78 8.2 Introduction to the Nervous System
Created by CK-12 Foundation/ Adapted by Christine Miller
In the Blink of an Eye
As you drive into a parking lot, a boy on a skateboard suddenly flies in front of your car across your field of vision. You see the boy in the nick of time and react immediately. You slam on the brakes and steer sharply to the right — all in the blink of an eye. You avoid a collision, but just barely. You’re shaken up, but thankful that no one was hurt. How did you respond so quickly? Rapid responses like this are controlled by your nervous system.
Overview of the Nervous System
The nervous system, illustrated in the sketch below, is the human organ system that coordinates all of the body’s voluntary and involuntary actions, by transmitting electrical signals to and from different parts of the body. Specifically, the nervous system extracts information from the internal and external environments, using sensory receptors. Usually, it then sends signals encoding this information to the brain, which processes the information to determine an appropriate response. Finally, the brain sends signals to muscles, organs, or glands to bring about the response. In the example above, your eyes detected the boy, the information traveled to your brain, and your brain told your body to act so as to avoid a collision.
Signals of the Nervous System
The signals sent by the nervous system are electrical signals called nerve impulses, and they are transmitted by special nervous system cells called neurons (or nerve cells), like the one in Figure 8.2.3. Long projections (called axons) from neurons carry nerve impulses directly to specific target cells. A cell that receives nerve impulses from a neuron (typically a muscle or a gland) may be excited to perform a function, inhibited from carrying out an action, or otherwise controlled. In this way, the information transmitted by the nervous system is specific to particular cells and is transmitted very rapidly. In fact, the fastest nerve impulses travel at speeds greater than 100 metres per second! Compare this to the chemical messages carried by the hormones that are secreted into the blood by endocrine glands. These hormonal messages are “broadcast” to all the cells of the body, and they can travel only as quickly as the blood flows through the cardiovascular system.
This simple model of a nerve cell shows part of its long axon which carries nerve impulses to other cells. The multiple shorter projections are called dendrites, and they receive nerve impulses from other cells.
Organization of the Nervous System
As you might predict, the human nervous system is very complex. It has multiple divisions, beginning with its two main parts, the central nervous system (CNS) and the peripheral nervous system (PNS), as shown in the diagram below (Figure 8.2.4). The CNS includes the brain and spinal cord, and the PNS consists mainly of nerves, which are bundles of axons from neurons. The nerves of the PNS connect the CNS to the rest of the body.
The PNS can be further subdivided into two divisions, known as the autonomic and somatic nervous systems (Figure 8.2.5). These divisions control different types of functions, and they often interact with the CNS to carry out these functions. The somatic nervous system controls activities that are under voluntary control, such as turning a steering wheel. The autonomic nervous system controls activities that are not under voluntary control, such as digesting a meal. The autonomic nervous system has three main divisions: the sympathetic division (which controls the fight-or-flight response during emergencies), the parasympathetic division (which controls the routine “housekeeping” functions of the body at other times), and the enteric division (which provides local control of the digestive system).
8.2 Summary
- The nervous system is the human organ system that coordinates all of the body’s voluntary and involuntary actions, by transmitting signals to and from different parts of the body.
- The nervous system has two major divisions, called the central nervous system (CNS) and the peripheral nervous system (PNS). The CNS includes the brain and spinal cord, and the PNS consists mainly of nerves that connect the CNS with the rest of the body.
- The PNS can be subdivided into two major divisions: the somatic nervous system and the autonomic nervous system.The somatic system controls activities that are under voluntary control. The autonomic system controls activities that are not under voluntary control. The autonomic nervous system is further divided into the sympathetic division (which controls the fight-or-flight response), the parasympathetic division (which controls most routine involuntary responses), and the enteric division (which provides local control of the digestive system).
- Electrical signals sent by the nervous system are called nerve impulses. They are transmitted by special cells called neurons. Nerve impulses can travel to specific target cells very rapidly.
8.2 Review Questions
- List the general steps through which the nervous system generates an appropriate response to information from the internal and external environments.
- What are neurons?
- Compare and contrast the central and peripheral nervous systems.
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- Which major division of the peripheral nervous system allows you to walk to class? Which major division of the peripheral nervous system controls your heart rate?
- Identify the functions of the three main divisions of the autonomic nervous system.
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- What is an axon, and what is its function?
- Define nerve impulses.
- Explain generally how the brain and spinal cord can interact with and control the rest of the body.
- How are nerves and neurons related?
- What type of information from the outside environment do you think is detected by sensory receptors in your ears?
8.2 Explore More
The Nervous System, Part 1: Crash Course A&P #8, CrashCourse, 2015.
Engineering the Human Nervous System: Megan Moynahan at TEDxBrussels,
TEDx Talks, 2013.
Attributions
Figure 8.2.1
Skateboard_1613 by Autoria propia on Wikimedia Commons is released into the public domain (https://en.wikipedia.org/wiki/Public_domain) (Derivative work of this file: SkateboardinDog.jpg)
Figure 8.2.2
Nervous_system_diagram.svg by The Emirr on Wikimedia Commons is used under a CC BY 3.0 (https://creativecommons.org/licenses/by/3.0/deed.en) license.
Figure 8.2.3
MultipolarNeuron by BruceBlaus on Wikimedia Commons is used under a CC BY 3.0 (https://creativecommons.org/licenses/by/3.0/deed.en) license.
Figure 8.2.4
Overview_of_Nervous_System by OpenStax on Wikimedia Commons is used under a CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/deed.en) license.
Figure 8.2.5
Divisions of the Nervous System by CK-12 Foundation is used under the CC BY-NC 3.0 (https://creativecommons.org/licenses/by-nc/3.0/) license.
References
Betts, J. G., Young, K.A., Wise, J.A., Johnson, E., Poe, B., Kruse, D.H., Korol, O., Johnson, J.E., Womble, M., DeSaix, P. (2013, April 25). Figure 12.2 Central and peripheral nervous system [digital image]. In Anatomy and Physiology (Section 12.1). OpenStax. https://openstax.org/books/anatomy-and-physiology/pages/12-1-basic-structure-and-function-of-the-nervous-system
Brainard, J/ CK-12 Foundation. (2016). Figure 5 [digital image]. In CK-12 College Human Biology (Section 10.2) [online Flexbook]. CK12.org. https://www.ck12.org/book/ck-12-college-human-biology/section/10.2/
CrashCourse. (2015, February 23). The nervous system, Part 1: Crash Course A&P #8. YouTube. https://www.youtube.com/watch?v=qPix_X-9t7E&feature=youtu.be
TEDx Talks. (2013, November 3). Engineering the human nervous system: Megan Moynahan at TEDxBrussels. YouTube. https://www.youtube.com/watch?v=Nsxw5_Iz7mY&feature=youtu.be
The highly complex body system of an animal that coordinates its actions and sensory information by transmitting signals to and from different parts of its body. The nervous system detects environmental changes that impact the body, then works in tandem with the endocrine system to respond to such events.
Actions which take place according to the one's desire or are under control.
Actions which are not under one's conscious control.
Specialized nerve cell that responds to a particular type of stimulus such as light or chemicals by generating a nerve impulse.
The central nervous system organ inside the skull that is the control center of the nervous system.
A group of tissues in a living organism that have been adapted to perform a specific function. In higher animals, organs are grouped into organ systems; e.g., the esophagus, stomach, and liver are organs of the digestive system.
A signal transmitted along a nerve fiber.
A functional unit of the nervous system that transmits nerve impulses; also called a nerve cell.
One of two main divisions of the nervous system that includes the brain and spinal cord.
One of two major divisions of the nervous system that consists of all the nervous tissue that lies outside the central nervous system.
A thin, tubular bundle of central nervous system tissue that extends from the brainstem down the back to the pelvis and connects the brain with the peripheral nervous system.
A structure in the nervous system that consists of cable-like bundles of axons and makes up the majority of the peripheral nervous system.
A long extension of the cell body of a neuron that transmits nerve impulses to other cells.
A division of the peripheral nervous system that controls involuntary activities.
A division of the peripheral nervous system that controls voluntary activities.
The division of the autonomic nervous system that controls the fight-or-flight response.
The division of the autonomic nervous system that returns the body to normal after the fight-or-flight response and maintains homeostasis at other times.
A division of the autonomic nervous system that controls digestive functions.
division of the peripheral nervous system that controls involuntary activities
