sensory organs towards the central nervous system. After the brain processes this information, it usually sends signals through motor nerves to effector organs (muscles and glands) which can bring about an appropriate response. The multiplicity of connections between neurons, muscles, and glands allows the nervous system to coordinate, regulate, monitor, and integrate many body functions, and makes it possible for us to obtain, process, and react to meaningful information about our environment. More details of the physiology of the nervous system can be found in the next chapter.