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Photograph of an electric eel.
An electric eel flexes its muscles to create a voltage that stuns prey. (credit: chrisbb, Flickr)


Just as nerve impulses are transmitted by depolarization and repolarization of adjacent membrane, the depolarization that causes muscle contraction can also stimulate adjacent muscle cells to depolarize (fire) and contract. Thus, a depolarization wave can be sent across the heart, coordinating its rhythmic contractions and enabling it to perform its vital function of propelling blood through the circulatory system. [link] is a simplified graphic of a depolarization wave spreading across the heart from the sinoarterial (SA) node , the heart’s natural pacemaker.

The figure shows that the charge distribution on the outer surface of the heart changes from positive to negative during depolarization. This wave of depolarization, spreading from the upper right toward the lower left of the heart, is represented by a vector pointing in the direction of the wave. The components of this vector are measured by placing electrodes on the patient’s chest. The figure shows three electrodes, labeled R A, L A, and L L, placed to form a triangle around the heart. The electrode R A is close to the right atrium, L A is close to the left atrium, and L L is just below the heart. R A and L A form a pair called lead one, R A and L L form a second pair called lead two, and L A and L L form a third pair called lead three. Each pair of electrodes measures a component of the depolarization vector.
The outer surface of the heart changes from positive to negative during depolarization. This wave of depolarization is spreading from the top of the heart and is represented by a vector pointing in the direction of the wave. This vector is a voltage (potential difference) vector. Three electrodes, labeled RA, LA, and LL, are placed on the patient. Each pair (called leads I, II, and III) measures a component of the depolarization vector and is graphed in an ECG.

An electrocardiogram (ECG)    is a record of the voltages created by the wave of depolarization and subsequent repolarization in the heart. Voltages between pairs of electrodes placed on the chest are vector components of the voltage wave on the heart. Standard ECGs have 12 or more electrodes, but only three are shown in [link] for clarity. Decades ago, three-electrode ECGs were performed by placing electrodes on the left and right arms and the left leg. The voltage between the right arm and the left leg is called the lead II potential and is the most often graphed. We shall examine the lead II potential as an indicator of heart-muscle function and see that it is coordinated with arterial blood pressure as well.

Heart function and its four-chamber action are explored in Viscosity and Laminar Flow; Poiseuille’s Law . Basically, the right and left atria receive blood from the body and lungs, respectively, and pump the blood into the ventricles. The right and left ventricles, in turn, pump blood through the lungs and the rest of the body, respectively. Depolarization of the heart muscle causes it to contract. After contraction it is repolarized to ready it for the next beat. The ECG measures components of depolarization and repolarization of the heart muscle and can yield significant information on the functioning and malfunctioning of the heart.

[link] shows an ECG of the lead II potential and a graph of the corresponding arterial blood pressure. The major features are labeled P, Q, R, S, and T. The P wave is generated by the depolarization and contraction of the atria as they pump blood into the ventricles. The QRS complex is created by the depolarization of the ventricles as they pump blood to the lungs and body. Since the shape of the heart and the path of the depolarization wave are not simple, the QRS complex has this typical shape and time span. The lead II QRS signal also masks the repolarization of the atria, which occur at the same time. Finally, the T wave is generated by the repolarization of the ventricles and is followed by the next P wave in the next heartbeat. Arterial blood pressure varies with each part of the heartbeat, with systolic (maximum) pressure occurring closely after the QRS complex, which signals contraction of the ventricles.

This figure has two graphs, placed one below the other. The lower graph shows an E C G of the lead two potential, and the upper graph shows the corresponding changes in arterial blood pressure. In each case, time is plotted on the horizontal axis, in seconds. The vertical axis of the upper graph shows the arterial blood pressure in millimeters of mercury, and the vertical axis of the lower graph shows the lead two voltage in millivolts. The upper graph is roughly sinusoidal, showing the diastolic or minimum blood pressure at about eighty millimeters of mercury, and the systolic or maximum blood pressure at about one hundred twenty millimeters of mercury. For the lower graph, the main features are labeled P, Q, R, S, and T. The P wave is a smooth curve that rises from zero millivolts to a peak of about zero point two five millivolts and falls to just below zero millivolts when it reaches point Q. From point Q to point R, the voltage rises steeply to about one millivolt, and then drops equally sharply to point S, at negative zero point three millivolts. This is followed by the T wave, which is a smooth curve, broader than the P wave, with a peak of comparable height. All of this is completed in less than seven-tenths of a second, with the voltage returning to zero millivolts. After about one-tenth of a second, the cycle begins again. The systolic blood pressure follows soon after the QRS complex.
A lead II ECG with corresponding arterial blood pressure. The QRS complex is created by the depolarization and contraction of the ventricles and is followed shortly by the maximum or systolic blood pressure. See text for further description.

Taken together, the 12 leads of a state-of-the-art ECG can yield a wealth of information about the heart. For example, regions of damaged heart tissue, called infarcts, reflect electrical waves and are apparent in one or more lead potentials. Subtle changes due to slight or gradual damage to the heart are most readily detected by comparing a recent ECG to an older one. This is particularly the case since individual heart shape, size, and orientation can cause variations in ECGs from one individual to another. ECG technology has advanced to the point where a portable ECG monitor with a liquid crystal instant display and a printer can be carried to patients' homes or used in emergency vehicles. See [link] .

Photograph of a NASA scientist in an underwater habitat recording her vital signs using a portable device and a laptop computer.
This NASA scientist and NEEMO 5 aquanaut’s heart rate and other vital signs are being recorded by a portable device while living in an underwater habitat. (credit: NASA, Life Sciences Data Archive at Johnson Space Center, Houston, Texas)

Phet explorations: neuron


Stimulate a neuron and monitor what happens. Pause, rewind, and move forward in time in order to observe the ions as they move across the neuron membrane.

Section summary

  • Electric potentials in neurons and other cells are created by ionic concentration differences across semipermeable membranes.
  • Stimuli change the permeability and create action potentials that propagate along neurons.
  • Myelin sheaths speed this process and reduce the needed energy input.
  • This process in the heart can be measured with an electrocardiogram (ECG).

Conceptual questions

Note that in [link] , both the concentration gradient and the Coulomb force tend to move Na + size 12{"Na" rSup { size 8{+{}} } } {} ions into the cell. What prevents this?

Define depolarization, repolarization, and the action potential.

Explain the properties of myelinated nerves in terms of the insulating properties of myelin.


Integrated Concepts

Use the ECG in [link] to determine the heart rate in beats per minute assuming a constant time between beats.

80 beats/minute

Integrated Concepts

(a) Referring to [link] , find the time systolic pressure lags behind the middle of the QRS complex. (b) Discuss the reasons for the time lag.

Questions & Answers

where we get a research paper on Nano chemistry....?
Maira Reply
nanopartical of organic/inorganic / physical chemistry , pdf / thesis / review
what are the products of Nano chemistry?
Maira Reply
There are lots of products of nano chemistry... Like nano coatings.....carbon fiber.. And lots of others..
Even nanotechnology is pretty much all about chemistry... Its the chemistry on quantum or atomic level
no nanotechnology is also a part of physics and maths it requires angle formulas and some pressure regarding concepts
Preparation and Applications of Nanomaterial for Drug Delivery
Hafiz Reply
Application of nanotechnology in medicine
what is variations in raman spectra for nanomaterials
Jyoti Reply
ya I also want to know the raman spectra
I only see partial conversation and what's the question here!
Crow Reply
what about nanotechnology for water purification
RAW Reply
please someone correct me if I'm wrong but I think one can use nanoparticles, specially silver nanoparticles for water treatment.
yes that's correct
I think
Nasa has use it in the 60's, copper as water purification in the moon travel.
nanocopper obvius
what is the stm
Brian Reply
is there industrial application of fullrenes. What is the method to prepare fullrene on large scale.?
industrial application...? mmm I think on the medical side as drug carrier, but you should go deeper on your research, I may be wrong
How we are making nano material?
what is a peer
What is meant by 'nano scale'?
What is STMs full form?
scanning tunneling microscope
how nano science is used for hydrophobicity
Do u think that Graphene and Fullrene fiber can be used to make Air Plane body structure the lightest and strongest. Rafiq
what is differents between GO and RGO?
what is simplest way to understand the applications of nano robots used to detect the cancer affected cell of human body.? How this robot is carried to required site of body cell.? what will be the carrier material and how can be detected that correct delivery of drug is done Rafiq
analytical skills graphene is prepared to kill any type viruses .
Any one who tell me about Preparation and application of Nanomaterial for drug Delivery
what is Nano technology ?
Bob Reply
write examples of Nano molecule?
The nanotechnology is as new science, to scale nanometric
nanotechnology is the study, desing, synthesis, manipulation and application of materials and functional systems through control of matter at nanoscale
Is there any normative that regulates the use of silver nanoparticles?
Damian Reply
what king of growth are you checking .?
What fields keep nano created devices from performing or assimulating ? Magnetic fields ? Are do they assimilate ?
Stoney Reply
why we need to study biomolecules, molecular biology in nanotechnology?
Adin Reply
yes I'm doing my masters in nanotechnology, we are being studying all these domains as well..
what school?
biomolecules are e building blocks of every organics and inorganic materials.
how did you get the value of 2000N.What calculations are needed to arrive at it
Smarajit Reply
Privacy Information Security Software Version 1.1a
How we can toraidal magnetic field
Aditya Reply
How we can create polaidal magnetic field
Mykayuh Reply
Because I'm writing a report and I would like to be really precise for the references
Gre Reply
where did you find the research and the first image (ECG and Blood pressure synchronized)? Thank you!!
Gre Reply
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Source:  OpenStax, Physics 101. OpenStax CNX. Jan 07, 2013 Download for free at http://legacy.cnx.org/content/col11479/1.1
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