Overview of the Nervous System
When you have completed this section, you should be able to
- describe the major anatomical subdivisions of the nervous system;
- state the general functions of the nervous system and how these relate to the general classes of nerve cells; and
- describe the basic physiological properties of nerve cells that enable them to carry out their functions.
The fundamental purpose of the nervous system is (1) to receive information from receptors—cells and organs specialized to detect changes in the body and its external environment; (2) to process this information and determine the appropriate response, if any—a step called neural integration; and (3) to issue commands to effectors, cells and organs (mainly muscle and gland cells) that carry out the body's responses.
The nervous system has two major anatomical subdivisions (fig. 12.2):
- The central nervous system (CNS) consists of the brain and spinal cord, which are enclosed and protected by the cranium and vertebral column.
- The peripheral nervous system (PNS) consists of all the nervous system except the brain and spinal cord. It is composed of nerves and ganglia. A nerve is a bundle of nerve fibers wrapped in fibrous connective tissue. Nerves emerge from the CNS through foramina of the skull and vertebral column and carry signals to
- Figure 12.2 Subdivisions of the Nervous System.
Saladin: Anatomy & I 12. Nervous Tissue I Text I I © The McGraw-Hill
Physiology: The Unity of Companies, 2003 Form and Function, Third Edition
Chapter 12 Nervous Tissue 445
and from other organs of the body. A ganglion1 (plural, ganglia) is a knotlike swelling in a nerve where the cell bodies of neurons are concentrated.
The peripheral nervous system is functionally divided into sensory and motor divisions, and each of these is further divided into somatic and visceral subdivisions.
- The sensory (afferent2) division carries sensory signals by way of afferent nerve fibers from sensory receptors (cells and organs that detect stimuli) to the CNS.
- The visceral sensory division carries signals mainly from the viscera of the thoracic and abdominal cavities, such as the heart, lungs, stomach, and urinary bladder.
- The somatic3 sensory division carries signals from receptors in the skin, muscles, bones, and joints.
- The motor (efferent4) division carries motor signals by way of efferent nerve fibers from the CNS to effectors (cells and organs that carry out the body's responses, mainly gland and muscle cells).
- The visceral motor division (autonomic5 nervous system) carries signals to glands, cardiac muscle, and smooth muscle. We usually have no voluntary control over these effectors, and this system operates at an unconscious level. The responses of this system and its effectors are visceral reflexes. The autonomic nervous system has two further divisions:
- The sympathetic division tends to arouse the body for action, for example by accelerating the heartbeat and increasing respiratory airflow, but it inhibits digestion.
- The parasympathetic division tends to have a calming effect, slowing down the heartbeat, for example, but stimulating digestion.
- The somatic motor division carries signals to the skeletal muscles. This output produces muscular contractions that are under voluntary control as well as involuntary muscle contractions called somatic reflexes.
The foregoing terminology may give the impression that the body has several nervous systems—central, peripheral, sensory, motor, somatic, and visceral. These are just terms of convenience, however. There is only one nervous system, and these subsystems are interconnected parts of the whole.
Before You Go On
Answer the following questions to test your understanding of the preceding section:
- What is a receptor? Give two examples of effectors.
- Distinguish between the central and peripheral nervous systems, and between visceral and somatic divisions of the sensory and motor systems.
- What is another name for the visceral motor nervous system? What are the two subdivisions of this system?
Was this article helpful?
This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.
Get My Free Ebook