|MadSci Network: Anatomy|
The nervous system can be divided into several parts, the biggest division being between the central nervous system (CNS), which contains the brain and spinal cord, and the peripheral nervous system, which contains all of the nerves and ganglia outside of the CNS. The peripheral nervous system can be further divided into two sets of nerves: voluntary and autonomic. The voluntary nerves are what you use to walk, to write, to sing, or to do anything else that you can control. The autonomic nerves are what your brain uses to run all of the functions of your body that you do not control, like digesting food, or pumping blood through the heart, or shivering when its cold.
One of the functions of the autonomic nervous system is to react to stress by changing how the body operates, to better deal with whatever is causing the stress. Emotions such as sorrow, fear, or anger are translated as stress to the autonomic nervous system, because the only causes of these emotions in our primitive mammalian ancestors were stressful situations. All mammals (and most other vertebrates) have developed a "Fight or Flight" response to danger, which gives the individual the speed to run, or the strength to fight, when attacked by another animal. To accomplish this, the stress of being attacked triggers the autonomic nervous system to increase the flow of oxygen and sugar to the muscles, so that they can be used more actively.
The increase in oxygen is accomplished at several levels: blood flow to the muscles is increased by making the heart beat harder and faster, and by decreasing blood flow to the internal organs; absorption of oxygen into the blood is increased by making the lungs breath faster; and air intake into the lungs is increased by opening the throat and mouth. The side effects of this are panting, heart pounding, and nausea. In the case of crying, which is the autonomic response to sorrow or grief, most of these effects are easily recognized.
An important part of opening the throat to allow the lungs to get more air involves expanding the glottis - the muscle which controls the opening from the back of the throat (pharynx) into the voice box (larynx) - so that more air can pass through it. So part of the autonomic response of crying is to use throat muscles to open the glottis as wide as possible. This doesn't pose a problem until you want to swallow; swallowing involves closing the glottis, so that food doesn't get into your larynx. So if you try to swallow while you are crying, the muscles for swallowing are fighting against the muscles for crying, and this tug-of-war over the glottis is perceived as a lump behind your larynx which makes it difficult to swallow.
As an aside, the autonomic nervous system has two parts: sympathetic and parasympathetic. These function together much like the gas and brake pedals in a car. The sympathetic nervous system activate the heightened response to stress, and the parasympathetic nervous system reduces stress activity and promotes more recouperative activities. One of the chief purposes of the parasympathetic nervous system is to coordinate and run digestion, which includes swallowing. So the lump in your throat is caused by both the sympathetic and parasympathetic nerves of the throat being activated at the same time.
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