Myocardial Contractility

Question 1

syncytium

Answer 1

•the atria (or ventricles) acting as one large cell

Question 2

intercalated discs

Answer 2

• the areas of contact between cells
• desmosomes (velcro)
• gap junctions (allow for sharing of onformation)

Question 3

calcium ions

Answer 3

• their influx through slow calcium channels accounts for the prolonged plateau phase and absolute refractory period that enable cardiac muscle to function properly
• Ca++ ions also combine with the regulatory protein troponin in the troponin-tropomyosin complex - removes inhibition that prevents heads of myosin from forming cross bridges with the active sites on actin ***virtually identical to that of skeletal muscle mechanism
• approx. 20% of Ca++ required for contraction is supplied by the influx during the plateau phase, the rest is released from storage in the sarcoplasmic reticulum

Question 4

calcium induced calcium release

Answer 4

• L-type channels in T tubules - voltage gated, respond to action potential, let a small amount of Ca++ in which triggers the opening of nearby Ca++ release channels in the lateral sacs of the sarcoplasmic reticulum
• in skeletal muscle, AP releases stored Ca++, modulate force by the number of fibers that get activated
• cardiac muscle modulates force by the amount of Ca++ released into the cytoplasm

Question 5

positive inotropes

Answer 5

• sympathetic stimulation
• epinephrine
• dopamine
• isoproterenol
• hypercalcemia
• digoxin

Question 6

negative inotropes

Answer 6

• parasympathetic stimulation
• hypoxia
• hyperkalemia
• propranolol
• calcium channel blockers

Question 7

Law of Laplace

Answer 7

•wall tension is directly related to the product of the intraventricular pressure and internal radius and inversely related to wall thickness

T = P x R
•T = tension
•P = pressure
•R = lumen radius

o = T/h
•o = stress
•h = wall thickness