Frog+Heart+Discussion

Frog+Heart+Discussion - Electrical and Mechanical...

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Electrical and Mechanical Properties of  the Frog Heart Discussion #5 NPB 101L 10/30/2006
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Cardiac Muscle Neighboring cells are coupled by gap junctions. Depolarizing current flows through these junctions, and if depolarization reaches threshold, an action potential is generated in those cells.   Cardiac muscle is  striated, but muscle  cells are shorter than  skeletal muscle cells.   Adjacent cells are  joined end to end at  intercalated disks.
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The Human Heart
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From the body From the lungs Truncus arteriosus
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Three-Chambered Heart The frog heart contains two atria and one ventricle Conus arteriosus arises from the ventricle Contains a spiral valve that directs the oxygenated and deoxygenated blood into appropriate channels The superior and inferior vena cavae empty into the sinus venosus, which empties into the right atrium the sinus venosus, The sinus venosus is the pacemaker of the heart Both oxygenated and deoxygenated blood resides in the ventricle
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Cardiac Action Potential Originates in the sinoatrial node, located in the right atrium. The cells in the SA node are the pacemaker cells, meaning that they can depolarize spontaneously and fire off action potentials at a regular rate. Intrinsic heart rate is usually between 60 and 100 beats per minute The action potential propagates from cell to cell via gap junctions and spreads throughout the right and left atrium Electrical activity can spread quickly from the right atrium to the left by way of Bachman’s bundle, so the two atria are depolarized at essentially the same time
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Cardiac Action Potential Cont. The action potential does not spread directly from the atria to the ventricles. Once it reaches the AV node, the signal is carried by a network of specialized conducting cells known as the His- Purkinje fiber system This fiber system conducts the action potential onto the ventricular muscle in an orderly sequence Once the ventricular muscle is electrically activated, it enters an effective refractory period (ERP). The ERP is equivalent to the absolute refractory period seen in nerve and skeletal muscle During this time, an additional electrical stimulus cannot generate an action potential Once the cells begin to repolarize, the relative refractory period (RRP) begins. Another electrical stimulus will produce an action potential, but it will be smaller
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Conduction Pathway Through the Heart SA node A Atrium A AV node A Bundle of His/Purkinje fibers A Ventricles
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Underlying the Cardiac Action Potential There are four important time-dependent and voltage-gated membrane currents that give rise to the action potential 1. The Na + current (I Na ) – inward current responsible for the rapid depolarizing phase in atrial and ventricular muscle and in Purkinje fibers 2. The Ca 2+ current (I ca ) – inward current responsible for the rapid depolarizing phase in the SA and AV nodes 3. The K
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Frog+Heart+Discussion - Electrical and Mechanical...

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