The nervous system has three specific functions:
1. Sensory input. Sensory receptors present in skin and organs respond to external and internal stimuli by generating nerve impulses that travel to the brain and spinal cord.
2. Integration. The brain and spinal cord sum up the data received from all over the body and send out nerve impulses.
3. Motor output. The nerve impulses from the brain and spinal cord go to the effectors, which are muscles and glands. Muscle contractions and gland secretions are responses to stimuli received by sensory receptors.
Divisions of the Nervous System
The nervous system has two major divisions: the central nervous system and the peripheral nervous system (Fig. 8.1). The central nervous system (CNS) includes the brain and spinal cord, which have a central location-they lie in the midline of the body. The peripheral nervous system (PNS), which is further divided into the somatic division and the autonomic division, includes all the cranial and spinal nerves. Nerves have a peripheral location in the body, meaning that they project out from the central nervous system. The division between the central nervous system and the peripheral nervous system is arbitrary; the two systems work together.
Figure 8.1 Organization of the nervous system in humans. a. This pictorial representation shows the central nervous system (CNS, composed of brain and spinal cord) and some of the nerves of the peripheral nervous system (PNS). b. The CNS communicates with the PNS. In the somatic system, nerves conduct impulses from sensory receptors located in the skin and internal organs to the CNS; nerves also conduct motor impulses from the CNS to the skeletal muscles. In the autonomic system, consisting of the sympathetic and parasympathetic divisions, motor impulses travel to smooth muscle, cardiac muscle, and glands.
Although exceedingly complex, nervous tissue is made up of just two principal types of cells: (1) neurons, also called nerve cells, which transmit nerve impulses; and (2) neuroglia, which supports and nourishes neurons.
Neurons vary in appearance, but all of them have just three parts: a cell body, dendrite(s), and an axon. In Figure 8.2a, the cell body contains the nucleus as well as other organelles. In motor neurons, the dendrites are the many short extensions that receive signals from sensory receptors or other neurons. At the dendrites, signals can result in nerve impulses that are then conducted by an axon. The axon is the portion of a neuron that conducts nerve impulses. Any long axon is also called a nerve fiber. Long axons are covered by a white myelin sheath formed from the membranes of tightly spiraled neuroglia. In the PNS, a neuroglial cell called a neurolemmocyte (Schwann cell) performs this function, leaving gaps called neurofibril nodes (nodes of Ranvier). Another type of neuroglial cell performs a similar function in the CNS.
Types of Neurons
Neurons can be classified according to their function and shape. Motor neurons take nerve impulses from the CNS to muscles or glands. Motor neurons are said to be multipolar because they have many dendrites and a single axon (Fig. 8.2a). Motor neurons cause muscle fibers to contract or glands to secrete, and therefore they are said to innervate these structures.
Sensory neurons take nerve impulses from sensory receptors to the CNS. The sensory receptor, which is the distal end of the long axon of a sensory neuron, may be as simple as a naked nerve ending (a pain receptor), or it may be a part of a highly complex organ, such as the eye or ear. Almost all sensory neurons have a structure that is termed unipolar (Fig. 8.2b). In unipolar neurons, the extension from the cell body divides into a branch that comes to the periphery and another that goes to the CNS. Because both branches are long and myelinated and transmit nerve impulses, it is now generally accepted to refer to them collectively as an axon.
Interneurons, also known as association neurons, occur entirely within the CNS. Interneurons, which are typically multipolar (Fig. 8.2c), convey nerve impulses between various parts of the CNS. Some lie between sensory neurons and motor neurons, and some take messages from one side of the spinal cord to the other or from the brain to the cord, and vice versa. They also form complex pathways in the brain where processes accounting for thinking, memory, and language occur.
Figure 8.2 Neuron anatomy. a. Motor neuron. Note the branched dendrites and the single, long axon, which branches only near its tip. b. Sensory neuron with dendritelike structures projecting from the peripheral end of the axon. c. Interneuron (from the cortex of the cerebellum) with highly branched dendrites.