19.2 Cardiac Muscle and Electrical Activity

atrioventricular bundle branches

(also, left or right bundle branches) specialized myocardial conductile cells that arise from the bifurcation of the atrioventricular bundle and pass through the interventricular septum; lead to the Purkinje fibers and also to the right papillary muscle via the moderator band

Calcium ions play two critical roles in the physiology of cardiac muscle:

-influx through slow calcium channels accounts for the prolonged plateau phase - absolute refractory period that enable cardiac muscle to function properly

Action Potential in Cardiac Contractile Cells

-more stable resting phase than conductive cells at approximately −80 mV for cells in the atria and −90 mV for cells in the ventricles -rapid depolarization, followed by a plateau phase and then repolarization -long refractory periods required for the cardiac muscle cells to pump blood effectively before they are capable of firing for a second time

Membrane Potentials and Ion Movement in Cardiac Conductive Cells

-rise slowly from an initial value of −60 mV up to about -40 mV -slow influx of Na+ prepotential -Rapid influx of Ca2+ depolarization -outflux of K+ repolarization -Threshold

atrioventricular bundle

a bundle of modified heart muscle that transmits the cardiac impulse from the atrioventricular node to the ventricles causing them to contract

atrioventricular (AV) node

a node of specialized heart muscle located in the septal wall of the right atrium; receives impulses from the sinoatrial node and transmits them to the atrioventricular bundle

cardiac conduction system

a system of specialized muscle tissues that conducts electrical impulses that stimulate the heart to beat -triggered by the fastest cells - Components: sinoatrial node, atrioventricular node, atrioventricular bundle, atrioventricular bundle branches, and Purkinje cells

Cardiac muscle undergoes

aerobic respiration patterns, primarily metabolizing lipids and carbohydrates

heart block (AV)

an interference with the normal conduction of electric impulses that control activity of the heart muscle -SA nodal blocks occur within the SA node -AV nodal blocks occur within the AV node -Infra-Hisian blocks involve the bundle of His

cardiac muscle cells

branching chains of cells, uni- or binucleate striations(Dark A+ light I bands); intercalated discs(support the synchronized contraction of the muscle)

tetany

condition in which muscle remains involuntarily contracted, not compatible with life, since it would prevent the heart from pumping blood

myocardial contractile cells

conduct impulses and are responsible for contractions that pump blood through the body -99% of the cells in the atria and ventricles

artificial pacemaker

electronic device that stimulates impulse initiation within the heart

Heart rate is modulated by

endocrine and nervous systems

Purkinje fibers

fibers in the ventricles that transmit impulses to the right and left ventricles, causing them to contract

autorhythmicity

heart's ability to control its own contractions

a second-degree or incomplete block occurs when

impulses from the SA node reach the AV node and continue, while others do not -the ECG would reveal some P waves not followed by a QRS complex, while others would appear normal

A first-degree or partial block indicates a delay

in conduction between the SA and AV nodes - can be recognized on the ECG as an abnormally long PR interval

There are two major types of cardiac muscle cells:

myocardial contractile cells and myocardial conducting cells

Electrocardiogram (ECG)

recording of the electrical changes that occur in the myocardium during a cardiac cycle

Structure of Cardiac Muscle

short, branched cells, one central nucleus and 2 or more nuclei in come cells

myocardial conducting cells

specialized cells that transmit electrical impulses throughout the heart and trigger contraction by the myocardial contractile cells

Sinoatrial (SA) Node

the pacemaker; highly specialized, neurological tissue impeded in the wall of the right atrium; responsible for initiating electrical conduction of the heartbeat, causing the atria to contract and firing conduction of impulses to the AV node

third-degree or complete block

there is no correlation between atrial activity (the P wave) and ventricular activity (the QRS complex).

Cardiac Muscle Metabolism

uses fatty acid oxidation in both the well-fed and fasting states