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Heart activity
• Modulation of autorhythmic cells
• Electrical activity co-ordinates heart
contraction
• Pacemakers
• Electrocardiogram shows heart electrical
activity
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Modulation of autorhythmic cells
• NE (sympathetic) and E (adrenal
hormone)
• Autorhythmic cells have beta1 receptors
• Cyclic AMP levels increase
• Properties of If and Ca++ channels altered
• More rapid Na+ and Ca++ entry
• Rapid action potential
• Rapid contractions
Modulation of autorhythmic cells
• Parasympathetic, acetyl choline
• Muscarinic receptors
• K+ channels open mb hyperpolarizes
cell less excitable
• Ca++ channel less likely to open slower
depolarization  cell is less excitable
Electrical activity in the heart
• Begins with action potential in
autorhythmic cells.
• Gap junctions spread depolarization
through intercalated discs
The heart’s main pacemaker is in the sinoatrial node (SA node)
where a group of autorhythmic cells are located at the junction of the
vena cava and the right atrium
The conductive
system of the heart
consists of noncontractile
autorhythmic cells
Action potentials spread across atria.
Action potentials encounter fibrous
Tissue at the junction of the atria and
Ventricles.
Action potential reach the ventricles
Through the AV node, then spread
Rapidly down the bundle of His.
Prukinje fibers rapidly transmit
Impulses up the ventricle
All contractile cells at apex
Contract simultaneously
Why does the AV node delay action
potentials?
• This allows the atria to complete
contraction and allows the ventricles to fill
before the ventricles contract
Autorhythmic cells
• All cells of the conducting fibers are
autorhythmic
• The cells of the SA node set the heart rate
because their rhythm is fastest.
• They set the pace.
• If the SA node is damaged, the other
pacemakers still function, the pace lowers.
The electrocardiogram shows the
electrical activity of the heart.
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ECG
Electrodes placed on the skin
On arms and leg
ECG shows summated electrical activity
during heart activity
• Indirectly shows contraction and relaxation
of atria and ventricles
Einthoven’s triangle.
Location of electrode placement
For ECG
The electrocardiogram. Know the points.
ECG
• P wave, atria depolarize
• QRS complex, wave of ventricular
depolarization
• T wave, ventricles repolarize
The heart cycle
• Period from the beginning of one heart
beat to the beginning of the next heart
beat
• Two phases:
– Diastole, period of cardiac muscle relaxation
– Systole, period of cardiac muscle contraction
• The atria and ventricles do not contract
and relax at the same time.
5 phases of the heart cycle
• Heart at rest (atrial and ventricular
diastole)
• Completion of ventricular filling (atrial
systole)
• Early ventricular contraction (first heart
sound)
• The heart pumps (ventricular ejection)
• Ventricular relaxation (second heart
sound)