Anatomy and Physiology: Sympathetic: Yikes!
Our autonomic nervous system (ANS) is responsible for our unconscious control of our body, as opposed to our somatic nervous system, which is our thoughts, our basic conscious perception of the senses, and our movements and other reactions to those senses. The ANS is a very old section of the brain, which, despite all that it controls, does not require as much energy to function. Much of this control is centered in the medulla oblongata, which is at the base of the brain stem, just superior to the spinal cord.
Although the processing is centered in the medulla, other sections of the brain are involved. The thalamus, as a switching station for sensory input, and an emotional center (due to the limbic system), sends input to the hypothalamus. The hypothalamus is the real control center, sending instructions to the pons (higher respiratory function) and the medulla, which in turn sends instructions along the sympathetic and parasympathetic pathways. In the simplest sense, the body needs to be able to gear up in response to periods of stress or danger. Equally important is the ability to slow down when the danger is over, in order to focus on those areas ignored when under threat.
Gearing Up Organs
If you were in my classroom I would either suddenly scream out loud or surreptitiously drop a large metal lab stool. Anything to make you jump! That loud surprise is enough to get the average person's sympathetic division kicked into high gear (I could release a live tiger, but I want to keep my job!). Think about your body's reaction to such a jump, especially these organs: heart, lungs, and brain (at least that part associated with alertness).
Flex Your Muscles
Myoglobin? Hemoglobin? It might help if you remember that hemoglobin has four subunits, each of which carries a single O2 molecule (four in all). Each of those four subunits is similar in structure to a myoglobin molecule, which of course can carry only one O2 molecule. Incidentally, the higher quantity of myoglobin in beef than chicken is what makes red meat red!
Here's where you need to put the pieces together. I'll get you started. One of the connections in the sympathetic pathway involves stimulating the adrenal glands to release epinephrine (adrenaline). It is the epinephrine that gives you that sense of terror: elevated heart rate, deeper respiration, heightened senses (enough to make you jumpy). You've experienced the what, but why does it happen? To answer that, you need to think about oxygen and glucose, our quick energy source.
Have you ever felt like your intestines are all knotted up when you were under stress? Those cramps are the result of peristalsis, or the movement of food along the GI tract coming to a halt. You can blame it on the sympathetic division of your ANS!
Since oxygen is so important for extracting energy from glucose, it makes sense that you would breathe deeply: The more deeply you breathe, the more oxygen you get. The more oxygen, the more energy. So why the fast heart rate? Easy! To get the oxygen to the muscles and to get rid of all the carbon dioxide you are releasing from your muscles. As a matter of fact, oxygen is easier to take up in the muscles due to the presence of myoglobin, which functions, like hemoglobin, to carry the oxygen.
You might remember that the body stores some of the excess glucose as the polysaccharide (multiple sugar) glycogen. In times of stress the majority of the glucose is used by the muscles, so it makes perfect sense that glycogen is stored there, especially with all that myoglobin.
The other place where glycogen is stored is the liver. Glycogen is released as a result of the action of epinephrine from the adrenal glands, which are also stimulated by the sympathetic division. Epinephrine also works to dilate the airways in the lungs and increase the heart rate. In addition to all these subconscious actions, the brain becomes more alert, no doubt to anticipate and react to danger.
Not all organs are stimulated, however, as certain actions need to be set aside during an emergency in order to place the energy where it is most needed. The kidneys are put on hold, leaving the filtering of waste until later. It may seem paradoxical, but the digestive system spends the inning in the dugout, too. On the surface it seems that such an action is foolhardy, as you need energy in an emergency, but the energy you can absorb in a short time is very small, considering the energy needed for the rest of the digestive process. For this reason, the body opts for using energy reserves (glycogen).
The sympathetic pathways follow the general arrangement of the spinal nerves, but after leaving the spinal cord they decide to go their own way (see Figure 20.10). This makes a certain amount of sense, as the spinal nerves have branches that do need to reach the skin, stimulate the muscles, and so on. The sympathetic branches are concerned with the other organs, such as those in the thoracic and abdominopelvic cavities.
Remember those pesky anterior and posterior roots, and how the spinal nerve branched off in front, making a sympathetic ganglion (called a paravertebral ganglion)? All the nerve fibers before the ganglion have the rather unromantic name preganglionic fibers. The postganglionic fibers (after the ganglion) are also called sympathetic nerves! As if that's not enough, the ganglia are connected vertically, forming a pathway parallel to the spinal cord called the sympathetic chain (or trunk). Along the way, a cardiac and pulmonary plexus can be found, as well as three ganglia (called prevertebral ganglia): the celiac (to and from the stomach, pancreas, liver, and adrenals), the superior mesenteric (to and from the small and large intestine), and the inferior mesenteric (to and from the rectum, kidneys, urinary bladder, and reproductive organs).
Excerpted from The Complete Idiot's Guide to Anatomy and Physiology 2004 by Michael J. Vieira Lazaroff. All rights reserved including the right of reproduction in whole or in part in any form. Used by arrangement with Alpha Books, a member of Penguin Group (USA) Inc.