The heart is regulated by both neural and endocrine control, yet it is capable of initiating its own action potential followed by muscular contraction. The conductive cells within the heart establish the heart rate and transmit it through the myocardium. The contractile cells contract and propel the blood. The normal path of transmission for the conductive cells is the sinoatrial (SA) node, internodal pathways, atrioventricular (AV) node, atrioventricular (AV) bundle of His, bundle branches, and Purkinje fibers. The action potential for the conductive cells consists of a prepotential phase with a slow influx of Na+ followed by a rapid influx of Ca2+ and outflux of K+. Contractile cells have an action potential with an extended plateau phase that results in an extended refractory period to allow complete contraction for the heart to pump blood effectively. Recognizable points on the ECG include the P wave that corresponds to atrial depolarization, the QRS complex that corresponds to ventricular depolarization, and the T wave that corresponds to ventricular repolarization.
artificial pacemaker
medical device that transmits electrical signals to the heart to ensure that it contracts and pumps blood to the body
atrioventricular bundle
(also, bundle of His) group of specialized myocardial conductile cells that transmit the impulse from the AV node through the interventricular septum; form the left and right atrioventricular bundle branches
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
atrioventricular (AV) node
clump of myocardial cells located in the inferior portion of the right atrium within the atrioventricular septum; receives the impulse from the SA node, pauses, and then transmits it into specialized conducting cells within the interventricular septum
autorhythmicity
ability of cardiac muscle to initiate its own electrical impulse that triggers the mechanical contraction that pumps blood at a fixed pace without nervous or endocrine control
Bachmann’s bundle
(also, interatrial band) group of specialized conducting cells that transmit the impulse directly from the SA node in the right atrium to the left atrium
bundle of His
(also, atrioventricular bundle) group of specialized myocardial conductile cells that transmit the impulse from the AV node through the interventricular septum; form the left and right atrioventricular bundle branches
electrocardiogram (ECG)
surface recording of the electrical activity of the heart that can be used for diagnosis of irregular heart function; also abbreviated as EKG
heart block
interruption in the normal conduction pathway
interatrial band
(also, Bachmann’s bundle) group of specialized conducting cells that transmit the impulse directly from the SA node in the right atrium to the left atrium
intercalated disc
physical junction between adjacent cardiac muscle cells; consisting of desmosomes, specialized linking proteoglycans, and gap junctions that allow passage of ions between the two cells
internodal pathways
specialized conductile cells within the atria that transmit the impulse from the SA node throughout the myocardial cells of the atrium and to the AV node
myocardial conducting cells
specialized cells that transmit electrical impulses throughout the heart and trigger contraction by the myocardial contractile cells
myocardial contractile cells
bulk of the cardiac muscle cells in the atria and ventricles that conduct impulses and contract to propel blood
P wave
component of the electrocardiogram that represents the depolarization of the atria
pacemaker
cluster of specialized myocardial cells known as the SA node that initiates the sinus rhythm
prepotential depolarization
(also, spontaneous depolarization) mechanism that accounts for the autorhythmic property of cardiac muscle; the membrane potential increases as sodium ions diffuse through the always-open sodium ion channels and causes the electrical potential to rise
Purkinje fibers
specialized myocardial conduction fibers that arise from the bundle branches and spread the impulse to the myocardial contraction fibers of the ventricles
QRS complex
component of the electrocardiogram that represents the depolarization of the ventricles and includes, as a component, the repolarization of the atria
sinoatrial (SA) node
known as the pacemaker, a specialized clump of myocardial conducting cells located in the superior portion of the right atrium that has the highest inherent rate of depolarization that then spreads throughout the heart
sinus rhythm
normal contractile pattern of the heart
spontaneous depolarization
(also, prepotential depolarization) the mechanism that accounts for the autorhythmic property of cardiac muscle; the membrane potential increases as sodium ions diffuse through the always-open sodium ion channels and causes the electrical potential to rise
T wave
component of the electrocardiogram that represents the repolarization of the ventricles
1. Which of the following is unique to cardiac muscle cells?
A) Only cardiac muscle contains a sarcoplasmic reticulum.
B) Only cardiac muscle has gap junctions.
C) Only cardiac muscle is capable of autorhythmicity
D) Only cardiac muscle has a high concentration of mitochondria.
C
2. The influx of which ion accounts for the plateau phase?
A) sodium
B) potassium
C) chloride
D) calcium
D
3. Which portion of the ECG corresponds to repolarization of the atria?
A) P wave
B) QRS complex
C) T wave
D) none of the above: atrial repolarization is masked by ventricular depolarization
D
4. Which component of the heart conduction system would have the slowest rate of firing?
A) atrioventricular node
B) atrioventricular bundle
C) bundle branches
D) Purkinje fibers
D
1. Why is the plateau phase so critical to cardiac muscle function?
It prevents additional impulses from spreading through the heart prematurely, thereby allowing the muscle sufficient time to contract and pump blood effectively.
2. How does the delay of the impulse at the atrioventricular node contribute to cardiac function?
It ensures sufficient time for the atrial muscle to contract and pump blood into the ventricles prior to the impulse being conducted into the lower chambers.
3. How do gap junctions and intercalated disks aid contraction of the heart?
Gap junctions within the intercalated disks allow impulses to spread from one cardiac muscle cell to another, allowing sodium, potassium, and calcium ions to flow between adjacent cells, propagating the action potential, and ensuring coordinated contractions.
4. Why do the cardiac muscles cells demonstrate autorhythmicity?
Without a true resting potential, there is a slow influx of sodium ions through slow channels that produces a prepotential that gradually reaches threshold.