Unit Six. Animal Life
22. The Animal Body and How It Moves
Nerve cells carry information rapidly from one vertebrate organ to another. Nerve tissue, the fourth major class of vertebrate tissue, is composed of two kinds of cells: (1) neurons, which are specialized for the transmission of nerve impulses, and (2) supporting glial cells, which supply the neurons with nutrients, support, and insulation.
Neurons have a highly specialized cell architecture that enables them to conduct signals rapidly throughout the body. Their plasma membranes are rich in ion-selective channels that maintain a voltage difference between the interior and the exterior of the cell, the equivalent of a battery. When ion channels in a local area of the membrane open, ions flood in from the exterior, temporarily wiping out the charge difference. This process, called depolarization, tends to open nearby voltage-sensitive channels in the neuron membrane, resulting in a wave of electrical activity that travels down the entire length of the neuron as a nerve impulse.
Each neuron is composed of three parts, as illustrated in figure 22.7: (1) a cell body, which contains the nucleus; (2) threadlike extensions called dendrites extending from the cell body, which act as antennae, bringing nerve impulses to the cell body from other cells or sensory systems; and (3) a single, long extension called an axon, which carries nerve impulses away from the cell body. Axons often carry nerve impulses for considerable distances: The axons that extend from the skull to the pelvis in a giraffe are about 3 meters long!
Figure 22.7. Neurons carry nerve impulses.
Neurons carry nerve impulses, which are electrical signals, from their initiation in dendrites, to the cell body, and down the length of the axon, where they may pass the signal to a neighboring cell.
The body contains neurons of various sizes and shapes. Some are tiny and have only a few projections, others are bushy and have more projections, and still others have extensions that are meters long. However, all fit into one of three general categories of neurons as shown in table 22.5. Sensory neurons 1 generally carry electrical impulses from the body to the central nervous system (CNS), the brain and spinal cord. Motor neurons 2 generally carry electrical impulses from the central nervous system to the muscles. Association neurons 3 occur within the central nervous system and act as a “connector” between sensory and motor neurons. These will be discussed in more detail in chapter 28. Neurons are not normally in direct contact with one another. Instead, a tiny gap called a synapse separates them. Neurons communicate with other neurons by passing chemical signals called neurotransmitters across the gap.
TABLE 22.5. TYPES OF NEURONS
Vertebrate nerves appear as fine white threads when viewed with the naked eye, but they are actually composed of bundles of axons. Like a telephone trunk cable, nerves include large numbers of independent communication channels— bundles composed of hundreds of axons, each connecting a nerve cell to a muscle fiber or other type of cell. It is important not to confuse a nerve with a neuron. A nerve is made up of the axons of many neurons, just as a cable is made of many wires.
Key Learning Outcome 22.6. Nerve tissue provides the vertebrate body with a means of communication and coordination.