Functional and Anatomical Organization of the Nervous Tissue
Anatomy & Physiology I
By the end of this section the student will be able to:
The nervous system has three major functions; sensory, integration, and motor. The sensory function is involved with detecting changes in the internal and external environment. The integration function involves processing the sensory information and formulating a response. The motor function is involved with controlling effectors such as muscles and glands.
These functions would be displayed if you ordered a pizza one night when you were very hungry. Simply anticipating the pizza would make your mouth water. When you heard the delivery person pull in the drive, your entire digestive system would kick into action. Your stomach would start to secrete gastric juice and increase its muscular activity. The way you smell and see the pizza would cause your digestive system to swing into full gear, and that first bite would be the culmination of your anticipation. The motor responses (salivation and increased gastric activity) would be appropriate for the sensory input of seeing, smelling, tasting, and anticipating the food.
Suppose you opened the door expecting the pizza delivery person but found an angry grizzly bear instead. You would see the bear, hear its growl, and smell its breath. Your entire body would instantly be prepared to fight or flee (I suggest the later). Your heart rate would increase, your breathing rate would increase, your pupils would dilate, and you would experience a massive rush of epinephrine from the adrenal glands. I suspect you would scream slam the door shut and run to another room in your house. Again, this would be an appropriate motor response to the given situation.
In both cases above, your nervous system detected these environmental factors, processed the sensory input (integration), and sent out the motor responses to your muscles and glands. Consider what might happen if we "had our wires crossed" and the grizzly bear was at the door, we would salivate, experience increased gastric activity, and try to hand the bear a check. Or we might scream, slam the door, and run from the pizza person with the pizza. Of course, we do not have "wires" to cross but it is an interesting way to demonstrate what might happen if integration did not work properly.
Although we do not have electrical "wires" that run through our body (normally); we do have several similarities to electrical devices. Think about an amplifier for a stereo system. The amplifier simply amplifies the sound that comes into it from its input devices such as CD players, cassette decks, DVD players, and video cassette players. The amplifier then sends signals to the speakers. There are wires that go into the amplifier and wires that come out of the amplifier. The wires that go in are input wires from the various sound-producing components. The wires that go out conduct the amplified sound information to the speakers. You can change the qualities of the sound with the amplifier such as volume, tone, left right balance, base and treble balance, etc.
Your nervous system is similar to the speaker analogy. Your nervous system has two major divisions, the central nervous system (CNS) and the peripheral nervous system (PNS). The CNS is composed of the brain and the spinal cord. This division receives sensory input, integrates it, and sends out the appropriate motor output. The peripheral nervous system makes up the rest of the nervous system. The peripheral nervous system contains nerves that conduct information into and out of the CNS. The PNS itself has two major divisions, the sensory division and the motor division. The sensory division senses both the internal environment and the external environment. It is sometimes called the afferent division because it conducts information into the CNS. The motor division is the efferent division because it conducts information out of the CNS to effectors such as muscles and glands.
Figure 1. Sensory or afferent neurons (represented here by a blue arrow) take sensory information to the CNS (represented here by a purple downwardarrow) while Motor or efferent neurons (represented here by a red arrow) send signals from the CNS to the muscles and glands.
The motor division has two subdivisions, the somatic division and the autonomic division. The somatic division is the part that you can consciously control. The autonomic division controls smooth muscle, cardiac muscle, and glands. The autonomic nervous system has two divisions, the sympathetic division and the parasympathetic division. The sympathetic division prepares your body for stress. It would be the division involved if you came face to face with an angry bear. The parasympathetic division is the division involved with times of low stress - imagine being kicked back on the beach in a nice comfy chair at sun set with no aggravations at all.
Figure 2. The Divisions of the Nervous System. The Central Nervous System receives information from the Peripheral Nervous System's Sensory division and sends output through the Motor division of the PNS. The Motor division includes the Autonomic and Somatic divisions. The ANS includes the Sympathetic and Parasympathetic divisions.
In our stereo analogy, the amplifier would represent the CNS because it receives input signals and produces output signals. The stereo components that send sound into the amplifier would represent sensory organs and their input (afferent) wires, which go into the amplifier, would represent sensory nerves. The speakers would be the effectors because they are what the amplifier controls. The wires going out of the speakers (efferent) would represent the motor nerves. Of course in a stereo system, there are all sorts of different types of wires, cables, and connectors that are used to hook the components together. Inside each component, there is a staggering array of other wires, circuit boards and who knows what all else. Again, nervous systems are not composed of wires, but there are different types of cells.
The nervous system can be divided into two parts mostly on the basis of a functional difference in responses. The somatic nervous system (SNS) is responsible for conscious perception and voluntary motor responses. Voluntary motor response means the contraction of skeletal muscle, but those contractions are not always voluntary in the sense that you have to want to perform them. Some somatic motor responses are reflexes, and often happen without a conscious decision to perform them. If your friend jumps out from behind a corner and yells "Boo!" you will be startled and you might scream or leap back. You didn't decide to do that, and you may not have wanted to give your friend a reason to laugh at your expense, but it is a reflex involving skeletal muscle contractions. Other motor responses become automatic (in other words, unconscious) as a person learns motor skills (referred to as "habit learning" or "procedural memory").
The autonomic nervous system (ANS) is responsible for involuntary control of the body, usually for the sake of homeostasis (regulation of the internal environment). Sensory input for autonomic functions can be from sensory structures tuned to external or internal environmental stimuli. The motor output extends to smooth and cardiac muscle as well as glandular tissue. The role of the autonomic system is to regulate the organ systems of the body, which usually means to control homeostasis. Sweat glands, for example, are controlled by the autonomic system. When you are hot, sweating helps cool your body down. That is a homeostatic mechanism. But when you are nervous, you might start sweating also. That is not homeostatic, it is the physiological response to an emotional state.
There is another division of the nervous system that describes functional responses. The enteric nervous system (ENS) is responsible for controlling the smooth muscle and glandular tissue in your digestive system. It is a large part of the PNS, and is not dependent on the CNS. It is sometimes valid, however, to consider the enteric system to be a part of the autonomic system because the neural structures that make up the enteric system are a component of the autonomic output that regulates digestion. There are some differences between the two, but for our purposes here there will be a good bit of overlap.
Figure 3. Somatic structures include the spinal nerves, both motor and sensory fibers, as well as the sensory ganglia (posterior root ganglia and cranial nerve ganglia). Autonomic structures are found in the nerves also, but include the sympathetic and parasympathetic ganglia. The enteric nervous system includes the nervous tissue within the organs of the digestive tract.
The nervous system can be separated into divisions on the basis of anatomy and physiology. The anatomical divisions are the central and peripheral nervous systems. The CNS is the brain and spinal cord. The PNS is everything else. Functionally, the nervous system can be divided into those regions that are responsible for sensation, those that are responsible for integration, and those that are responsible for generating responses. All of these functional areas are found in both the central and peripheral anatomy.
Considering the anatomical regions of the nervous system, there are specific names for the structures within each division. A localized collection of neuron cell bodies is referred to as a nucleus in the CNS and as a ganglion in the PNS. A bundle of axons is referred to as a tract in the CNS and as a nerve in the PNS. Whereas nuclei and ganglia are specifically in the central or peripheral divisions, axons can cross the boundary between the two. A single axon can be part of a nerve and a tract. The name for that specific structure depends on its location.
Nervous tissue can also be described as gray matter and white matter on the basis of its appearance in unstained tissue. These descriptions are more often used in the CNS. Gray matter is where nuclei are found and white matter is where tracts are found. In the PNS, ganglia are basically gray matter and nerves are white matter.
The nervous system can also be divided on the basis of how it controls the body. The somatic nervous system (SNS) is responsible for functions that result in moving skeletal muscles. Any sensory or integrative functions that result in the movement of skeletal muscle would be considered somatic. The autonomic nervous system (ANS) is responsible for functions that affect cardiac or smooth muscle tissue, or that cause glands to produce their secretions. Autonomic functions are distributed between central and peripheral regions of the nervous system. The sensations that lead to autonomic functions can be the same sensations that are part of initiating somatic responses. Somatic and autonomic integrative functions may overlap as well.
A special division of the nervous system is the enteric nervous system, which is responsible for controlling the digestive organs. Parts of the autonomic nervous system overlap with the enteric nervous system. The enteric nervous system is exclusively found in the periphery because it is the nervous tissue in the organs of the digestive system.
Glossary
Except where otherwise noted, this work by The Community College Consortium for Bioscience Credentials is licensed under a Creative Commons Attribution 4.0 International License.
Text from Michael Ayers, M.S.and from BioBook licensed under CC BY NC SA and Boundless Biology Open Textbook licensed under CC BY SA.
Other text from OpenStaxCollege licensed under CC BY 3.0. Modified by Alice Rudolph, M.A. and Andrea Doub, M.S. for c3bc.
Instructional Design by Courtney A. Harrington, Ph.D., Helen Dollyhite, M.A. and Caroline Smith, M.A. for c3bc.
Media by Brittany Clark, Jose DeCastro, Jordan Campbell and Antonio Davis for c3bc.
This product was funded by a grant awarded by the U.S. Department of Labor's Employment and Training Administration. The product was created by the grantee and does not necessarily reflect the official position of the U.S. Department of Labor. The Department of Labor makes no guarantees, warranties, or assurances of any kind, express or implied, with respect to such information, including any information on linked sites and including, but not limited to, accuracy of the information or its completeness, timeliness, usefulness, adequacy, continued availability, or ownership.