Cardiovascular System Module 4: Cardiac Muscle and Electrical Activity
By the end of this section, you will be able to:

Cardiac muscle shares a few characteristics with both skeletal muscle and smooth muscle, but it has some unique properties of its own. The most important property is its ability to create an electrical signal that causes its own cells to contract. This property is known as autorhythmicity. Neither smooth nor skeletal muscle can do this. Even though cardiac muscle has autorhythmicity, heart rate is controlled by the endocrine and nervous systems.

Structure of Cardiac Muscle

Compared to the giant cylinders of skeletal muscle, cardiac muscle cells, or cardiomyocytes, are considerably smaller. Cardiac muscle also demonstrates striations, the alternating pattern of dark and light bands. Mitochondria are plentiful, providing energy for the contractions of the heart. Typically, cardiomyocytes have a single, central nucleus, but two or more nuclei may be found in some cells.

Cardiac muscle cells branch freely, and are often Y shaped. A connection between two adjoining cells is an intercalated disc, which helps support the coordinated contraction of the muscle ([link]b). The cell membranes from adjacent cells bind together at the intercalated discs. They help to synchronize the contraction ([link]c). Synchronizing the action of the muscle cells help the heart to pump blood more efficiently. Strong connective tissue also helps to bind the cells together. It is important to create a strong bond between cells so they can resist the stress that comes with their constant contractions.

Cardiac Muscle
(a) Cardiac muscle cells have myofibrils composed of myofilaments arranged in sarcomeres, T tubules to transmit the impulse from the sarcolemma to the interior of the cell, numerous mitochondria for energy, and intercalated discs that are found at the junction of different cardiac muscle cells. (b) A photomicrograph of cardiac muscle cells shows the nuclei and intercalated discs. (c) An intercalated disc connects cardiac muscle cells and consists of desmosomes and gap junctions. LM × 1600. (Micrograph provided by the Regents of the University of Michigan Medical School © 2012)
The top left panel of this figure shows the cross structure of cardiac muscle with the major parts labeled. The top right panel shows a micrograph of cardiac muscle. The bottom panel shows the structure of intercalated discs.

Conduction System of the Heart

If embryonic heart cells are separated into a Petri dish and kept alive, each is capable of generating its own electrical impulse followed by contraction. A fully developed adult heart maintains the capability of generating its own electrical impulse. The components of the cardiac conduction system include the sinoatrial node, the atrioventricular node, the atrioventricular bundle, the atrioventricular bundle branches, and the Purkinje cells ([link]).

Conduction System of the Heart
Specialized conducting components of the heart include the sinoatrial node, the internodal pathways, the atrioventricular node, the atrioventricular bundle, the right and left bundle branches, and the Purkinje fibers.
This image shows the anterior view of the frontal section of the heart with the major parts labeled.

Sinoatrial (SA) Node

Normal cardiac rhythm is established by the sinoatrial (SA) node, a specialized clump of myocardial cells located in the superior and posterior walls of the right atrium . The SA node is known as the pacemaker of the heart. It initiates the sinus rhythm, or normal electrical pattern followed by contraction of the heart.

Cardiac Conduction
(1) The sinoatrial (SA) node and the remainder of the conduction system are at rest. (2) The SA node initiates the action potential, which sweeps across the atria. (3) After reaching the atrioventricular node, there is a delay of approximately 100 ms that allows the atria to complete pumping blood before the impulse is transmitted to the atrioventricular bundle. (4) Following the delay, the impulse travels through the atrioventricular bundle and bundle branches to the Purkinje fibers, and also reaches the right papillary muscle via the moderator band. (5) The impulse spreads to the contractile fibers of the ventricle. (6) Ventricular contraction begins.
This image shows the different stages in the conduction cycle of the heart.

Atrioventricular (AV) Node

The atrioventricular (AV) node is a second clump of specialized myocardial conductive cells, located at the bottom of the right atrium within the atrioventricular septum. The septum prevents the impulse from spreading directly to the ventricles without passing through the AV node. It takes the impulse approximately 100 ms to pass through the node. This pause is critical to heart function, as it allows the atria to complete their contraction that pumps blood into the ventricles before the impulse is transmitted to the cells of the ventricle itself.

Atrioventricular Bundle (Bundle of His), Bundle Branches, and Purkinje Fibers

Continuing from the AV node, the atrioventricular bundle, or bundle of His, which follows along the interventricular septum before dividing into two atrioventricular bundle branches, commonly called the left and right bundle branches. The left bundle branch supplies the left ventricle, and the right bundle branch the right ventricle. Since the left ventricle is much larger than the right, the left bundle branch is also considerably larger than the right.

The Purkinje fibers (purr 'kin' gee) are additional myocardial conductive fibers that spread the impulse to the ventricles. They extend throughout the myocardium from the apex of the heart toward the atrioventricular septum and the base of the heart. Since the electrical stimulus begins at the apex, the contraction also begins at the apex and travels toward the base of the heart, similar to squeezing a tube of toothpaste from the bottom. This allows the blood to be pumped out of the ventricles and into the aorta and pulmonary trunk.

Electrocardiogram

By careful placement of surface electrodes on the body, it is possible to record the complex electrical signal of the heart. This tracing of the electrical signal is the electrocardiogram (ECG), also commonly abbreviated EKG (K coming kardiology, from the German term for cardiology). Careful analysis of the ECG reveals a detailed picture of both normal and abnormal heart function, and is an important clinical diagnostic tool. The standard electrocardiograph (the instrument that generates an ECG) uses 3, 5, or 12 leads. The greater the number of leads an electrocardiograph uses, the more information the ECG provides. The term “lead” is used to refer to the cable from the electrode to the electrical recorder.

Standard Placement of ECG Leads
In a 12-lead ECG, six electrodes are placed on the chest, and four electrodes are placed on the limbs.
This diagram shows the points where electrodes are placed on the body for an ECG.

A normal ECG tracing is presented in [link]. Each component, segment, and interval is labeled and corresponds to important electrical events, such as the action potential. demonstrating the relationship between these events and contraction in the heart.

The Action PotentialThe heartbeat is initiated by an electrical stimulus known as an action potential. When the heart receives a signal from the action potential the heart muscle cells depolarize (contract). When this signal fades, the muscle cells will repolarize (relax).

There are five prominent points on the ECG: the P wave, the QRS complex, and the T wave. The small P wave represents the depolarization (contraction) of the atria. The atria begin contracting approximately 25 ms after the start of the P wave. The large QRS complex represents the depolarization (contraction) of the ventricles, which requires a much stronger electrical signal because of the larger size of the ventricular cardiac muscle. During the QRS complex the atria are repolarizing (relaxing). The ventricles begin to contract as the QRS reaches the peak of the R wave. Lastly, the T wave represents the repolarization (relaxation) of the ventricles.

The major segments and intervals of an ECG tracing are indicated in [link]. Segments are defined as the regions between two waves. Intervals include one segment plus one or more waves. For example, the PR segment begins at the end of the P wave and ends at the beginning of the QRS complex. The PR interval starts at the beginning of the P wave and ends with the beginning of the QRS complex. The PR interval is more clinically relevant, as it measures the duration from the beginning of atrial depolarization (the P wave) to the initiation of the QRS complex. Since the Q wave may be difficult to view in some tracings, the measurement is often extended to the R that is more easily visible. Should there be a delay in passage of the impulse from the SA node to the AV node, it would be visible in the PR interval. [link] correlates events of heart contraction to the corresponding segments and intervals of an ECG.

Electrocardiogram
A normal tracing shows the P wave, QRS complex, and T wave. Also indicated are the PR, QT, QRS, and ST intervals, plus the P-R and S-T segments.
This figure shows a graph of millivolts over time and the heart cycles during an ECG.
ECG Tracing Correlated to the Cardiac Cycle
This diagram correlates an ECG tracing with the electrical and mechanical events of a heart contraction. Each segment of an ECG tracing corresponds to one event in the cardiac cycle.
This diagram shows the different stages of heart contraction and relaxation along with the stages in the QT cycle.

A heart block refers to an interruption in the normal conduction pathway. The nomenclature for these is very straightforward. SA nodal blocks occur within the SA node. AV nodal blocks occur within the AV node. Infra-Hisian blocks involve the bundle of His. Bundle branch blocks occur within either the left or right atrioventricular bundle branches. Hemiblocks are partial and occur within one or more fascicles of the atrioventricular bundle branch. Clinically, the most common types are the AV nodal and infra-Hisian blocks.

A heart block refers to an interruption in the normal conduction pathway. This results in an abnormal heart rhythm known a an arrhythmia. When arrhythmias become a chronic problem, a cardiologist can implant an artificial pacemaker, which delivers electrical impulses to the heart muscle to ensure that the heart continues to contract and pump blood effectively. These artificial pacemakers are programmable by the cardiologists and can either provide stimulation temporarily upon demand or on a continuous basis. Some devices also contain built-in defibrillators.

Chapter Review

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. 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.

Glossary

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