The nervous system is a complex structure made up of two organs: the brain and the spinal cord. Only tools like the microscope or electrophysiology can distinguish the various regions that make it because they are responsible for so many different functions.
Let's recap the Central and Peripheral Nervous Systems
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, and the peripheral nervous system (PNS) is everything else. The brain fits inside the cranial cavity of the skull, and the spinal cord fits inside the vertebral cavity of the spine. 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, some elements of the peripheral nervous system are within the cranial or vertebral cavities. The peripheral nervous system is so named because it is on the periphery, meaning beyond the brain and spinal cord. The line between central and peripheral is not always the same because it depends on different parts of the nervous system.
Nervous tissue, present in both the CNS and PNS, contains two basic types of cells: neurons and glial cells. A glial cell is one of many types of cells that help the neurons do their jobs by making a framework of tissue. In terms of how the nervous system works to communicate, the neuron is the more important of the two cells. To comprehend how the different parts of the nervous system work, you need to understand properly how a neuron is put together. Neurons are cells, so they have a soma, which is the body of the cell. They also have extensions of the cell, which are called processes. There is one important process that every neuron has called an axon, which is the fiber that connects a neuron with its target. Another type of process that branches off from the soma is the dendrite.
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Dendrites are responsible for receiving most of the input from other neurons. When you look at nervous tissue, some parts are mostly made up of cell bodies, and other parts are mostly made up of axons. These two regions within the structures of the nervous system are often referred to as "gray matter" (the regions with many cell bodies and dendrites) or "white matter" (the regions with many axons). 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 the 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 a lipid-rich substance called myelin. Lipids can appear as white (“fatty”) material, much like the fat on a raw piece of chicken or beef. 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, often a microscope is used and the tissue is stained with artificial colors. That is not to say that central nervous system tissue cannot be stained and viewed under a microscope, but unstained tissue is most likely to come from the CNS—for example, a frontal section of the brain or a cross-section of the spinal cord.
No matter how stained or unstained the tissue looks, the cell bodies of neurons or axons can be found in separate structures that need to be named. Those names are specific to whether the structure is central or peripheral. A localized collection of neuron cell bodies in the CNS is referred to as a nucleus. In the PNS, a cluster of neuron cell bodies is referred to as a ganglion. It is the center of an atom, where protons and neutrons are found; it is the center of a cell, where the DNA is found; and it is the center of some functions in the CNS. There is also a potentially confusing use of the word ganglion (plural: "ganglia") that has a historical explanation. In the central nervous system, there is a group of nuclei that are connected and were once called the basal ganglia before “ganglion” became accepted as a description for a peripheral structure. Some sources refer to this group of nuclei as the “basal nuclei” to avoid confusion.
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. There is an important point to make about these terms, which is that they can both be used to refer to the same bundle of axons. When those axons are in the PNS, the term is "nerve," but if they are in the CNS, the term is "tissue." The most obvious example of this is the axons that project from the retina into 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. There is a specific place where the name changes, which is the optic chiasm, but they are still the same axons.
Functional Divisions of the Nervous System
The nervous system can also be divided based on its functions, but anatomical divisions and functional divisions are different. The CNS and the PNS both do the same things, but they do them in different parts of the brain (like the cerebral cortex or the hypothalamus) or in different parts of the body (called ganglia). The problem with trying to fit functional differences into anatomical divisions is that sometimes the same structure can be part of several functions. For example, the optic nerve carries signals from the retina that are either used for the conscious perception of visual stimuli, which takes place in the cerebral cortex, or for the reflexive responses of smooth muscle tissue that are processed through the hypothalamus.
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. Secondly, control of the body can be somatic or autonomic—divisions that are largely defined by the structures that are involved in the response. There is also a region of the peripheral nervous system called the enteric nervous system that is responsible for a specific set of functions within the realm of autonomic control related to gastrointestinal functions.
What are the Basic Functions
The nervous system is involved in receiving information about the environment around us (sensation) and generating responses to that information (motor responses). There are parts of the nervous system that are in charge of feeling (sensory functions) and acting (motor functions). But there is a third function that needs to be included. Sensory information needs to be put together with other sensations, memories, feelings, and learning (cognition). Some regions of the nervous system are termed integration or association areas. Integration is the process of putting together information from the senses with higher cognitive functions like memories, learning, and emotions to make a response.
Sensation. The first major function of the nervous system is sensation—receiving information about the environment to gain input about what is happening outside the body (or, sometimes, within the body). The nervous system's sensory functions pick up on a change from homeostasis or a specific event in the environment. This is called a "stimulus." 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 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.
Response. The nervous system makes a response to stimuli that sensory structures pick up on. An obvious response would be the movement of muscles, such as withdrawing a hand from a hot stove, but there are broader uses for the term. The nervous system can cause the contraction of all three types of muscle tissue. For example, skeletal muscle contracts to move the skeleton, cardiac muscle is affected when the heart rate goes up during exercise, and smooth muscle contracts when the digestive system moves food along the digestive tract. Responses also include the neural control of glands in the body, 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 put into two groups: those that are voluntary or thought out (like skeletal muscle contraction) and those that are not (like smooth muscle contraction, heart muscle regulation, and gland activation). Voluntary responses are governed by the somatic nervous system, and involuntary responses are governed by the autonomic nervous system, which is discussed in the next section.
Integration. The nervous system processes the stimuli that sensory structures receive and communicates with them. 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. 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.
How they control the body
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The nervous system can be divided into two parts, mostly based on functional differences 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 they 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”).
Without the person being aware of it, the autonomic nervous system (ANS) controls the body. This is usually done to keep the body's internal environment stable, which is called homeostasis. Sensory input for autonomic functions can come from structures that are tuned to stimuli from the inside or outside of the body. The motor output extends to smooth and cardiac muscles as well as glandular tissue. The role of the autonomic system is to regulate the organ systems of the body, which usually means controlling homeostasis. The autonomic system, for example, regulates sweat glands. When you are hot, sweating helps cool your body down. That is a homeostatic mechanism. But when you are nervous, you might also start sweating. That is not homeostasis; it is the physiological response to an emotional state.
Another division of the nervous system describes functional responses. The smooth muscle and glandular tissue in your digestive system are controlled by the enteric nervous system (ENS). 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.
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