Download Depolarization - Little Shop of Physics

Depolarization
Muscle and nerve cells have a strong
electric character. Under normal
circumstances, the cell is negative inside,
positive outside; the potential difference
across the cell membrane is about -70 mV.
When a nerve cell “fires” or a muscle cell
contracts, this potential changes. Channels
open to allow sodium ions to come into the
cell, changing the potential to +40 mV inside
the cell.
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Dipole potential
The potential difference between two points in the field of a dipole is proportional
to the component of the dipole moment along the line connecting the points.
The electric field of the heart
As the heart beats, a wave of depolarization sweeps across the tissue. Cells that have not
yet depolarized are +; cells that have depolarized are -. Net result: a dipole moment.
The dipole moment of
the heart at its largest.
A basic electrocardiogram
As the heart beats, the dipole moment changes. The largest magnitude dipole
moment occurs when all of the heart muscle save the left ventricle has depolarized.
At any instant, the potential difference recorded between two electrodes is the
component of the dipole moment vector of the heart.
The electric field of
the heart
As the heart beats, the dipole
moment changes. A measurement of
the potential difference between two
points sweeps out a curve showing
the varying component of the dipole
moment vector.
The orientation of the heart
Focus on the largest peak of the electrocardiogram; this comes during the ventricular depolarization.
Depending on how the electrodes are placed, the component of the dipole moment at this instant
will be large or small. You can tell the orientation by trying different electrode arrangements.
!V = V2 " V1
!V = V2 " V1
!V = V2 " V1
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