|MadSci Network: General Biology|
Hello Khash! Thank you for your question. You asked: "I want to know how and why the body and brain using the nervous system controls the heart rate." The heart is basically one big muscle with one important job: keep blood moving forward. The brain needs blood, the rest of the body needs blood, and even the heart itself needs blood (there are blood vessels in the heart muscle that the heart pumps to). Depending on what kind of situation your body is going through, be it eating or trying to get out of the way of a speeding car, your organs need a particular amount of blood in order to have oxygen for producing energy. The brain therefore signals the heart accordingly. The autonomous nervous system of the body is responsible for maintaining balance in our body systems. Two main divisions of this system, the sympathetic and the parasympathetic systems, both affect the heart. Let's look at these briefly: Parasympathetic Nervous System - (commonly known for inducing the "Rest and Digest" State). These nerve fibers come from the very top and very bottom of the spine (collectively referred to as the craniosacral levels) and secrete acetylcholine. Sympathetic Nervous System - (commonly known for inducing the "Fight or Flight" Response). These nerve fibers come from the thoracic and lumbar regions of the spine and secrete norepinephrine. These two systems work in cooperation throughout the day. If I scarf a Big Mac, my parasympathetic system will kick in and innervate my stomach, bladder, and salivary glands to focus my body's energies on processing the food. My heart rate will slow (since a fast heart rate during digestion is useless and would only waste energy that my digestive system could use). If, while I'm walking along eating my Big Mac, I accidentally step in front of a speeding car, my sympathetic system will suddenly kick in, and I'll attempt to run away. OK, what does this have to do with the heart? Well, the neat thing about the sympathetic nervous system is that it not only innervates the heart to increase the rate of signal conduction, but it also innervates a really cool organ, the Adrenal Medulla, which secretes hormones such as Epinephrine and Norepinephrine (epi. and norepi.) that do amazing things to the body. That weird rush that I would get if I suddenly found myself in front of a speeding car is my sympathetic system activating. The sensation is a combination of my blood vessels suddenly doing things that shifts the distribution of my blood around. Here's what happens: The sympathetic system directly affects the heart by increasing the rate of signal conduction. This then leads to an increase in both the speed and strength of the heart beat (heart rate and contractility) to get more blood moving forward. ***One note: Your question dealt with heart rate specifically. It seems logical that a heart beating faster would pump more blood forward, so that increasing the heart rate would be the best bet for helping the body. Actually, a heart simply beating faster eats up more energy since it's using more oxygen to keep itself pumping frantically. A more economical response by the brain is to signal the heart to increase the strength of each heart beat as well. This is very important. Now, a stronger heart beat can pump more blood forward per each beat, ultimately conserving energy. The sympathetic system can also affect the heart by doing things to other body parts which then affect the heart. Blood vessels in the skin and mucosa, for example, tighten up due to the effects of epi. and norepi.(you've seen this: if I see a ghost in my basement, you'd see me turn pale --- that's because the vessels in my skin and lips suddenly constricted, so I lose my pinkish color and blanch as blood flow is reduced). By doing this, less blood goes to these places, but more blood is saved for the heart, lungs, and brain. Likewise, other blood vessels in the muscles and lungs widen (dilate) in order to contain more blood to transport more oxygen. In total, your muscles, lungs, brain, and heart will be getting more oxygen from the blood by various vessel either constricting or dilating --- these organs are useful when you're trying frantically to scramble out of the way of that oncoming car...! The heart rate is affected by numerous other factors. Changes in blood pressures by changes in blood volume or by vessel dilation/constriction alter the stretch of certain arteries which ultimately leads to the brain speeding up or slowing down the heart rate as necessary. Certain drugs and toxins can speed, slow, or even completely block signal conduction to those nerves in the heart, ultimately changing the heart rate. One particular drug, Atropine, unbalances that sympathetic-parasympathetic balance by hiding the parasympathetic response so it ultimately looks like the body is only experiencing a sympathetic response. Hope this helped to answer your question! I kept my answer fairly general, but if you have further questions regarding the specific chemical processes involved with signal transduction in the brain and the heart, feel free to email me at firstname.lastname@example.org or submit another question to MadSci. Sincerely, G. Monreal For information available on the web, please consult the following excellent sites: http://paralia.com/athina/Heart/BeatRegulation.html http://sun.science.wayne.edu/~bio669/Cardio.html http://www.oucom.ohiou.edu/CVPhysiology/E010.htm
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