• ‫ي‬
Dr /Noha Elsayed
Objectives
• Define membrane potential, function and
recognize steps of action potential .
• Define conductivity and automaticity.
• Characteristics of conduction in heart
Is the difference in electric potential between the
interior & the exterior of biological cells
All eukaryotic cells maintain more negative charge in
the interior than the exterior
1. Provide power to cells to operate their function
2. Used for transmitting signals between different
parts of the body
In all body cells, the membrane potential is held at
relatively stable values in their baseline states called
resting potential
Opening & closing of ion channels ---- cause ions to
pass from inside & outside the cell → leading to
depolarization or hyperpolarization
Depolarization
Hyperpolarization
If the interior is more
positive
If the interior is less
positive
Is an event in which the membrane potential of a cell
rapidly rises & falls.
Resting state
Both sodium & potassium channels
are closed & membrane is in resting
state
Depolarization
phase
Activation gates of sodium channels
open, but potassium channels
remain closed & so sodium ions
rush into the cells leading to more
+ve charge inside the cells
Repolarization phase
Back to resting state
Inactivation gates close
the sodium channels, but
potassium channels open
allowing potassium ions to
leave the cells ---- leading
to decrease in +ve charge
inside the cell
Automaticity (Autorhythmicity)
 Some tissues or cells have the ability to produce
spontaneous rhythmic excitation without
external stimulus.
 Different intrinsic rhythm of rhythmic cells
 Purkinje fiber, 15 – 40 /min
 Atrioventricular node 40 – 60 /min
 Sinoatrial node 90 – 100 /min
 normal pacemaker
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Conducting System of Heart
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characteristics of conduction in heart
 Delay in transmission at the A-V node (150 –200 ms) –
sequence of the atrial and ventricular contraction –
physiological importance
 Rapid transmission of impulses in the Purkinje system –
synchronize contraction of entire ventricles – physiological
importance
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THANK YOU
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