AnatomySAnode

: Sinoatrial Node (SA node)
: Location; The sinoatrial node (SA node) consists of a cluster of specialized cells that have pacemaker activity (automaticity). These cells are responsible for initiating the electrical impulse that stimulates the heart muscles to contract rhythmically. The SA node is located high on the right atrium close to whether the superior vena cava enters the right atrium.

Sinus rhythm

The SA ryhthm is the normal pacemaker of the heart, firing at about 60-100 beats per minute. A heart controlled by the SA node is said to be in normal sinus rhythm. The electrical impulse from the SA node spreads over the right and left atria and causes atrial contraction. The impulses are also conducted to the atrioventicular (AV) node. It takes about 0.03 seconds for the impulse to travel from the SA to AV node. A few people believe that there are three internodal tracts (anterior, middle and posterior) that conduct the impulse to the AV node. The existence of these three tracts is controversial since their presence has not been firmly proven.

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Fast and slow sinus rhythm

In sinus rhythm, every P-wave is followed by a QRS complex, the R-R interval is regular and the P-R interval is less than 0.2 seconds (one big box on the EKG paper). A fast sinus rhythm, faster than 100 beats a minute, is known as sinus tachycardia while a slow rhythm, slower than 60 beats a minute, is known as sinus bradycardia.

Innervation by the Autonomic Nervous System

The SA node under the influence of the autonomic nervous system. The sympathetic system innervates the heart and causes increases the heart rate via B₁ adrenergic receptors, for instance in fight or fright.; The parasympathetic system, via the vagus nerve, slows the heart rate and establishes the resting heart rate of about 60-70 beats per minute. If parasympathetic actiivty is blocked by anti-cholinergic drugs or the vagal nerve is cut, the heart rate increases. If parasympathetic stimulation is increased, for instance by massaging the carotid sinus (baroreceptors), the heart rate decreases.
: Why does the rhythm originate in the SA node?; The rhythm originates from the SA node because the SA node depolarizes more frequently (60-100 beats per minute) than the AV node (40-60 beats per minute) and ventricular conducting system (30-40 beats per minute) so the AV node and ventricular conducting system are 'captured' by the sinus impulse and driven at 60-100 beast per minute.

Last Updated: 01/03/00.
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