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Reflexes

 

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Overview
Sneezing, coughing, and blinking are simple reflexes. But they aren't as simple as they seem. It's true that reflexes have a simple definition. They're involuntary actions or movements that occur in response to a stimulus. And you may not think that much goes on when you yawn, for example. But you might be surprised. Each reflex action can involve countless complex communications and intricate coordination among nerve cells called neurons. These neurons are found in the central nervous system, which includes your brain and spinal cord, and the peripheral nervous system, which includes all the nerves that reach your body's extremities. Not all neurons are alike, however. Sensory (or afferent ) neurons carry messages to the brain and spinal cord. Motor (or efferent) neurons carry messages away from the brain and spinal cord. They tell muscles to contract or relax and spur glands into action. Interneurons send messages between nerve cells within the brain, spinal cord, and the periphery. These busy characters make up over 99 percent of the more than 10 billion neurons in our nervous system. But why is the brain involved in reflex actions at all? As part of the nervous system, the brain has specialized functions, only some of which control thought or voluntary movement. The brain stem, for example, manages involuntary reflexes such as breathing and keeping our balance. We don't consciously decide to do these things. But a part of our brain is still involved. Reflexes serve as primitive responses that protect our bodies from danger and help us adjust to our surroundings. We cough, for example, when an irritant enters our windpipe and we need to expel it through our mouth. We sneeze when we need to clear our nasal air passages of irritants and allergens. We blink when danger threatens the sensitive tissues of the eye and when we need to moisten and clean the cornea. (This reflex occurs 900 times an hour!) We yawn when nerves in the brain stem find there's too much carbon dioxide in the blood. A yawn makes the muscles in our mouth and throat contract and forces our mouth wide open, allowing us to expel carbon dioxide and take in a large amount of oxygen-rich air. Without these reflex actions, we would be unable to survive. So even though they may be simple, these reflexes are a really big deal.

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