The atria and ventricles work together, alternately contracting and relaxing to pump blood through your heart. The electrical system of your heart is the power source that makes this possible. Show
Your heartbeat is triggered by electrical impulses that travel down a special pathway through your heart:
At rest, a normal heart beats around 50 to 99 times a minute. Exercise, emotions, fever and some medications can cause your heart to beat faster, sometimes to well over 100 beats per minute. How fast does the normal heart beat?How fast the heart beats depends on the body's need for oxygen-rich blood. At rest, the SA node causes your heart to beat about 50 to 100 times each minute. During activity or excitement, your body needs more oxygen-rich blood; the heart rate rises to well over 100 beats per minute. Medications and some medical conditions may affect how fast your heart-rate is at rest and with exercise. How do you know how fast your heart is beating?You can tell how fast your heart is beating (your heart rate) by feeling your pulse. Your heart-rate is the amount of times your heart beats in one minute. You will need a watch with a second hand. Place your index and middle finger of your hand on the inner wrist of the other arm, just below the base of the thumb. You should feel a tapping or pulsing against your fingers. Count the number of taps you feel in 10 seconds. Multiply that number by 6 to find out your heart-rate for one minute: Pulse in 10 seconds x 6 = \__ beats per minute (your heart-rate) When feeling your pulse, you can also tell if your heart rhythm is regular or not. Normal Heart Beat1. The SA node sets the rate and rhythm of your heartbeat. 2. The SA node fires an impulse. The impulse spreads through the walls of the right and left atria, causing them to contract. This forces blood into the ventricles. 3. The impulse travels to the AV node. Here, the impulse slows for a moment before going on to the ventricles. 4. The impulse travels through a pathway of fibers called the His-Purkinje network. This network sends the impulse into the ventricles and causes them to contract. This forces blood out of the heart to the lungs and body. 5. The SA node fires another impulse. The cycle begins again.
Clin Anat. 2009 Jan;22(1):99-113.
An organized rhythmic contraction of the heart requires adequate propagation of electrical impulses along the conduction pathway. Of note, the impulses in the His-Purkinje system travel in such a way that papillary muscle contraction precedes that of the ventricles, thereby preventing regurgitation of blood flow through the AV valves. ElectrophysiologyPhysiol Rev. 2005 Oct;85(4):1205-53. Ion channelsHeart Rhythm. 2010 Jan;7(1):117-26.
Properties of cardiac ion channels
Note: The different types of cardiac ion channels are discussed below, throughout the description of the phases of action potentials in different cardiac cells. Action potentials and impulse conductionPhysiol Rev. 2005 Oct;85(4):1205-53. Action potential: electrical stimulation created by a sequence of ion fluxes through specialized channels in the membrane (sarcolemma) of cardiomyocytes that leads to cardiac contraction. Action potential in cardiomyocytesThe action potential in typical cardiomyocytes is composed of 5 phases (0-4), beginning and ending with phase 4. Phase 4: The resting phase
Phase 0: Depolarization
Phase 1: Early repolarization
Phase 2: The plateau phase
Phase 3: Repolarization
Action potential in cardiac pacemaker cellsPharmacol Ther. 2005 Jul;107(1):59-79.
Table 1. Cardiac cell types displaying pacemaker behavior.
The sequence of events for pacemaker action potential:
Implications of pacemaker activity on global cardiac depolarization
Refractory period
Sequence of depolarization
Excitation-contraction couplingNature. 2002 Jan 10;415(6868):198-205. Excitation-contraction coupling represents the process by which an electrical action potential leads to contraction of cardiac muscle cells. This is achieved by converting a chemical signal into mechanical energy via the action of contractile proteins. Calcium is the crucial mediator that couples electrical excitation to physical contraction by cycling in and out of the myocyte’s cytosol during each action potential. Contractile proteinsMain contractile elements:
Regulatory elements:
Calcium-induced calcium release (CICR)The initial influx of Ca2+ into myocytes through L-type Ca2+ channels during phase 2 of the action potential is insufficient to trigger contraction of myofibrils. This signal is amplified by the CICR mechanism, which triggers much greater release of Ca2+ from the sarcoplasmic reticulum.
Contractile cycle
Myocyte relaxationAs with myocyte contraction, this process is synchronized with the electrical activity of the cell.
Neural modulation of contractilityAdv Physiol Educ. 2011 Mar;35(1):28-32.
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