In this lesson, you will learn how the nervous system is categorized into anatomical and physiological divisions. Specifically, this lesson will cover:
The picture you have in your mind of the nervous system probably includes the brain, the organ composed primarily of nervous tissue contained within the cranium, and the spinal cord, the organ composed primarily of nervous tissue within the vertebral column. That suggests it is made of two organs—and you may not even think of the spinal cord as an organ—but the nervous system is a very complex structure. Within the brain, many different and separate regions are responsible for many different and separate functions. It is as if the nervous system is composed of many organs that all look similar and can only be differentiated using tools such as the microscope or electrophysiology. In comparison, it is easy to see that the stomach is different than the esophagus or the liver, so you can imagine the digestive system as a collection of specific organs.
The nervous system can be divided into two major regions: the central and peripheral nervous systems. The central nervous system (CNS) is the brain and spinal cord (the central axis), and the peripheral nervous system (PNS) is the structure outside of the central axis, nerves, and related structures. The brain is contained within the cranial cavity of the skull, and the spinal cord is contained within the vertebral cavity of the vertebral column. It is a bit of an oversimplification to say that the CNS is what is inside these two cavities and the peripheral nervous system is outside of them, but that is one way to start to think about it. In actuality, there are some elements of the peripheral nervous system that are within the cranial or vertebral cavities and some elements of the central nervous system that are outside of them. Depending on different aspects of the nervous system, the dividing line between central and peripheral is not necessarily universal.
Central and Peripheral Nervous System - The structures of the PNS are referred to as ganglia and nerves, which can be seen as distinct structures. The equivalent structures in the CNS are not obvious from this overall perspective and are best examined in prepared tissue under the microscope.
Nervous tissue is present in both the CNS and PNS. Recall that nervous tissue contains two basic types of cells: neurons and neuroglial cells, sometimes referred to as glial cells or glia (glia, glue). Remember that neurons function to generate and propagate electrochemical signals throughout the body while glial cells provide support for neurons.
In order to understand the organization patterns of the nervous system, it is important to review the structure of a neuron. Recall that a neuron is composed of a cell body, or soma, with two types of processes that extend outwards. Dendrites branch off of the cell body and monitor the electrochemical activity in their surrounding area, bringing electrical signals into the soma. The axon extends away from the cell body and propagates an electrical signal onto the next cell. Some neurons will have a wrapping called myelin around portions of the axon.
At the end of the axon are axon terminals which form connections with and transfer electrochemical signals to other cells.
The Neuron - The cell body of a neuron, also called the soma, contains the nucleus and mitochondria. The dendrites transfer the electrochemical signal to the soma. The axon carries the electrochemical signal away to another excitable cell. The axon terminals for the synapse (connection) to the next excitable cell.
Looking at nervous tissue, there are regions that predominantly contain cell bodies and regions that are largely composed of just axons. These two regions within nervous system structures are often referred to as gray matter (the regions with many cell bodies and dendrites) or white matter (the regions with many axons).
key concept
The image below demonstrates the appearance of these regions in the brain and spinal cord. The colors ascribed to these regions are what would be seen in “fresh,” or unstained, nervous tissue. Gray matter is not necessarily gray. It can be pinkish because of blood content, or even slightly tan, depending on how long the tissue has been preserved. But the white matter is white because axons are insulated by myelin which is lipid-rich. Lipids can appear as white (“fatty”) material, much like the fat on a raw piece of chicken or beef. Actually, the gray matter may have that color ascribed to it because next to the white matter, it is just darker—hence, gray.
The distinction between gray matter and white matter is most often applied to central nervous tissue, which has large regions that can be seen with the unaided eye. When looking at peripheral structures, a microscope is often used and the tissue is stained with artificial colors. That is not to say that central nervous tissue cannot be stained and viewed under a microscope but unstained tissue is most likely from the CNS—for example, a frontal section of the brain or a cross-section of the spinal cord.
Gray Matter and White Matter - A brain removed during an autopsy, with a partial section removed, shows white matter surrounded by gray matter. Gray matter makes up the outer cortex of the brain.
Regardless of the appearance of stained or unstained tissue, the cell bodies of neurons or axons can be located in discrete anatomical structures. Those structures are named based on whether they are located in the central or peripheral nervous system. A localized collection of neuron cell bodies in the CNS is referred to as a nucleus (CNS).
In the PNS, a cluster of neuron cell bodies is referred to as a ganglion. Admittedly, the term ‘nucleus’ has multiple meanings in this anatomy and physiology course alone. The table below outlines those definitions to help you better understand its uses.
What Is a Nucleus? - (a) The nucleus of an atom contains its protons and neutrons. (b) The nucleus of a cell is the organelle that contains DNA. (c) A nucleus in the CNS is a localized center of function with the cell bodies of several neurons, shown here circled in red.
The terminology applied to bundles of axons also differs depending on location. A bundle of axons, or fibers, found in the CNS is called a tract whereas the same thing in the PNS would be called a nerve. It is important to note that the same bundle of axons can be referred to as both a tract and nerve if it spans both central and peripheral regions.
EXAMPLE
One example of this is the axons that project from the retina in your eye to the brain. Those axons are called the optic nerve as they leave the eye, but when they are inside the cranium, they are referred to as the optic tract. This naming scheme can be applied to roads as well. A single road from point A to point B in different towns may have different names on either side of the town, county, or state line.
Table: Structures of the CNS and PNS
CNS
PNS
Group of Neuron Cell Bodies (i.e., gray matter)
Nucleus
Ganglion
Bundle of Axons (i.e., white matter)
Tract
Nerve
terms to know
Brain
A nervous system organ located in the cranial cavity and primarily composed of nervous tissue.
Spinal Cord
A nervous system organ located in the vertebral cavity and primarily composed of nervous tissue.
Central Nervous System (CNS)
The anatomical division of the nervous system that consists of the brain and spinal cord.
Peripheral Nervous System (PNS)
The anatomical division of the nervous system that consists of structures outside of the brain and spinal cord.
Gray Matter
Regions of nervous tissue with many cell bodies and dendrites.
White Matter
Regions of nervous tissue with many axons.
Nucleus (CNS)
A group of neuron cell bodies in the central nervous system.
Ganglion
A group of neuron cell bodies in the peripheral nervous system.
Tract
A bundle of axons in the central nervous system.
Nerve
A bundle of axons in the peripheral nervous system.
2. Functional Divisions of the Nervous System
In a separate method, the nervous system can be divided on the basis of its functions. The CNS and the PNS both contribute to the same functions, but those functions can be attributed to different regions of the brain (such as the cerebral cortex or the hypothalamus) or to different structures in the periphery. Furthermore, the same structure can sometimes be part of several functions.
There are two ways to consider how the nervous system is divided functionally. First, the basic functions of the nervous system are sensation, integration, and response. In other words, the system must detect a change, process that information, and respond accordingly. Secondly, control of the body can be conscious or subconscious, a division that is largely defined by the structures that are involved in the response.
2a. Basic Functions
Sensation: The first major function of the nervous system is sensation—the detection of information about the environment to gain input about what is happening outside the body (or, sometimes, within the body) and the translation of that into electrochemical signals. The sensory functions of the nervous system register the presence of a change from homeostasis or a particular event in the external or internal environment, known as a stimulus.
reflect
The senses we think of most are the “big five”: taste, smell, touch, sight, and hearing. The stimuli for taste and smell are both chemical substances (molecules, compounds, ions, etc.), touch is physical or mechanical stimuli that interact with the skin, sight is light stimuli, and hearing is the perception of sound, which is a physical stimulus similar to some aspects of touch. There are actually more senses than just those, but that list represents the major senses. Those five are all senses that receive stimuli from the outside world and of which there is conscious perception. Additional sensory stimuli might be from the internal environment (inside the body), such as the stretch of an organ wall or the concentration of certain ions in the blood.
Integration: Stimuli that are received by sensory structures are communicated to the nervous system where that information is processed. This is called integration. Stimuli are compared with, or integrated with, other stimuli, memories of previous stimuli, or the state of a person at a particular time. This leads to the specific response that will be generated.
EXAMPLE
Seeing a baseball pitched to a batter will not automatically cause the batter to swing. The trajectory of the ball and its speed will need to be considered. Maybe the count is three balls and one strike, and the batter wants to let this pitch go by in the hope of getting a walk to first base. Or, maybe the batter’s team is so far ahead that it would be fun to just swing away.
Response: The nervous system produces a response, or internal event, on the basis of the stimuli perceived by sensory structures. An obvious response would be the movement of muscles, such as withdrawing a hand from a hot stove, but there are broader uses of the term. The nervous system can cause the contraction of all three types of muscle tissue.
EXAMPLE
Skeletal muscle contracts to move the skeleton, cardiac muscle is influenced as heart rate increases during exercise, and smooth muscle contracts as the digestive system moves food along the digestive tract. Responses also include the neural control of glands in the body as well, such as the production and secretion of sweat by the eccrine and merocrine sweat glands found in the skin to lower body temperature.
Responses can be divided into those that are voluntary or conscious (contraction of skeletal muscle) and those that are involuntary (contraction of smooth muscles, regulation of cardiac muscle, and activation of glands).
2b. Controlling the Body
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 a functional division of the nervous system which is responsible for conscious perception and voluntary motor responses. Voluntary motor response means the contraction of skeletal muscle. At times, these motor responses are intentional such as reaching for a cup and filling it with water when you realize you are thirsty. At other times, these motor responses are performed unintentionally such as screaming or jumping back when a friend jumps out from behind a corner and yells “Boo!” In both cases, it is skeletal muscle that is activated in response to the received stimuli. The stimuli that elicit these types of responses are also referred to as somatic stimuli.
The autonomic nervous system (ANS) is a functional division of the nervous system which is responsible for involuntary perception and 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.
Somatic, Autonomic, and Enteric Structures of the Nervous System - 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.
IN CONTEXT Everyday Connection - How Much of Your Brain Do You Use?
Have you ever heard the claim that humans only use 10 percent of their brains? Maybe you have seen an advertisement on a website saying that there is a secret to unlocking the full potential of your mind—as if there were 90 percent of your brain sitting idle, just waiting for you to use it. If you see an ad like that, don’t click. It isn’t true.
An easy way to see how much of the brain a person uses is to take measurements of brain activity while performing a task. An example of this kind of measurement is functional magnetic resonance imaging (fMRI), which generates a map of the most active areas and can be generated and presented in three dimensions. This procedure is different from the standard MRI technique because it is measuring changes in the tissue in time with an experimental condition or event.
fMRI - This fMRI shows activation of the visual cortex in response to visual stimuli.
The underlying assumption is that active nervous tissue will have greater blood flow. By having the subject perform a visual task, activity all over the brain can be measured. Consider this possible experiment: the subject is told to look at a screen with a black dot in the middle (a fixation point). A photograph of a face is projected on the screen away from the center. The subject has to look at the photograph and decipher what it is. The subject has been instructed to push a button if the photograph is of someone they recognize. The photograph might be of a celebrity, so the subject would press the button, or it might be of a random person unknown to the subject, so the subject would not press the button.
In this task, visual sensory areas would be active, integrating areas would be active, motor areas responsible for moving the eyes would be active, and motor areas for pressing the button with a finger would be active. Those areas are distributed all around the brain and the fMRI images would show activity in more than just 10 percent of the brain (some evidence suggests that about 80 percent of the brain is using energy—based on blood flow to the tissue—during well-defined tasks similar to the one suggested above). This task does not even include all of the functions the brain performs. There is no language response, the body is mostly lying still in the MRI machine, and it does not consider the autonomic functions that would be ongoing in the background.
terms to know
Sensation
A nervous system function in which sensory information about the environment is detected translated into electrochemical signals.
Integration
A nervous system function in which various stimuli are compared and/or integrated to produce a response.
Response
A nervous system function in which an event is caused to occur as a consequence of a specific stimulus or stimuli.
Somatic Nervous System (SNS)
A functional division of the nervous system which is responsible for conscious perception and voluntary motor responses.
Autonomic Nervous System (ANS)
A functional division of the nervous system which is responsible for involuntary perception and control of the body.
Enteric Nervous System (ENS)
A division of the autonomic nervous system which is responsible for controlling the smooth muscle and glandular tissue in your digestive system.
summary
In this lesson, you learned about the ways in which the nervous system is divided. The nervous system can be divided anatomically into the central and peripheral nervous systems. Physiologically, it also forms functional divisions of the nervous system which are categorized as the basic functions of sensation, integration, and response or those that deal with controlling the body voluntarily or involuntarily.
