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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)

Electrocardiograms

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

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?

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Define depolarization, repolarization, and the action potential.

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

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Problems&Exercises

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

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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.

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Questions & Answers

What is electric
Manasseh Reply
electric means?
ghulam
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PhysicswithMrV
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becuse the d .c cannot travel for long distance trnsmission
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the
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sound is directly proportional to the temperature.
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PhysicswithMrV
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hi pls help me with this question A ball is projected vertically upwards from the top of a tower 60m high with a velocity of 30ms1.what is the maximum height above the ground level?how long does it take to reach the ground level?
mahmoud
please guys help, what is the difference between concave lens and convex lens
Vincent Reply
convex lens brings rays of light to a focus while concave diverges rays of light
Christian
for mmHg to kPa yes
Matthew
it depends on the size
Matthew Reply
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Vincent
a lens which diverge the ray of light
rinzuala
concave diverges light
Matthew
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Vincent
A diverging lens
Yusuf
What is isotope
Yusuf
each of two or more forms of the same element that contain equal numbers of protons but different numbers of neutrons in their nuclei, and hence differ in relative atomic mass but not in chemical properties; in particular, a radioactive form of an element. "some elements have only one stable isotope
Karthi
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I would like to know this too
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Olufunsho
What is light wave
Sakeenah Reply
What is wave
Sakeenah
What is light
Sakeenah
okay
True
explain how neurons communicate feed and stimulate
Jeff
Great science students
Omo
A wave is a disturbance which travels through the medium transferring energy from one form to another without causing any permanent displacement of d medium itself
OGOR
Light is a form o wave
OGOR
Neurons communicate by sending message through nerves in coordination
OGOR
What are petrochemicals, give two examples
OGOR
light has dual nature, particle as well as wave. when we want to explain phenomena like Interference of light, then we consider light as wave.
Lalita
what is it as in the form of it or how to visualize it or what it contains
Matthew
particles of light are like small packets of energy called photons, and flow or motion of photons is wave like
Lalita
light is just the energy of which photons emit
Matthew
the wave is how they travel
Matthew
photons do not emitt energy, they are energy. They are massless particles.
Lalita
a wave is a disturbance through the medium. Have you ever thrown a stone in still water? the disturbance produced travels in form of wave, the wave produced by throwing stone in still water are circular in nature.
Lalita
a photon does contain mass when in motion. it doesnt contain mass when at rest
Matthew
when would it ever be at rest
Bob
a wave is a disturbance of which energy travels
Matthew
that's darkness. darkness has no mass because the photons within in aren't moving or producing energy
Matthew
Hi guys. Please I've been trying to understand the concept of SHM, but it's not been really easy, could someone please explain it to me or suggest a site I could visit? Thank you.
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Matthew
effective mass of photons only comes into picture when we consider it accelerating in gravitational field, mass of photon has no meaning as it is always travelling with speed of light and is never at rest. with that high speed, Energy and momentum are equivalent. and darkness is absense of photons.
Lalita
darkness is absense of light. not the presence of 'resting photons'. photons are never at rest.
Lalita
photons are present in darkness but don't give off any light because they are stationary with no mass or energy. once a force makes them move again they will gain mass and give off light
Matthew
this theory is presented in Einsteins theory of special relativity
Matthew
A.The velocity Vo for the streamline flow of liquid in a small tube depends on the radius r of the tube,the density and the viscosity iter of the liquid .use the dimensional analysis to obtain an expression for the velocity . B.Given that Vo =r square ×p all over 4×iter ×l
True
A.The velocity Vo for the streamline flow of liquid in a small tube depends on the radius r of the tube,the density (rho)and the viscosity (iter)of the liquid. Use the method of dimensional analysis to obtain an expression for the velocity . B.Given that Vo =r square x p all over 4 x iter x l
True
Matthew, photons ARE light. there is no such thing as a photon that isn't moving. in fact the speed they move at is called C (for constant) in physics. through a vacuum they always travel at this speed no matter what. they can not slow down; except in another medium.
Brad
The reason why a photon can go at this speed is BECAUSE it had no mass. nothing can go this speed or faster because it needs to have no mass or negative mass. that's why it's called the constant.
Brad
when a photon hits something that is opaque, this is the only way to "stop"it. it isn't merely stopped but absorbed and turned into heat energy, then the remaining energy is reflected in different wavelengths. that reflection is what we call color. the darker something is, the less photons are ther
Brad
e. complete blackness is the absolute absence of photons altogether. I believe what you're referring to is not speed, but wavelength, which is indirectly proportional to the amount of energy a particular photon is made up of.
Brad
in order for a photon to have zero wavelength, it must (at least theoretically) have infinite energy.
Brad
about mass: you may have photons confused with electrons. elections have a mass so small that people say they are without mass, but they do. it is called electron mass or Me-.
Brad
you may also be getting electrons and photons confused because of the cherenkov effect. that is what happens when a particle travels faster than light IN THAT PARTICULAR MEDIUM. I emphasize that because no other particle besides photons can go the speed of c.
Brad
when a particle goes faster than light in a particular medium, a blue light is emitted, called cherenkov radiation. this is why nuclear reactors glow blue.
Brad
nuclear reactors release so much energy that when they emit electrons, those electrons are given enough energy to go faster than light in that medium (in this case water), releasing blue light. if you put the reactor in air or a vacuum, this effect wouldn't happen because the speed of light in air
Brad
is very close to c, which is the universal speed limit. I'd you did go faster than c, time would go backwards and you would have infinite theoretical mass and probably spagghettify, like with a black hole.
Brad
*if
Brad
*electrons
Brad
light waves can travel through a vacuum, and do not require a medium. In empty space, the wave does not dissipate (grow smaller) no matter how far it travels, because the wave is not interacting with anything else.
Salim
Please is there any instructional material for sounds Waves, Echo, light waves
Salami
how far there is hot topic that is boarding me now
Abraham
linear motion
Ahmed
kinematic
Abraham
tell us about it
Akinsanya
kinematic
Emma
kinematics disscuss the motion without cuases ...
ghulam
wow I like what am seeing here I need someone to brush me up in physics in fact I'll say I know nothing
Godslight
How does the Geiger tube works
Salma Reply
pls he do we find for tension
Belinda Reply
tension is equal to the weight of the object. so for example if something weighs 45 Newtons then the tension in the Rope holding it is 45 Newtons. and because it is in equilibrium if the object is 45N and there are three ropes holding it there would be 15 N of tension in each to equal the weight
Shii
does that work for you?
Shii
tnx
Belinda
very correct
Kudzy
Practice Key Terms 4

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Source:  OpenStax, College physics. OpenStax CNX. Jul 27, 2015 Download for free at http://legacy.cnx.org/content/col11406/1.9
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