L4 Contractile and Autorhythmic Myocardium

Question 1

what are the differences in membrane potential between contractile and autorhythmic myocardium?

Answer 1

contractile - stable at -90 mV

autorhythmic - unstable pacemaker potential; usually starts at -60 mV

Question 2

what are the differences in events leading to threshold potential?

Answer 2

contractile - depolarisation enters via gap junction

autorhythmic - net Na+ entry through If channels; reinforced by Ca2+ entry

Question 3

what are the differences in rising phase of action potential?

Answer 3

contractile - Na+ entry

autorhythmic - Ca2+ entry

Question 4

what are the differences in repolarisation phase?

Answer 4

contractile - extended plateau caused by Ca2+ entry; rapid phase caused by K+ efflux

autorhythmic - rapid; caused by K+ efflux

Question 5

what are the differences in hyperpolarisation?

Answer 5

contractile - none; resting potential is -90 mV, the equillibrium potential for K+

autorhythmic - normally none; when repolarisation hits -60mV, the If channels open again. ACh can hypoerpolarise the cell

Question 6

what are the differences in duration of action potential?

Answer 6

contractile - extended: 200+ msec

autorhythmic - Variable; generally 150 + msec

Question 7

what are the differences in refractory period?

Answer 7

contractile - long because resetting of Na+ channel gates delayed until end of action potential

autorhythmic - not significant in normal function