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Lecture 1

BIOC34H3 Lecture Notes - Lecture 1: Purkinje Fibers, Vagus Nerve, Ectopic Pacemaker

Biological Sciences
Course Code
Stephen Reid

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BIOC34 Winter 2017
Assignment 1: PhysioEx Lab 6 (Cardiovascular Physiology)
Activity 1:
Results: When I applied the electrical stimuli to the heart, the amplitude of the ventricular
systole waves increased. When applying a single stimulus rapidly at 20V, I obtained a double
peak on the trace, which contained an extra-systole of the ventricle. During the application of
multiple stimuli, the effects of stimulation on the contractile activity displayed a wave
summation, and tetanus, and thus the amplitude could not increase due to wave summation.
Large waves represented ventricular contraction.
Question: The refractory period of cardiac muscle cells is significantly longer than the
refractory period of skeletal muscle cells. Given your knowledge of the cardiac cycle, what
is the reason for (or advantage of) cardiac muscle cells having a lengthy refractory period.
When an area of the heart depolarises, it goes into a refractory period in which it will not respond
to any stimulus. It prevents the recycling of electrical activity. The advantage of the lengthy
refractory period is that it gives time for the heart to reset to its natural rhythm, allowing it to
spend more time relaxing (in diastole). If cardiac muscle cells emerge from their refractory
period earlier than they should, then this becomes abnormal because the waves of depolarization
are continuously regenerated. This makes it impossible for the contraction of the muscles at
different times to produce coordinated pumping action. During absolute refractory of the cardiac
muscle cells, systole (contraction) of the cardiac muscle cells occurs simultaneously. Thus,
during this refractory period, no other contraction can occur except for systole. The long
refractory period of cardiac muscle cells allows for the heart to become excited before
contraction occurs, allowing the heart to pump blood and then relax and fill the ventricles with
Activity 2
Results: When the vagus nerve was electrically stimulated, it slowed down heart beat per minute.
There was slower contraction. As I increased the number of stimuli from 20 stimuli/sec to 50
stimuli/sec, and applied multiple stimuli on the vagus nerve, the heart rate decreased and
eventually the heart stopped beating.
2a) What is vagal escape? Vagal escape is the resumption of the heartbeat after stopping
momentarily, due to the reduction in blood pressure.
2b) Describe two possible mechanisms of vagal escape? Two possible mechanisms of vagal
escape include the parasympathetic and sympathetic system. In vagal escape, there is a reduction
in blood pressure because of muscarinic stimulation, which is compensated for stimulation from
the sympathetic system and increases the heart rate. Another mechanism is the initiation of a
rhythm by the Purkinje fibers. The parasympathetic (vagus) nerve would deactivate the SA node
and AV node. The Purkinje fiber would start the heart again, acting as an ectopic pacemaker and
establish its own rhythm.
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