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 fire, making it the pacemaker of the heart. It has a pacemaker potential that is constantly being depolarized by a steady inflow of sodium ions. During exercise this pacemaker potential must depolarize quicker to increase heart rate and supply the body with more oxygenated blood.
It must reach its threshold of -40mV in order for its calcium channels to open up. The inflow of calcium ions further depolarizes the conducting cells (Saladin, pg 729). The rapid depolarization of the SA Node during exercise has a direct effect on the contractile cells of the heart. The contractile cells have a resting potential of -90mV and they are depolarized from gap junctions. These gap junctions connect them to conducting cells, and allow them to receive sodium ions. The faster depolarization rate of conducting cells causes sodium ions to flow through gap junctions at a higher rate.
This causes contractile cells to reach 30 mV faster and allows the slow calcium channels of the plasma membrane to open up (Saladin, pg Bio 214- TH 1- Spring 2013 – Prof. Kabrhel Name ______________ 730-731). Calcium binds to calmodulin which allows for the binding of actin and myosin. This causes the cells to contract. This is known as ventricular contraction in the cardiac cycle and is ST Segment of the ECG (Topic 2b, 27). Exercise effects cardiac output directly. Cardiac output is the amount of blood ejected by each ventricle in 1 minute. It is determined by heart rate and stroke volume.
During exercise cardiac output is 21 Liters/minute. This is because exercise is indirectly a chronotropic agent that calls upon the sympathetic nervous system (Saladin, pg 740). Exercise also increases stroke volume, which is the amount of blood ejected during the ventricular ejection phase of the cardiac cycle. During exercise preload is increased because veins deliver more deoxygenated blood back to the heart. The more blood returns to the heart the more the myocardium stretches, which results in a greater force of contraction. The greater the force of contraction, the greater the stroke volume will be.
Also, proprioceptors located on muscles and joints provide the heart with information. These receptors tell the heart to increase blood flow to make up for the added stress on the body caused by exercise (Saladin, pg 741-742). After exercise the body has to recover and return to normal function. The ability for the body to rest or recover after exercise is directly related to the heart. The parasympathetic nervous system reaches the heart through the vagus cranial nerve.
Preganglionic fibers of the vagus nerve synapse directly with postganglionic epicardial surfaces of the heart wall (Saladin, pg 728). The vagus nerve has inhibitory effects on the SA Node and it reduces heart rate. The parasympathetic nervous system produces the neurotransmitter acetylcholine (ACh) that can bind directly to muscarinic receptors on the plasma membrane of nodal cells. ACh binds directly to potassium gated channels and allows potassium ions to leave the cell. This hyperpolarizes the cells and causes the SA Node to fire less often.
This establishes the vagus tone or resting heart rate of 70-80 bpm. The inhibitory actions of the parasympathetic nervous system occur faster because it does not require the use of secondary messengers (Saladin, pg 740). Autorhythmic cells or conducting cells fire less frequently during rest. The pacemaker potential of the SA Node takes longer to reach threshold (-40mV) because of the parasympathetic nervous system. This causes less heart beats per minute and it also has a direct effect on the contractile cells (Saladin, pg 729). The contractile cells receive a slower inflow of sodium ions from the autorhythmic cells when the body is at rest.
This causes a slower depolarization of the contractile cells. This means that it will take contractile cells longer to reach their threshold, Bio 214- TH 1- Spring 2013 – Prof. Kabrhel Name ______________ and delays the slow calcium channels from opening. This causes time between contractions of the heart to increase and it also causes repolarization to take longer. During rest the cardiac cycle takes longer to be completed (Topic 2b, slide 25-29). Cardiac output is decreased when the body is resting; it is on 4-6 Liters/minute. When the body is resting both the heart rate and the stroke volume are decreased.
The inhibitory effects of the parasympathetic nervous system have a direct effect on reducing the heart rate. The stroke volume is reduced because there is significantly less blood returning to the heart via veins during rest. This makes the contraction of myocardium less because of the lack of stretching during the preload phase. During rest the heart does not have to work as hard to supply the entire body with oxygenated blood as frequently. This causes a significant decrease in the work load of the heart (Saladin, pg 743).