Download Electrical Activity of the Heart

Cardio 4 – Electrical Activity of the Heart
Anil Chopra
1. Describe the main structures of the human heart.
2. Describe the structure of a typical cardiac monocyte.
Cardiac monocytes are found exclusively in the heart and are shorter and less circular
than skeletal muscle. They have a centrally located nucleus among the myofibrils.
They are usually connected end to end. They have transverse thickenings and
thinnings called intercalated disks. They have gap junctions which allow action
potentials to spread across the syncitium so that they all contract together.
3. Briefly describe the pathways of the heart that subserve the normal orderly
passage of electrical activity through it.
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Heart beat initiated at SAN (Sino-atrial node).
Excitation spreads over atria to produce atrial systole. The ventricles are filled
with blood.
AVN (atrio-ventricular node) receives depolarisation after a delay.
AVN conducts excitation down network of Purkinjie fibres (via the Bundle of
His) which travel down the base of the ventricles and up the walls of the
ventricles.
Excitation then spreads in such a way that the ventricles contract from the base
(apex) upward.
4. Sketch intracellular action potential for:
a) Sino-atrial node
Pre-potential slope determines how quick it can depolarise
again. This can be affected by sympathetic
and parasympathetic stimulation.
Threshold level is -50mV which is reached
quicker or slower depending on what type
of stimulation it receives
No potential
Contain voltagesensitive Ca2+ channels
Myogenic rhythm is slower if
SAN is damaged. AVN can do
its job but natural rhythm is
slower.
Longer action potential
5. State that the sino-atrial node is the normal pacemaker and explain why and how
this is so.
SAN cells do not have a stable resting potential. They constantly depolarise and
repolarise. When the depolarisation gets as high as -50mV it triggers the threshold at
which point an action potential is generated. AVN also has a small pre-potential but
it is myogenically slower than the SAN.
6. Describe how activity in the SA node spreads to both atria.
It is easily spread along the atria by passing through the gap junctions from
monocytes to monocyte via the intercalated disks which have low electrical
resistance.
7. Explain why transmission of electrical activity from the atria to the ventricles
normally only occurs at the Atrio ventricular node.
There are fibres in between the atria and ventricles called the annulus fibrosus
which is an electrical insulator. This therefore forces the electrical impulse to
travel to the AVN. This delay allows time for the ventricles.
8. Describe how electrical activity is transmitted to all parts of the ventricles
through the Bundle of His and Purkinje fibres.
The bundle of His transfers the impulse through the annulus fibrosus down the
left and right branches of the Purkinje system. This distributes the impulse over
the inner walls and therefore cause ventricular contraction from the apea upward.
9. Explain why the ventricular action potential has a long duration and relate this
to the function of the ventricles.
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When neighbouring tissues cause the cells in the ventricles to reach the threshold
level, voltage-gates FAST SODIUM channels open causing a large influx of Na+
ions.
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These then cause depolarisation
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SLOW CALCIUM channels will also
open causing a slower influx of Ca2+
channels into the cytosol. This causes the
repolarisation to be slow.
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This is vital so that not only is all the
blood expelled from the ventricles, the
contraction is strong enough to force the
blood a long distance and also, it is vital
for the pumping action of the heart. The
refractory period is longer is longer than
the actual contraction itself and means that the heart cannot produce a fused
tetanus.
10. Describe ECG waveforms using PQRST nomenclature, state the electrical
events each one represents.
Wave of depolarisation moves
TOWARD +VE  Upward
AWAY FROM +VE  Downward