The Purkinje fibers (// pur-KIN-jee; Purkinje tissue or subendocardial branches) are located in the inner ventricular walls of the heart, just beneath the endocardium in a space called the subendocardium. The Purkinje fibers are specialized conducting fibers composed of electrically excitable cells. They are larger than cardiomyocytes with fewer myofibrils and many mitochondria. They conduct cardiac action potentials more quickly and efficiently than any other cells in the heart. Purkinje fibers allow the heart's conduction system to create synchronized contractions of its ventricles, and are essential for maintaining a consistent heart rhythm.
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Purkinje fibers are a unique cardiac end-organ. Further histologic examination reveals that these fibers are split in ventricles walls. The electrical origin of atrial Purkinje fibers arrives from the sinoatrial node.
The Purkinje fibers are further specialized to rapidly conduct impulses (having numerous fast voltage-gated sodium channels and mitochondria, and fewer myofibrils, than the surrounding muscle tissue). Purkinje fibers take up stain differently from the surrounding muscle cells because of having relatively fewer myofibrils than other cardiac cells. The presence of glycogen around the nucleus causes Purkinje fibers to appear, on a slide, lighter and larger than their neighbors, being arranged along the longitudinal direction (parallel to the cardiac vector). They are often binucleated cells.
Heart rate is governed by many influences from the autonomic nervous system. The Purkinje fibers do not have any known role in setting heart rate unless the SA node is compromised (when they can act as pacemaker cells). They are influenced by electrical discharge from the sinoatrial node.
During the ventricular contraction portion of the cardiac cycle, the Purkinje fibers carry the contraction impulse from both the left and right bundle branch to the myocardium of the ventricles. This causes the muscle tissue of the ventricles to contract. This generates force to eject blood out of the heart, either to the pulmonary circulation from the right ventricle, or to the systemic circulation from the left ventricle.
Purkinje fibers also have the ability of firing at a rate of 20-40 beats per minute if upstream conduction or pacemaking ability is compromised. In contrast, the SA node in normal state can fire at 60-100 beats per minute. In short, they generate action potentials, but at a slower rate than the sinoatrial node. This capability is normally suppressed. Thus, they serve as the last resort when other pacemakers fail. When a Purkinje fiber does fire, it is called a premature ventricular contraction or PVC, or in other situations can be a ventricular escape. It plays a vital role in the circulatory system.
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