Page 146 - 84 human physiolofy part-1

Chapter 8

The Heart's Electrical Conduction System

The heart is primarily made up of muscle tissue. A network of nerve fibers coordinates the
contraction and relaxation of the cardiac muscle tissue to obtain an efficient, wave-like pumping action
of the heart

Control of Heartbeat

The heart contains two cardiac pacemakers that spontaneously cause the heart to beat. These can
be controlled by the autonomic nervous system and circulating adrenaline. If the cardiac muscles just
contracted and relaxed randomly at a natural rhythm the cycle would become disordered and the heart
would become unable to carry on its function of being a pump. Sometimes when the heart undergoes
great damage to one part of the cardiac muscle or the person incurs an electric shock, the cardiac cycle
can become uncoordinated and chaotic. Some parts of the heart will contract whilst others will relax so
that instead of contracting and relaxing as a whole, the heart will flutter abnormally. This is called
fibrillation and can be fatal if not treated within 1 minute.

SA Node
The sinoatrial node (abbreviated SA node or SAN, also called the sinus node) is the impulse generating
(pacemaker) tissue located in the right atrium of the heart. Although all of the heart's cells possess the
ability to generate the electrical impulses (or action potentials) that trigger cardiac contraction, the
sinoatrial node is what normally initiates it, simply because it generates impulses slightly faster than the
other areas with pacemaker potential. Because cardiac myocytes, like all nerve cells, have refractory
periods following contraction during which additional contractions cannot be triggered, their
pacemaker potential is overridden by the sinoatrial node. The SA node emits a new impulse before
either the AV or purkinje fibers reach threshold. The sinoatrial node (SA node) is a group of cells
positioned on the wall of the right atrium, near the entrance of the superior vena cava. These cells are
modified cardiac myocytes. They possess some contractile filaments, though they do not contract. Cells
in the SA node will naturally discharge (create action potentials) at about 70-80 times/minute. Because
the sinoatrial node is responsible for the rest of the heart's electrical activity, it is sometimes called the
primary pacemaker. If the SA node doesn't function, or the impulse generated in the SA node is
blocked before it travels down the electrical conduction system, a group of cells further down the heart
will become the heart's pacemaker. These cells form the atrioventricular node (AV node), which is an
area between the right atrium and ventricle, within the atrial septum. The impulses from the AV node
will maintain a slower heart rate (about 40-60 beats per a minute). When there is a pathology in the AV
node or purkinje fibers, an ectopic pacemaker can occur in different parts of the heart. The ectopic
pacemaker typically discharges faster than the SA node and causes an abnormal sequence of
contraction. The SA node is richly innervated by vagal and sympathetic fibers. This makes the SA node
susceptible to autonomic influences. Stimulation of the vagus nerve causes decrease in the SA node
rate (thereby causing decrease in the heart rate). Stimulation via sympathetic fibers causes increase in
the SA node rate (thereby increasing the heart rate). The sympathetic nerves are distributed to all parts
of the heart, especially in ventricular muscles. The parasympathetic nerves mainly control SA and AV
nodes, some atrial muscle and ventricular muscle. Parasympathetic stimulation from the vagal nerves
decreases the rate of the AV node by causing the release of acetylcholine at vagal endings which in turn
increases the K+ permeability of the cardiac muscle fiber. Vagal stimulation can block transmission
through AV junction or stop SA node contraction which is called "ventricular escape." When this
happens, the purkinje fibers in the AV bundle develops a rhythm of their own. In the majority of
patients, the SA node receives blood from the right coronary artery, meaning that a myocardial

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