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? Such rapid responses are controlled by your nervous system.
Overview of the Nervous System
The nervous system, which is 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. It then usually 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 instructed your body to act so as to avoid a collision.
Figure 2. The human nervous system consists of the brain and spinal cord (central nervous system) and a network of branching nerves that travel throughout the body (peripheral nervous system). Some of the major nerves in the peripheral system are identified in this drawing.
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 named neurons, or nerve cells, like the one in the figure below (all the parts of a neuron are explained in the next section). Dendrites of a neuron receive nerve impulse from other cells. Long projection (called axons) from neurons carry nerve impulses directly to specific target cells. Schwann cells wrapped around the axon are called glial cells. They create myelin sheath which allows the nerve impulse to travel very rapidly through the axons. A cell that receives nerve impulses from a neuron 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 meters 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.
Figure 3. This simple model of a neuron shows 1 Dendrite, 2 Axon, 3 Nodes of Ranvier, 4 Axon Terminals, 5 Schwann cell (Myelin Sheath), 6 Cell body, and 7 Nucleus
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. 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. You can learn much more about the CNS by reading the concept Central Nervous System.
Figure 4. The nervous system is divided into the peripheral and central systems. The peripheral nervous system is divided into the autonomic and somatic systems. The autonomic nervous system is divided into the sympathetic and parasympathetic systems.
The PNS is divided into two major parts, called the autonomic and somatic nervous systems. 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 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. You can learn more about the PNS and its subdivisions by reading the concept Peripheral Nervous System.
- 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 also has 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 for digestive processes.
- Signals sent by the nervous system are electrical signals called nerve impulses. They are transmitted by special cells called neurons. Nerve impulses can travel to specific target cells very rapidly.
- List the general steps by 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.
- 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 divisions of the autonomic nervous system.
- What is an axon and what is its function?
- True or False. A nerve impulse always causes the target cell to perform an action.
- True or False. The spinal cord is not considered part of the peripheral nervous system.
- Define nerve impulses.
- Explain why signals in the nervous system are generally more targeted and specific than signals in the endocrine system.
- Explain generally how the brain and spinal cord can interact with and control the rest of the body.
- ___________ actions are performed without the person thinking about them.
- The fight-or-flight response is controlled by the:
A. autonomic nervous system
B. somatic nervous system
C. central nervous system
D. parasympathetic nervous system
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?
Professor Megan Moynahan is working with teams of world-renowned researchers who are developing an artificial nervous system, with the hope that it will give people with paralysis the ability to do what would otherwise be the impossible for them. Watch her fascinating TED talk on the research in the video below.
Check out this handwritten tutorial and general overview of the divisions of our nervous system:
Credit: By Autoria propia. (Fruto de mi trabajo.) [Public domain], via Wikimedia Commons;
License: CC BY-NC 3.0
License: CC BY-NC 3.0
License: CC BY-NC 3.0
Credit: Suzanne Wakim;