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Refractory period of cardiac muscle
Learning Objectives
At the end of the practical, the student should be able to:
Define Refractory Period?
Differentiate between absolute and relative refractory periods?
Explain what is a cardiac cycle?
Determine the extent to which it is possible to re-stimulate cardiac
contraction during a cardiac cycle?
Refractory Period
The period that follows effective
stimulation, during which excitable tissue
fails to respond to a stimulus of threshold
Absolute Refractory Period.
Relative Refractory Period.
Absolute Refractory Period:
During the absolute refractory period, a second stimulus (no matter how strong)
will not excite the muscle fiber.
Relative Refractory Period:
During the relative refractory period, the
muscle fiber can respond only to a
strong stimulus.
Cardiac Cycle
A complete contraction and relaxation episode in the heart.
The cardiac cycle consists of:
A Systolic Phase and
A Diastolic Phase.
One complete cardiac cycle forms
the common "lub-dub" pattern
recognized as the heart beat.
Exercise : Refractory period of cardiac
The aim is to determine the extent to
which it is possible to re-stimulate
cardiac contraction during a cardiac
• To be performed only if time allows and
instructed to do so.
• If you are using the ADInstruments
Bridge Amp please use this 'Zero
Bridge Amp' link to correct for any
residual offset voltage again.
Connect the positive and negative BNC connectors of the Animal Nerve
Stimulating Electrode to the analog Outputs on the Power Lab.
• Check that the number of pulses
in the Stimulator panel is set to 1.
• Click Start to begin recording.
• Gently place the tips of the
stimulating electrode on the
surface of the frog heart.
• By clicking the Stimulate button
on the Stimulator panel, attempt
to elicit a second contraction from
the frog heart at each of the
points in the cardiac cycle
illustrated in the figure below.
• Approximate points during the
cardiac cycle at which you should
attempt to stimulate the heart.
Stimulator Panel
• The Stimulator panel allows the control of the voltage (current in some
experiments) of a pulse, the number of pulses and the interval between pulses
applied to a tissue.
• In some experiments a Stimulate button may be visible. This allows the control
of stimulation independently from sampling.
Stimulate Button
• The Stimulate button is only available while recording. It allows the user to
stimulate at any time during a recording. Stimulation can be halted by clicking
the Stop button in the
LabTutor Data Panel.
• In the table, enter "yes" if stimulation resulted in an extra
contraction (extra-systole) with a stimulus delivered at the
point in the contraction cycle or "no" if it did not.