Rhythymic Excitation and Coordination of the Heart: Cardiology.
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Grado
Physiology (FLGX317)
Institución
North-West University (NWU)
Book
Guyton and Hall Textbook of Medical Physiology
This document provides a in-depth summary of the chapters from the textbooks in relation to the above topic. The topic focuses and discusses the muscle cells, excitation-contraction coupling, the conduction system, the SA node, internodal pathways, Parkinje system and various other factors that are...
Topic: Rhythmic Excitation
and Coordination of the Heart
Course: Cardiology
Study Unit 2
Questions Notes
Explain the Action Potential in the ventricular muscle fiber averages about
characteristics of an 105 mv- the intracellular potential rises from a negative- -85
action potential in the mv, to a positive- +20 mv.
heart muscle After the initial spike, membrane remains depolarized for about
0.2 secs, exhibiting a plateau, followed by a rapid
repolarization.
Presence of plateau cases contraction to last 15 secs longer in
cardiac cells.
Characteristics of action potential in cardiac cells:
Action potentials is caused by two types of channels that are
opened:
o Fast Na+ channels
o Slow Ca+ channels, also called Calcium-Sodium
channels.
The fast Na+ channels remain open for only a few thousandths
of a second and close abruptly.
The Slow Ca+ channels remain open for several tenths of a
second. During this time, large quantities of calcium and
sodium flow to the interior of the muscle fiber and this
maintains the prolonged period of depolarization- causing the
plateau in the action potential.
The calcium ions that enter the plateau phase activate the
muscle contractile process.
Immediately after the AP, the permeability of the cardiac
muscle membrane for potassium ions decreases about 5 fold-
this may be due to the excess calcium influx.
, The decreased potassium permeability greatly decreases the
outflux of positively charged potassium ions during AP plateau
and thus prevents early return of the action potential voltage
to its resting level.
The permeability for K increases greatly, once the Ca-Na
channels close. And this rapid loos of K returns the membrane
potential to its resting level, thus ending the action potential.
Action potential is initiated in the SA node
AP spreads throughout the myocardium, passing from cell to
cell through gap junctions
Depolarization first occurs in the cells of the atria
Conduction rapid enough that the left and right atria contract
at the same time
The AP is transferred to the AV node- located at the base of the
right atrium.
Conducted rapidly through the internodal pathways
Analyse the excitation-
AP flows down the interventricular septum and conducted
contraction coupling in
through the Bundle of His/ atrioventricular bundle.
the heart muscle cells
The bundle of His divides into a right and left branch which
separate at the apex of the heart and enter the walls of the
ventricles.
These pathways consist the Purkinje fibers
Purkinje fibers conduct AP rapidly to myocytes in the
ventricles.
This causes rapid depolarization of the ventricles
simultaneously and ensure single coordinated contraction.
SA node- sinoatrial node where normal rhythmical impulses are
generated
Explain the organisation Internodal pathways that conduct impulses from the SA node to the
of the specialized AV node.
excitation and AV node- atrioventricular node where impulses from the atria are
conduction system in the delayed before passing into the ventricles.
heart AV bundle- Bundle of His which conduct impulses form the atria to the
ventricles; left and right bundle branches of Purkinje fibres- conduct
cardiac impulses to all parts of the ventricles.
Discuss the self- High Na ion conc. in the ECF outside the nodal pathway, and open Na
excitation of the sino- channels; Na ions outside of the fibres tend to leak to the inside. Thus,
atrial node (SA node) between heartbeats the influx of Na causes causes a slow rise in the
and evaluate the RMP in the positive direction.
consequences of this When the threshold voltage- -40mv, is reached; L-type calcium
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