Cardiac pacemaker
The contraction of heart
The heart is a myogenic muscular organ found in all animals with a circulatory system , that is responsible for pumping blood throughout the blood vessels by repeated, rhythmic contractions...

 (cardiac) muscle in all animals with hearts is initiated by chemical impulses. The rate at which these impulses fire controls the heart rate. The cell
Cell (biology)
The cell is the basic structural and functional unit of all known living organisms. It is the smallest unit of life that is classified as a living thing, and is often called the building block of life. The Alberts text discusses how the "cellular building blocks" move to shape developing embryos....

s that create these rhythm
Rhythm may be generally defined as a "movement marked by the regulated succession of strong and weak elements, or of opposite or different conditions." This general meaning of regular recurrence or pattern in time may be applied to a wide variety of cyclical natural phenomena having a periodicity or...

ical impulses are called pacemaker cells, and they directly control the heart rate
Heart rate
Heart rate is the number of heartbeats per unit of time, typically expressed as beats per minute . Heart rate can vary as the body's need to absorb oxygen and excrete carbon dioxide changes, such as during exercise or sleep....


In human
Humans are the only living species in the Homo genus...

s, and occasionally in other animals, a mechanical device called an artificial pacemaker
Artificial pacemaker
A pacemaker is a medical device that uses electrical impulses, delivered by electrodes contacting the heart muscles, to regulate the beating of the heart...

 (or simply "pacemaker") may be used after damage to the body's intrinsic conduction system to produce these impulses synthetically.

The pacemaker is located in the wall of the right atrium
Right atrium
The right atrium is one of four chambers in the hearts of mammals and archosaurs...


Primary (SA node)

One percent of the Cardiomyocytes in the myocardium possess the ability to generate electrical impulses (or action potential
Action potential
In physiology, an action potential is a short-lasting event in which the electrical membrane potential of a cell rapidly rises and falls, following a consistent trajectory. Action potentials occur in several types of animal cells, called excitable cells, which include neurons, muscle cells, and...

A specialized portion of the heart, called the sinoatrial node
Sinoatrial node
The sinoatrial node is the impulse-generating tissue located in the right atrium of the heart, and thus the generator of normal sinus rhythm. It is a group of cells positioned on the wall of the right atrium, near the entrance of the superior vena cava...

, is responsible for atrial propagation of this potential.

The sinoatrial node (SA node) is a group of cells positioned on the wall of the right atrium
Right atrium
The right atrium is one of four chambers in the hearts of mammals and archosaurs...

, near the entrance of the superior vena cava
Superior vena cava
The superior vena cava is truly superior, a large diameter, yet short, vein that carries deoxygenated blood from the upper half of the body to the heart's right atrium...

. These cells are modified cardiomyocyte. They possess rudimentary contractile filaments, but contract relatively weakly.

Cells in the SA node spontaneously depolarize
In biology, depolarization is a change in a cell's membrane potential, making it more positive, or less negative. In neurons and some other cells, a large enough depolarization may result in an action potential...

, resulting in contraction, approximately 100 times per minute. This native rate is constantly modified by the activity of sympathetic
Sympathetic nervous system
The sympathetic nervous system is one of the three parts of the autonomic nervous system, along with the enteric and parasympathetic systems. Its general action is to mobilize the body's nervous system fight-or-flight response...

 and parasympathetic
Parasympathetic nervous system
The parasympathetic nervous system is one of the two main divisions of the autonomic nervous system . The ANS is responsible for regulation of internal organs and glands, which occurs unconsciously...

 nerve fibers, so that the average resting cardiac rate in adult humans is about 70 beats per minute. Because the sinoatrial node is responsible for the rest of the heart's electrical activity, it is sometimes called the primary pacemaker.

Secondary (AV junction & Bundle of His)

If the SA node does not function, a group of cells further down the heart will become the heart's pacemaker, this is known as an ectopic pacemaker
Ectopic pacemaker
An ectopic pacemaker or ectopic focus is an excitable group of cells that causes a premature heart beat outside the normally functioning SA node of the human heart. Acute occurrence is usually non-life threatening, but chronic occurrence can progress into tachycardia, bradycardia or ventricular...

. These cells form the atrioventricular node (AV node), which is an area between the left atria and the right ventricles, within the atrial septum.

The cells of the AV node normally discharge at about 40-60 beats per minute, and are called the secondary pacemaker.

Further down the electrical conducting system of the heart is the Bundle of His. The left and right branches of this bundle, and the Purkinje fibres, will also produce a spontaneous action potential at a rate of 30-40 beats per minute, if the SA and AV node both do not function. The reason the SA node controls the whole heart is that its action potentials are released most often to the heart's muscle cells; this will produce contraction. The action potential generated by the SA node passes down the cardiac conduction system, and arrives before the other cells have had a chance to generate their own spontaneous action potential. This is the electrical conduction system of the heart|normal conduction of electrical activity within the heart.

Generation of action potentials

There are three main stages in the generation of an action potential in a pacemaker cell. Since the stages are analogous to contraction of cardiac muscle cells, they have the same naming system. This can lead to some confusion. There is no phase one or two, just phases zero, three and four.

Phase 4 - Pacemaker potential

The key to the rhythmic firing of pacemaker cells is that, unlike muscle
Muscle is a contractile tissue of animals and is derived from the mesodermal layer of embryonic germ cells. Muscle cells contain contractile filaments that move past each other and change the size of the cell. They are classified as skeletal, cardiac, or smooth muscles. Their function is to...

 and neuron
A neuron is an electrically excitable cell that processes and transmits information by electrical and chemical signaling. Chemical signaling occurs via synapses, specialized connections with other cells. Neurons connect to each other to form networks. Neurons are the core components of the nervous...

s, these cells will slowly depolarize by themselves.

As in all other cells, the resting potential of a pacemaker cell (-60mV to -70mV) is caused by a continuous outflow or "leak" of potassium
Potassium is the chemical element with the symbol K and atomic number 19. Elemental potassium is a soft silvery-white alkali metal that oxidizes rapidly in air and is very reactive with water, generating sufficient heat to ignite the hydrogen emitted in the reaction.Potassium and sodium are...

 ions through ion channel
Ion channel
Ion channels are pore-forming proteins that help establish and control the small voltage gradient across the plasma membrane of cells by allowing the flow of ions down their electrochemical gradient. They are present in the membranes that surround all biological cells...

Integral membrane protein
An integral membrane protein is a protein molecule that is permanently attached to the biological membrane. Proteins that cross the membrane are surrounded by "annular" lipids, which are defined as lipids that are in direct contact with a membrane protein...

 in the membrane
Cell membrane
The cell membrane or plasma membrane is a biological membrane that separates the interior of all cells from the outside environment. The cell membrane is selectively permeable to ions and organic molecules and controls the movement of substances in and out of cells. It basically protects the cell...

 that surrounds the cells. The difference is that this potassium permeability decreases as time goes on, partly causing the slow depolarization. As well as this, there is a slow inward flow of sodium
Sodium is a chemical element with the symbol Na and atomic number 11. It is a soft, silvery-white, highly reactive metal and is a member of the alkali metals; its only stable isotope is 23Na. It is an abundant element that exists in numerous minerals, most commonly as sodium chloride...

, called the funny current
Funny current
Funny current refers to a specific current in the heart....

, as well as an inward flow of calcium. This all serves to make the cell more positive.

This relatively slow depolarization continues until the threshold potential is reached. Threshold is between -40mV and -50mV. When threshold is reached, the cells enter phase 0.

Phase 0 - Upstroke

Though much faster than the depolarization caused by the funny current and decrease in potassium permeability above, the upstroke in a pacemaker cell is slow compared to that in an axon
An axon is a long, slender projection of a nerve cell, or neuron, that conducts electrical impulses away from the neuron's cell body or soma....


The SA and AV node do not have fast sodium channels like neurons, and the depolarization is mainly caused by a slow influx of calcium ions. (The funny current also increases). The calcium is let into the cell by voltage-sensitive calcium channels that open when the threshold is reached.

Phase 3 - Repolarization

The calcium channels are rapidly inactivated, soon after they open. Sodium permeability is also decreased.
Potassium permeability is increased, and the efflux of potassium (loss of positive ions) slowly repolarises the cell.

See also

  • Cardiac action potential
    Cardiac action potential
    In electrocardiography, the cardiac action potential is a specialized action potential in the heart, necessary for the electrical conduction system of the heart....

  • Electrical conduction system of the heart
    Electrical conduction system of the heart
    The normal intrinsic electrical conduction of the heart allows electrical propagation to be transmitted from the Sinoatrial Node through both atria and forward to the Atrioventricular Node. Normal/baseline physiology allows further propagation from the AV node to the ventricle or Purkinje Fibers...

  • Artificial pacemaker
    Artificial pacemaker
    A pacemaker is a medical device that uses electrical impulses, delivered by electrodes contacting the heart muscles, to regulate the beating of the heart...

The source of this article is wikipedia, the free encyclopedia.  The text of this article is licensed under the GFDL.