Regulation of Cardiac Function 2
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Why is an action potential in the heart so long?
• prevents tetany (muscular spasms) - prevents tachycardia occuring
What is calcium entry required for in cardiac muscle?
for tension development
What are the major differences between the action potentials of the ventricular muscle compared to the SA node?
The ventricles have a more rapid upstroke compared to the atrium. Ventricular myocytes have a more negative resting potential than the atrium.
What currents (ion movements) occur during the SA node action potential?
Calcium ion move inwards which causes the cell to depolarise which generates a slow-rising action potential. Potassium ions move out the cell promoting repolarisation
—– of the pacemaker determines heart rate
decay
What does a slow decay of the pacemaker mean?
A slower heart rate generated
Noradrenaline increases the speed of decay. How does it do this?
It activates adenlyl cylase which activates cGMP. acetlycholine inhibits adenlyl cylase
What is needed in the cell to allow the movement of ions?
Connexons
What are the two things that rate of conduction depends on?
• Resistance of gap junctions 2. Membrane capacitance (number of connexons)
Explain the conduction of the electrical activity in the heart
Initiation occurs at the SA node on atrial systole. Diffusion of this through myocytes. Conduction occurs via the AV node. Delay in this impulse so all the blood has left the atria. Contraction of the potential via the Bundle of His, left and right bundles and Purkinje fibres.
Contraction of the heart occurs from inside to outside or outside to inside?
Endocardium to epicardium (moves outwards)
Are there any nerve cells in the cardiac conduction pathway? What cell is present?
No Muscle cells
What feature of a cardiac muscle cell is important for rapid action potential conduction?
T tubules
What must be released in order for cardiac muscle to contract?
Ca2+ release. It is stored during diastole and must then be removes by the calcium-sodium exchanger.
When heart rate increases, what happens to the Ca2+ uptake after an action potential?
It must be taken up much faster.
What do chronotropic agents and iontotropic agents alter? Give examples of molecules which have positive and negative effects
Chronotrophic - alters heart rate Positive - noradrenaline via beta receptors Negative - acetylcholine, beta blockers, Inotropic - alters contractility Positive - noradrenalne Negative - hypoxia
What are the options for the number of leads you can use for a ECG recording?
3, 6 , 12
What causes an straight line up and a straight line down in an ECG?
Up - the depolarisation of the heart is occurring in direction of the anode Down - depolarisation occurs away from the anode