"The heart is happiest when it beats for others." Anonymous

Overview                       

Late in the 1800s, physiologists realized that muscles contract once provided with an electric stimulus. Most often the electrical precursor comes from motor neurons connecting either the brain or reflex loops through one's spine. Thus, most muscles are slaves to the nervous system.

The heart consists mostly of muscle cells. Remarkably, the heart is not dependent on the nervous system to pump or beat. The heart possesses its own highly specialized electrical system. This system is able to initiate an electrical impulse, automatically, and does so regularly 60-100 times a minute. This impulse influences neighboring cells electrically that in turn influence other cardiac cells. Almost instantaneously, the impulse becomes an electrical wave encompassing all chambers of the heart.

With the electrical stimulation of the muscle cells, the cells become smaller. The result: the heart as a whole becomes smaller or contracts. Blood is pumped out.  

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The Sinoatrial Node (SA Node)

The SA (sinoatrial) node is a clump of specialized cardiac cells located in the near the opening of the right atrium, where deoxygenated blood from the vena cava enters. These specialized cells have the ability to self-initiate an electrical impulse. This ability, called automaticity, makes the SA node a pacemaker site in the heart.

A pacemaker sets the pace or rate by which electrical waves move through the heart resulting in a contraction.Although other pacemaker sites exist in the heart, the SA node commonly 'fires' at a rate that is faster than other pacemakers. In the heart, the pacemaker which fires at the quickest pace, takes control of the heart rate. This is why the SA node is often the 'dominant pacemaker'.

Cardiac rhythms - a series of heart beats - that initiate from the SA node are called sinus rhythms.The SA node usually fires at a rate of 60-100 times each minute. The rate increases with the release of adrenaline (sympathetic nervous system) and slows down with Vagal nerve stimulation (parasympathetic nervous system).

For example, the when you are under stress of any kind, the adrenals release adrenaline (epinephrine) causing the SA node to fire more often. Note that the cardiac muscle cells also respond with more forceful contractions. Together, the heart begins to pump out increased volume of blood to meet the new demands.

Once the SA node initiates an electrical impulse, the resulting electrical wave moves its way across the right and left atria. Remember that the atrial septum with it fibrous tissue separates the right atrium from the left atrium. A tunnel is required to cross the atrial septum. This comes in the form of Bachman's Bundle. The wave takes approximately 3/100 of a second to cross the atrium to arrive at the AV node.   

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AV Junction (AV Node and the Bundle of His)

The AV node (atrioventricular) is a rounded bulbar structure of specialized cells similar to the SA node. It also possesses the ability to self-initiate an electrical impulse, serving as a back-up for the SA node in case the SA node slows down too much or stops firing. It usually does not initiate impulses, though, for its intrinsic firing rate is normally 40-60 each minute, slower than the SA node.

The AV node has a second important role besides acting as a back-up pacemaker. It significantly slows down the transmission of the electrical wave across its cells. The time taken to cross the small AV node is 10-12/100 of a second. This delay of the electrical wave in the AV node allows the atria to finish contracting prior to the ventricles contracting, thus providing for an atrial kick.

The Bundle of His continues from the AV node and passes through the fibrous tissue that separates the top and bottom of the heart. Therefore, the Bundle of His electrically connects the atria with the ventricle. This enables the all-important ventricles to contract, resulting in blood being pumped to the lungs (right ventricle) and to the rest of the body (left ventricle).

Like the AV node, the Bundle of His also has specialized pacemaker cells that are empowered with automaticity. The Bundle of His and the AV node's intrinsic rate is identical, 40-60 electrical impulses per minute. Together, the Bundle of His and the AV node are called the AV Junction or just 'the junction'. The Bundle of His, AV node, atria and SA node are all located above the ventricles. Cardiac rhythms originating from these areas (initial impulse generated here) are called supraventricular rhythms.

The Bundle Branches

Electrically, the ventricles are dynamic. The electrical wave must travel quickly across the large ventricles for a forceful, coordinated contraction. The Bundle Branches help with this task with two effective strategies:
· divide one electrical wave into 4 equally effective waves;
· the bundle branches are insulated, being encapsulating them with fibrous tissue.

The insulated 4 way electrical wave speeds across the ventricles in only 1/100 of a second. I refer to the Bundle Branches as the "Autobahn" of the heart.    

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The Purkinje Network

The Bundle Branches end in the Purkinje network, forming tens of thousands of connections with the cardiac muscle cells. Pacemaker cells are found sparsely in the ventricles. These specialized cells initiate electrical impulses at a rate of 20-40 each minute. They are called upon to fire if the SA and AV nodes are not functional, if the supraventricular impulse is blocked at the junction, or if the impulse is blocked across both Bundle Branches.

In other words, the ventricles will try to pace themselves if necessary. Note that a heart rate of 20-40 beats per minute may not produce enough cardiac output to sustain consciousness or even to sustain life. These pacemaker cells are the last bastion, and can stop firing at any time. When this happens, well… you probably get the picture.     

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