The Autonomic Nervous System
Before diving into ANS anatomy and physiology, it is important to have a general understanding of its location within the Nervous system as a whole. The image below displays this quite well.
We can see that the autonomic nervous system is a component of the peripheral nervous system, and that it belongs specifically to the motor division of the PNS.
This can be a bit confusing, because we tend to associate everything involving the body’s viscera (organs) with the ANS, including sensory information from the viscera (upset stomach, etc). This is not the case however. The Autonomic Nervous system is solely responsible for the innervation of smooth muscle, cardiac muscle, and glands. For this reason, it is strictly a motor arm of the peripheral nervous system. Sensory information travels to the spinal cord from the viscera along visceral sensory fibers that are separate from the ANS.
So, knowing what the ANS innervates (knowing what its effectors are) we can make a fairly educated guess about what its roles are. Knowing that it innervates smooth muscle, we can surmise that it plays a role in gastric motility, artery vasoconstriction/dilation, gland secretions, heart rate, and bronchiole constriction/dilation, among other things involving smooth muscle, cardiac muscle, and glands. These guesses are correct! The ANS helps maintain many internal functions including those listed above. Rather than list all of the ANS’s functions as a whole, lets take a look at the two divisions of the ANS and their functions.
Most organs of the body have dual innervation. This means that they are innervated by both divisions of the autonomic nervous system. These two division are usually antagonistic. This means that the divisions are in a constant battle over the control of their target organs. Whichever system happens to be stimulating its target organ at a higher rate, will exert a greater influence over that organ.
The two ANS divisions discussed above are the sympathetic division, and the parasympathetic division. Their functions are described below:
The Sympathetic Division
This system is responsible for preparing our body for emergency situations. Anytime you can recall being frightened or anxious, you can be fairly sure that your sympathetic nervous system was stimulated in response. We sometimes refer to this system as the fight or flight system, as it prepares our body to run from danger, or to combat whatever is threatening us. Knowing this, it is fairly easy to remember its effect on the body. Let’s take a look at these effects below:
1. Shunts blood away from structures that do not need large amounts of blood flow during an emergency (visceral organs, skin) and redirects this blood to the skeletal muscles and heart.
2. Increases the diameter of the bronchioles in the lungs by promoting vasodilation and therefore increasing ventilation.
3. Causes increased release of glucose into the blood to serve as fuel.
4. In general, the sympathetic division slows down non essential activities such as GI Motility (movement of the gastrointestinal tract)
The Parasympathetic Division
The other division of the ANS is sometimes referred to as the rest and digest division. Its job is to keep body energy as low as possible, and to promote activities that are not essential during emergencies, but necessary to maintain homeostasis such as GI motility.
The parasympathetic effects are typically opposite those of the the sympathetic division. It promotes blood flow to the digestive system, saliva production, bronchiole constriction, decreased heart rate, pupillary constriction, and other vital activities that are not necessarily important during short term emergencies.
Role of the Autonomic Nervous System Quiz
For each question determine if the activity listed is a function of the sympathetic or parasympathetic division. Keep in mind that not all activities here are listed above, you must use your knowledge of the general functions of the two divisions to determine some answers
1. Decreased HR
2. Pupillary dilation
3. GI motility suppression
4. constriction of cutaneous blood vessels
5. increased blood flow to heart and skeletal muscles
6. bronchiole constriction
7. increased release of digestive enzymes
8. increased release of glucose into the blood
9. eye lenses accommodate to near field vision (tough one)
10. Increased blood pressure