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The circus movement can occur around an anatomical or functional core. Either type may occur alone, or together. [citation needed] Anatomically defined re-entry has a fixed anatomic pathway. Anomalous conduction via accessory pathways (APs) creates the re-entry circuit (which are also called bypass tracts), that exists between the atria and ...
Such channels are important parts of the electrical conduction system of the heart and form a component of the natural pacemaker. First described in the late 1970s in Purkinje fibers and sinoatrial myocytes , the cardiac pacemaker "funny" (I f ) current has been extensively characterized and its role in cardiac pacemaking has been investigated.
Adenosine is a key factor in regulating the body's sleep-wake cycle. [40] Adenosine levels rise during periods of wakefulness and lowers during sleep. Higher adenosine levels correlate with a stronger feeling of sleepiness, also known as sleep drive or sleep pressure. [41]
Class IV drugs are calcium (Ca) channel blockers. They work by inhibiting the action potential of the SA and AV nodes. If the patient is stable, adenosine may be used for restoration of sinus rhythm in patients with macro-reentrant supraventricular tachycardias. It causes a short-lived cessation of conduction through the atrio-ventricular node ...
Due to their unusual property of being activated by very negative membrane potentials, the movement of ions through the HCN channels is referred to as the funny current (see below). [14] Another hypothesis regarding the pacemaker potential is the 'calcium clock'. Calcium is released from the sarcoplasmic reticulum within the cell.
The conduction system consists of specialized heart muscle cells, situated within the myocardium. [3] There is a skeleton of fibrous tissue that surrounds the conduction system which can be seen on an ECG. Dysfunction of the conduction system can cause irregular heart rhythms including rhythms that are too fast or too slow.
This makes adenosine a useful medication for treating and diagnosing tachyarrhythmias, or excessively fast heart rates. This effect on the A 1 receptor also explains why there is a brief moment of cardiac standstill when adenosine is administered as a rapid IV push during cardiac resuscitation.
However, in pacemaker cells, this potassium permeability (efflux) decreases as time goes on, causing a slow depolarization. In addition, there is a slow, continuous inward flow of sodium, called the "funny" or pacemaker current. These two relative ion concentration changes slowly depolarize (make more positive) the inside membrane potential ...