During exercise your body must adapt to the added stress and pressure you are putting onto it. The heart plays a very important role in maintaining homeostasis during exercise. When exercising your heart rate increases and the “Left Heart” have to pump out more oxygenated blood to the entire body. The sympathetic nervous system also plays an important role during exercise. The sympathetic pathway comes out of the lower cervical and upper thoracic segments of the spinal cord. The sympathetic nervous system has postganglionic fibers that pass through cardiac plexuses and continue through cardiac nerves till they finally reach the heart (Saladin, pg 728). These postganglionic fibers release a neurotransmitter known as norepinephrine. Norepinephrine binds to Beta-adrenergic fibers of the heart. This binding activates Cyclic AMP (cAMP) which is a secondary messenger. These cAMPs activate an enzyme that opens up Calcium channels in the plasma membrane of heart fibers. Cyclic AMP increases both depolarization and repolarization rates of the Sinoatrial (SA) Node of the heart. Because of this, during exercise the heart can reach 120-180 bpm. The inflow of calcium ions accelerates depolarization of the SA Node. Also, cAMPs increase the calcium ion uptake of the sarcoplasmic reticulum of the SA Node which causes a faster rate of repolarization. This accelerating of both depolarization and repolarization increases heart rate to adjust to the added stress during exercise. In addition to the sympathetic nervous system, the chromaffin cells of the adrenal medulla release norepinephrine and epinephrine which increase heart rate as well (Saladin 741).
During exercise autorhythmic or conducting cells such as the Sinoatrial (SA) Node, Atrioventricular (AV) Node, right/left bundle branches of the AV node, and the purkinje fibers have to stimulate a faster heart rate (Topic 2b, slide 10). The SA Node does not have a resting potential which allows it to spontaneously...
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