Parent Cells and the Heartbeat

At the moment of conception, when the incoming soul takes possession of the parent cells, the sperm and the ovum, it begins building the body it will occupy, from the heart, specifically from the sinoatrial node and the atrioventricular node.

The sinoatrial node, the heart's natural pacemaker, sends regular electrical impulses from the top chamber, the atrium, causing it to contract and pump blood into the bottom chamber, the ventricle, through the atrioventricular node, which controls the heart rate. The AV node serves as an electrical relay station, slowing the current sent by the SA node.

The following information is excerpted from the Encyclopaedia Britannica:

"Regular beating of the heart is achieved as a result of the inherent rhythmicity of cardiac muscle; no nerves are located within the heart itself, and no outside regulatory mechanisms are necessary to stimulate the muscle to contract rhythmically. That these rhythmic contractions originate in the cardiac muscle can be substantiated by observing cardiac development in the embryo; cardiac pulsations begin before adequate development of nerve fibers. In addition, it can be demonstrated in the laboratory that even fragments of cardiac muscle in tissue culture continue to contract rhythmically. Furthermore, there is no gradation in degree of contraction of the muscle fibers of the heart, as would be expected if they were primarily under nervous control.

"The mere possession of this intrinsic ability is not sufficient, however, to enable the heart to function efficiently. Proper function requires coordination, which is maintained by an elaborate conducting system within the heart that consists primarily of two small, specialized masses of tissue, or nodes, from which impulses originate and of nerve like conduits for the transmission of impulses, with terminal branches extending to the inner surface of the ventricles.

"A basic understanding of the method by which an impulse is transmitted is essential before the conduction system that exists within the heart can be properly understood. Electrical potentials exist across membranes of essentially all cells of the body — that is, there is an electrical potential gradient created, generally by an excess of negative ions immediately inside the cell membrane and an equal excess of positive ions on the outside of the membrane, also known as a resting potential. (Ions are atoms or groups of atoms in solution that carry positive or negative electrical charges.) Further, some cells, such as nerve and muscle cells, have the additional distinction of being "excitable" — i.e., capable of conducting impulses along their membranes.

"Any factor that suddenly increases the permeability of the cell membrane and allows positive ions to flow through the membrane to the inside, while negative ions flow to the outside, is likely to bring about a sequence of rapid changes in the membrane potential, lasting only a fraction of a second. This change is followed by the return of the membrane potential to its resting value. This sequence of changes in potential is called an action potential and is responsible for the initiation of impulses transmitted, in the case of the heart, along the muscle fibers and the special conducting tissue fibers. Electrical stimulation, application of chemicals, mechanical damage, heat, and cold are among the factors that can bring about a change in the state of a cell membrane, momentarily disturbing its normal resting state and creating an action potential.

"The action potential occurs in two separate stages called depolarization and repolarization. During the process of depolarization, the normal negative potential inside the muscle fibre is lost, and the membrane potential actually reverses, or becomes slightly positive inside and negative outside the membrane. This process proceeds as a wave along the length of the muscle fibers. It lasts only a fraction of a second before the positive ions begin to resume their original position on the outside of the membrane, necessary before another impulse can pass. This recovery process is termed repolarization. An action potential is necessary to generate each depolarization wave.

"The sinoatrial node possesses the ability to generate an action potential spontaneously. This highly important structure is a small strip of specialized muscle located in the posterior wall of the right atrium, immediately beneath the point of entry of the superior vena cava. After each action potential is generated in the sinoatrial node, the impulse immediately spreads through the atrial muscle in the form of a ripple pattern similar to the pattern of waves generated when a stone is thrown into a pool of water.

"A few specialized atrial fibers relay this potential, or impulse, to the atrioventricular node, located in the lower part of the right rear atrial wall. To permit sufficient time for complete contraction of the atria before subsequent simultaneous contraction of the ventricles, the impulse is delayed slightly in its passage through the atrioventricular node. The special conducting fibers that leave this node form the bundle of His, which terminates in the multiple branches of the Purkinje network in the left and right ventricles. (The bundle of His was named for the German cardiologist Wilhelm His, Jr., who described it in 1893.) The bundle of His passes between the atrioventricular valves and leads into the interventricular septum, where bundles of fibers in right and left branches project downward beneath the endocardium on either side of the septum. They then curve around the tip of the ventricles and back toward the atria along the side walls.

"Both the atrioventricular node, with an intrinsic beat of 40-60 beats per minute, and the Purkinje fibers, with an intrinsic beat of 15-40 beats per minute, are self-stimulating and capable of rhythmic contraction, but at a rate slower than that of the sinoatrial node, which possesses an intrinsic capacity to beat 72 times per minute. Therefore, since recovery time is faster in the sinoatrial node, it controls the rate of the heartbeat, serving as the primary pacemaker. The atrioventricular node, with its 40-60 beats per minute, is termed the secondary pacemaker.

The heart's rhythmical beat is initiated and regulated from centers within the organ. The primary pacemaker, the sinoatrial node, is a small mass of specialized muscle cells located at the juncture of the upper vena cava and the right atrium. Electrical impulses are emitted by this group of cells. The excitation spreads through the two atria and, by way of a band of fibers called the bundle of His, into the ventricles. The bundle of His has its beginning in a small mass of cells, the atrioventricular node, located beneath the lining of the right atrium.

"Normally initiated heart rhythm, originating in the sinoatrial node, is called sinus rhythm. Under stimulation from the central nervous system and other metabolic factors, heart rate may normally rise and fall, with a slight variation, in part related to respiratory activity. In young individuals in excellent physical condition, the resting heart rate may fall as low as 40 to 50 beats per minute, and, under stressful psychological stimulation, the heart rate may rise to as high as 200 beats per minute. These situations are to be differentiated from pathological variations in heart rate. Abnormal slowing of heart rate, or sinus bradycardia (a slow sinus rhythm with a rate below 60, caused by disturbance of the sinoatrial node) or acceleration of heart rate, or tachycardia (excessive rapidity in the action of the heart with a pulse rate of above 100 beats per minute) may occur in a wide variety of disease states and be symptomatic of the underlying disease.

"Extra beats arising from the atrium, the nodal tissues, or the ventricle are not in themselves abnormal, though beats arising from the ventricle are more often associated with organic heart disease. Occasional extra systoles (contractions) occur in many normal individuals. In cardiovascular disease they are much more common. They do not interfere with normal cardiovascular function if infrequent. It has been noted that continued psychological stress, excessive smoking, and drinking of large amounts of tea and coffee enhance the tendencies for premature contractions of the atria. Premature contractions of the ventricles are more ominous, especially those that occur after exercise. They have been found to be associated with coronary artery disease in a large percentage of instances. If frequent enough, they may be the harbinger of more serious ventricular arrhythmia.

"The normal rhythm of the heart (i.e., the heart rate) can be altered by neural activity. The heart is innervated by sympathetic and parasympathetic nerves, which have a profound effect on the resting potential and the rate of diastolic depolarization in the SA nodal region. The activity of the sympathetic nervous system may be increased by the activation of the sympathetic nerves innervating the heart or by the secretion of epinephrine and norepinephrine from the adrenal gland. This decreases the resting potential of the myocytes of the SA node while increasing the rate of diastolic depolarization. The result is an increase in the heart rate. Conversely, stimulating the parasympathetic nervous system (vagal nerves to the heart) increases the resting potential and decreases the rate of diastolic depolarization; under these circumstances the heart rate slows. The sympathetic nervous system is activated under conditions of fright or vigorous activity (the so-called "fight or flight" reaction), where the increase in force and rate of heart contraction are easily felt; the parasympathetic system exerts its influence during periods of rest."