The Nervous System

The picture you have in your mind of the nervous system probably includes the brain and the spinal cord.  Since the nervous system is just a couple of organs, you might think it to be a simple organ system, but this couldn’t be further from the truth. Within the human brain are some 86 billion neurons that process sensory information and determine behaviors in fractions of a second. The coordination of between the brain and the rest of the body is as result of connections between neurons in the spinal cord and through perihperal nerves. The nervous system can be divided into two parts: the central and peripheral nervous systems; let’s take a look at each of them.


TL; DR

  • The nervous system is responsible for (i) interpreting information from the environment (ii) integrating sensory information and (iii) coordinating behavior 
  • Neurons and glia are the two cell types found in the nervous system
  • The brain and spinal cord make up the central nervous system while nerves make up the peripheral nervous system.


Functions of the Nervous System


The nervous system is involved in receiving information about the environment around us (sensation) and generating responses to that information (motor responses). The nervous system can be divided into regions that are responsible for sensation (sensory functions) and for the response (motor functions). But there is a third function that needs to be included. Sensory input needs to be integrated with other sensations, as well as with memories, emotional state, or learning (cognition). Some regions of the nervous system are termed integration or association areas. The process of integration combines sensory perceptions and higher cognitive functions such as memories, learning, and emotion to produce a response.

Sensation

A major function of the nervous system is sensation. Receiving information about the environment to process what is happening is a necessary prerequisite for behavior. Information in the environment that triggers a sensory modalities is known as a stimulus. For example, let’s consider what the stimulus would be for each of the five senses:

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    • 👅 Taste or gustation is activated by a chemical stimulus (molecules, compounds, ions).
    • 👃 Smell or olfaction is also activated by a chemical stimulus (molecules, compounds, ions).
    • Touch or somatosensation is activated a physical or mechanical stimulus.
    • 👁️ Sight or vision is activated photons of light.
    • 👂 Hearing or audition is activated by sound waves.
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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

After receiving some sensory input, it’s also the job of the nervous system to produce some behavior in response to this information. An example might removing your hand (response) from a hot object (stimulus). To do this, the nervous system needs to communicate with the muscular system. However, neurons can also communicate with glands. One example would be the the production and secretion of sweat to lower body temperature.

 

Responses can be divided into those that are voluntary – as in contraction of skeletal muscle – or involuntary (contraction of smooth muscles, regulation of cardiac muscle, activation of glands). Voluntary responses are governed by the somatic nervous system and involuntary responses are governed by the autonomic nervous system, which are discussed in the next section.

 

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. 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, it would be fun to just swing away.

 
 

It’s often overlooked, but we actually have quite a number more than the tradition five senses. A few examples of ‘other’ senses are: pain, which is a unique sensory perception know as nocicpetion, or proprioception, which allows us to determine  body position is in 3D space.

 


 

Cells and Tissues of the Nervous System


Nervous tissue contains two basic types of cells: neurons and glia. There are different types of glia. Generally they maintain the environment that supports normal neuron function, while the neuron provides the  communicative function of the nervous system.


Structure of the Neuron


To describe the functional divisions of the nervous system, it is important to understand the structure of a neuron. Neurons are cells and therefore have a soma, or cell body, but they also have extensions of the cell; each extension is generally referred to as a process. 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. Dendrites are responsible for receiving most of the input from other neurons. Click the link for a more detailed look at the structure of the neuron.


Gray Matter and White Matter

 

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).  The images below show 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 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. Actually, gray matter may have that color ascribed to it because next to the white matter, it is just darker—hence, gray.

gray matter and white matter of the brain
 Author/Source: Dr. John Beal (LSU) | License: CC BY 2.5
Spinal Cord_crosssection
 Author/Source: OpenStax | License: CC BY 4.0

 


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 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 cross section of the spinal cord.

Neuron
 Author/Source: OpenStax | License: CC BY 4.0

Central Nervous System

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central nervous system

Peripheral Nervous System

peripheral nerves
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  1. Betts JG, Young KA, Wise JA, Johnson E, Poe B, Kruse DH, Korol O, Johnson JE, Womble M, DeSaix P. “12.1 Basic Structure and Function of the Nervous System.” Anatomy and Physiology. OpenStax, 2013. Houston, TX. https://openstax.org/books/anatomy-and-physiology/pages/12-1-basic-structure-and-function-of-the-nervous-systemLicense Terms: Edited & Adapted | Access for free at https://openstax.org/books/anatomy-and-physiology/pages/1-introduction.
  2. Spielman RM, Dumper K, Jenkins W, Lacombe A, Lovett M, Perlmutter. “”3.4 The Brain and Spinal Cord.” Psychology. OpenStax CNX. 2014. Houston, TX. https://openstax.org/books/psychology/pages/3-4-the-brain-and-spinal-cordLicense: CC BY 4.0 License Terms: Edited & Adapted | Access for free at https://openstax.org/books/psychology/pages/1-introduction.
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