L3 conduction system of the heart

INTERNATIONAL UNIVERSITY

Conduction System of the Heart

Dr. Sam Nang


• Cardiac muscle has an intrinsic rhythmicity that allows the heartbeat to originate in and be conducted through the heart without extrinsic stimulation.

• Specialised strands of interconnecting cardiac muscle tissue that coordinate cardiac contraction constitute the conduction system.


• The conduction system enables the cardiac cycle, which refers to the events surrounding the filling and emptying of the chambers of the heart.

• The conduction system consists of specialized tissues that generate and distribute electrical impulses through the heart.


The components of the conduction system are the:

• sinoatrial node (SA node),

• atrioventricular node (AV node),

• atrioventricular bundle (bundle of His [pronounced "hiss"]), and

• conduction myofibers (Purkinje [pur-kin je] fibers).

• None of these structures are macroscopic, but their locations can be noted.


• The SA node, or pacemaker, is located in the posterior wall of the right atrium.

• The SA node initiates the cardiac cycle by producing an electrical impulse that spreads over both atria, causing them to contract simultaneously and force blood into the ventricles.


• The basic depolarization rate of the SA node is 70-80 times per minute.

• The impulse then passes to the AV node, located in the inferior portion of the interatrial septum.

• From here, the impulse continues through the atrioventricular bundle, located at the top of the interventricular septum.


• The atrioventricular bundle divides into right and left bundle branches, which are continuous with the conduction myofibers within the ventricular walls.

• Stimulation of these fibers causes the ventricles to contract simultaneously.


• Contraction of the ventricles is referred to as systole .

• Systole, together with the tension of the elastic fibers and contraction of smooth muscles within the systemic arteries, accounts for the systolic pressure within arteries.


• Ventricular relaxation is called diastole .

• During diastole, the diastolic pressure within arteries can be recorded.

• The heartbeat can vary by as much as 20 or 30 beat per minute in 24 hours, but the heart maintains an average of about 70 beats per minute during a day.

• Blood pressure recorded at 120 over 80 in the morning can rise to 140 over 100 by evening.

• The normal body temperature of 98.6°F will fluctuate between 97° and 99°F over a 24 hour period.

• Although the heart does have an innate contraction pattern, it is also innervated by the autonomic nervous system in order to respond to the everchanging physiological needs of the body.

• The SA and AV nodes have both sympathetic and parasympathetic innervation.

• Sympathetic stimulation accelerates the heart rate and dilates the coronary arteries, enabling the heart to meet its own increased metabolic demands as well as those of the rest of the body.

• Parasympathetic stimulation has the opposite effect.

• Sympathetic innervation is through fibers from the cervical and upper thoracic ganglia.

• Parasympathetic innervation is through branches of the vagus nerves.

• Branches from the right vagus innervate the SA node, and branches from the left vagus innervate the AV node.

• Cardiac output is the volume of blood ejected by the heart into the systemic circulation each minute.

• It is determined by multiplying the stroke rate, or heart rate, by the stroke volume.

• The stroke volume is the amount of blood pumped from the heart into systemic circulation with each ventricular contraction, which amounts to about 70 ml of blood.

• A normal resting cardiac output ranges between 4.2 and 5.6 liters per minute.

• Exercise increases the heart rate, as do the hormones epinephrine and thyroxine.

• Atropine, caffeine, and camphor are drugs that have a stimulatory effect.

• Blood pressure and body temperature also have a profound effect on the heart rate.

THE END

Health & Medicine

L3 conduction system of the heart