With Every Beat of Your Heart
A unique feature of the heart is that it beats spontaneously and rhythmically on its own. Regardless of any nervous system signals, the heart starts beating before you are born and consistently and continually beats until your death.
Pacemaker and Conduction
Setting the beat for the heart is the task of the pacemaker tissue in the right atrium. Much like a metronome continues to click once started, the pacemaker, once generated, beats for a lifetime. Beginning as tissue in the embryonic sinus venosus, a primitive and embryonic chamber receiving blood from the body, it becomes incorporated into the right atrial wall as development proceeds. The pacemaker is named the sinoatrial node (SA node) to represent its embryonic origins and its final adult location.
Cells of the sinoatrial node are modified cardiac muscle connected to the muscle cells of the atrium via the gap junctions of the intercalated disks. (Gap junctions allow direct cell-to-cell contact; intercalated disks are features of cardiac muscle that allow for contraction.) When the cells of the SA node spontaneously generate an action potential, it spreads to all the cells of both atria through these junctions.
The muscles of the atria and the muscles of the ventricles are separated by a connective tissue ring called the annulus fibrosus that forms the foundational anchor for the valves and the septa of the heart, which prevent the SA node signal from spontaneously spreading to the ventricles. Additional conductive cells pick up and relay the electrical signal through the annulus downward to the ventricles.
Anatomy of a Word
septa
Septa is the plural form of septum, and a septum is a wall. In the heart, septa separate the four chambers of the heart. The septum that divides the left and right atria is the atrial septum or the interatrial septum. The interventricular septum (or ventricle septum) divides the ventricles.
In the right atrioventricular region, not far from the tricuspid valve, is another area of modified cardiac muscle called the atrioventricular node (AV node). The AV node cells are connected to the atrial muscle via gap junctions (just as the SA node is connected to the atrium), so they are stimulated by the spreading electrical signal from the SA node. After detecting the signal, the AV node pauses before relaying the signal to the ventricles. This allows the atria to contract just before the ventricles contract, and therefore enables the ventricles to fully fill with blood before the next contraction.
After pausing, the AV node cells relay the signal through modified muscle cells arranged into a bundle of fibers that run through the annulus fibrosus and down toward the apex of the ventricles. This AV bundle (bundle of His) transfers the electrical signal to the base of the heart, where the fibers spread throughout the ventricle and regenerate the electrical signal in the ventricular muscle leading to the contraction.
EKG
The health and state of cardiac function can be assessed indirectly and noninvasively by detecting the electrical changes that occur in the heart. Electrodes positioned on the chest can receive the electrical potentials and display them as a series of peaks or waves that correspond to the electrical activity in the different chambers of the heart. This display is termed an electrocardiogram (EKG).
Where does the acronym EKG come from?
EKG is used to designate an electrocardiogram because, in German, the heart is referred to as Kardia. Also, this convention helps prevent confusing EKG with other diagnostics test such as an echocardiogram (ECG) or an electroencephalogram (EEG).
The initial small peak seen at the start of a cardiac cycle is the P wave, and represents the depolarization of the atrial muscle that precedes the contraction of the atria. The next wave is the largest and sharpest. This is the QRS complex, named for the bottom starting point (Q), the top of the spike (R), and the bottom point (S) of the wave that occurs during ventricular depolarization (the depolarization that precedes the contraction of the ventricle). Following the QRS complex is an intermediate-sized wave, the T wave, and it occurs during repolarization of both the ventricles and the atria. Any changes in the size of the waves or the timing between peaks has diagnostic value for the cardiologist.
Heart Rate
Although the heart is capable of beating on its own, it does not possess the ability to know when to increase or decrease its rate based on activity level. This is the role of the autonomic nervous system. During heavy exercise, your heart must increase its rate to provide the needed materials including oxygen to the hard-working muscles. Neurons produce and secrete norepinephrine and the adrenal gland produces epinephrine (adrenaline). These molecules cause the cells of the pacemaker to increase the rate of firing and lead to an increase in overall heart rate. Conversely, during periods of inactivity, such as sleeping, other neurons secrete acetylcholine, which causes the pacemaker to slow down and decreases the rate.
Strength of Contraction
In addition to beating faster, the heart may contract harder to eject more blood per beat during periods of increased demand. This strength of contraction doesn’t depend on outside signals but is built into the cardiac muscle itself. During periods of normal activity, the actin and myosin filaments overlap in such a way that not every actin can form a cross bridge with myosin. Only as the heart increasingly fills with blood and the fibers are stretched farther apart can the actin fully engage the myosin and provide the most intense contraction. In this way, the design of the muscle fibers provide a built-in reserve to be used when the heart is filled with more blood, which is during times of increased demand and results in more blood being pumped out of the heart.