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Sympathetic Nervous System: Definition, Function & Effects

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  • 0:01 Sympathetic Nervous…
  • 0:38 Autonomic Nervous…
  • 1:36 Hormones Involved in…
  • 2:29 Physiological Changes
  • 4:28 Lesson Summary
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Lesson Transcript
Instructor: John Williams
The autonomic nervous system is the portion that controls all of the involuntary functions of the body. This includes the body's response to emergency situations. In this article, we discuss the sympathetic nervous system and its responses under these circumstances.

Sympathetic Nervous System in Action

Imagine this: you are driving home, doing 75 miles an hour in a 55 mile an hour zone. Suddenly, you see the blue lights of a police car pulling up behind you. Your body starts to respond:

You begin to sweat heavily!
Your heart begins to beat faster and faster!
Your breathing starts to increase!

You, my friend, have just entered into sympathetic nervous activation.

In this lesson, we'll discuss the purpose of the sympathetic nervous system, what physiological changes your body experiences under sympathetic control, and why those changes are necessary in situations such as the one we just mentioned.

The Autonomic Nervous System at a Glance

The autonomic nervous system is the portion of the nervous system that regulates involuntary processes. In short, these are the processes that you do not purposely control. It's divided into two basic segments: the parasympathetic and the sympathetic nervous systems.

The parasympathetic nervous system (nicknamed 'the rest and digest system') is the segment that assists with normal autonomic functions. In other words, you are normally in parasympathetic nervous system control for most of your existence.

However, under emergency and stressful situations, the body begins to change in order to prepare you for the pending circumstances. This is known as the sympathetic nervous system, which is nicknamed 'fight or flight.' These physiological changes are designed to give you an added advantage for surviving emergency situations or making it through highly stressful situations. In a sense, this system gives you superpowers that are beyond your normal capabilities.

Hormones Involved in Sympathetic Nervous Activation

Norepinephrine and epinephrine are two hormones that are released in response to emergencies within the body. These two hormones often lead to what is common called an 'adrenaline rush,' which is the feeling of urgency that comes during stressful situations. Norepinephrine and epinephrine help the body to perform at optimal levels for such events.

During the initial activation of the sympathetic system, norepinephrine gets released first as a preparatory hormone that gets the body ready for the first stages of the emergency event. This also allows the body to return to normal quickly if the perceived situation is a false alarm. However, in the event that the circumstances require improved performance, then epinephrine will be released to augment, or increase, these physiological effects. These hormones work synergistically (or in combination) to activate the body.

Physiological Changes

Let's imagine for a second that you are being chased by a tiger. In order to survive this event, your body has to provide you certain capabilities. When emergency situations occur, such as this, several key changes will take place to prepare for the event.

Stimulatory Effects

First, blood flow will increase to all essential tissues. This includes increased blood flow to the skeletal muscle, which will be needed to escape the situation and/or fight off the threat. In order to do this, the heart must be stimulated to contract faster and harder, which will be accomplished by norepinephrine and epinephrine. Secondly, more oxygen will be needed to produce ATP (or energy), so breathing rates will increase. Finally, glycogen, which is a storage form of glucose, will be broken down so that glucose molecules can be used for energy. These are the primary responses that will take place in the body.

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