Frog Heart-Mechanical, Electrical, Pharmacological Properties Labs Fall 2011

Frog Heart-Mechanical, Electrical, Pharmacological Properties Labs Fall 2011

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BIO 335 Frog Heart Frog Heart-1 FROG HEART Introduction The heart is an organ influenced by a variety of neurotransmitter, hormonal and excitation-contraction coupling features which differ from those of skeletal muscle. The action potentials initiating each cardiac cycle are autogenic, a feature not present in skeletal muscle. Further, the rate and strength of each cardiac ventricular muscle contraction is subject to modification by neurotransmitters, by the hormone epinephrine, by alteration of the ionic environment in the extracellular fluid space, and by altering the amount of stretch (i.e., the volume of blood in the ventricle) of ventricular muscle fibers. The electrical activity in the frog heart is initiated in pacemaker cells in the sinus venosus. (Note that in mammals the analogous cells are located in sino-atrial (SA) node.) The autorhythmic pacemaker cells in the sinus venosus spontaneously generate action potentials that propagate along fibers in the cardiac conducting system to cardiac muscle fibers in the right atrium and left atrium and to conducting system cells located at the atrio- ventricular (AV) node. (The conduction pathway in the frog heart is similar to that of the mammalian heart.) Following a conduction delay at the A-V node, due to slow conduction of fibers in the AV node, action potentials are conducted rapidly along the interior of the ventricular wall by Purkinje fibers. These are large diameter, modified and specialized non- contractile cardiac fibers. The action potentials spread from Purkinje fibers to ventricular muscle fibers, resulting in contraction of the ventricle (NOTE: Frogs have a single ventricle). In the mammalian heart, the pathway is different due to the presence of a right and left ventricle, with conduction proceeding from the A-V node to fibers in the right and left Bundles of His, and then to Purkinje fibers in the right and left ventricles. Action potentials in Purkinje fibers spread to the right and left ventricular muscle fibers via gap junctions, ultimately resulting in simultaneous right and left ventricular muscle cell contraction. The electrical activity associated with the above sequence of events can be recorded from the surface of the body. One such electrical record is the electrocardiogram, ECG. The Electrocardiogram (ECG) Cardiac action potentials, their sequential propagation, and their spread of excitation from the base of the heart to the apex of the heart, generate an electric field. The electrocardiogram (ECG) is a measure of the spatial differences in an electric field (i.e., electric differences between active regions of the myocardium that are negative extracellularly vs. the regions of the myocardium in a resting state of extracellular positivity.) It is important to remember that individual action potentials generate electrical differences between the inside and outside of cells, whereas the electrocardiogram arises from electrical differences between large excited (depolarized) and
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This note was uploaded on 12/06/2011 for the course BIO 335 taught by Professor Cabot during the Fall '08 term at SUNY Stony Brook.

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Frog Heart-Mechanical, Electrical, Pharmacological Properties Labs Fall 2011

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