Ionic basis of AP for contractile myocyte, EC Coupling, & ECG

Question 1

what are the 3 electrolytes that are involved in an AP in contractile myocytes?

Answer 1

K, Na, and Ca

Question 2

describe phase 0 of an AP in a contractile myocyte

Answer 2

Known as rapid depolarization
phase
With influx of Na+ through the fast Na+ channels (voltage-gated Na+ channels)
i.e. Fast activation after reaching threshold potential @ approx. -65 mV
From approx. –90 mV to +20 mV in 3 to 5 msec

Question 3

describe phase 1 of an AP in a contractile myocyte

Answer 3

Known as early repolarization
phase
With rapid inactivation of the fast Na+ channels
Together with the activation of the transient outward K+ current (fast activated inactivated K+ channels)
i.e. Brief efflux of K+

Question 4

describe phase 2 of an AP in a contractile myocyte

Answer 4

Called the plateau phase
It is the balance between K+ efflux and Ca2+ influx
K+ efflux:
through K+ channels (also known as delayed rectifier K+ channels)
Ca2+ influx:
through L-type (long-lasting) Ca2+ channels which open @ ≈ - 40 mV
Duration ~175 msec

Question 5

describe phase 3 of an AP in a contractile myocyte

Answer 5

repolarization phase
Starts with the inactivation of the L-type Ca2+ channels
With the efflux of K+ exceed the influx of Ca2+ i.e. Inside of the cell membrane becomes
progressively more negative Duration ~ 75 msec
Cell is in refractory (unexcitable) during phases 0,1,2, & part of phase 3

Question 6

describe phase 4 of an AP in a contractile myocyte

Answer 6

Restoration of ionic concentrations
Na+ and K+ by Na+-K+ pumps (Na+-K+ ATPase)
With 2 K+ entering & 3 Na+ leaving the cell
Ca2+ by both the Na+-Ca2+ exchangers and ATP-driven Ca2+ pumps

Question 7

what type of junctions occur at contractile myocytes, and what types of receptors do they have?

Answer 7

neuromuscular junctions (chemical synapses), and receptors for Epi and NE

Question 8

Cardiac sarcolemma contains gap junctions (electrical synapses) that couple with neighboring cells which enhance the propagation of these impulses

Answer 8

ya

Question 9

what is the general path of EC coupling?

Answer 9

excitation (AP from autorhyth cells); depol of T tubule membrane (sarcolemma); increase in cytosolic Ca; muscle contraction

Question 10

2 ways to increase IC Ca

Answer 10

from ECS or the SR

Question 11

where does the excitation (AP) come from that depols the T tub?

Answer 11

Propagation of action potential through the Purkinje fibers or adjacent cells (gap junctions) of the ventricles

Question 12

general pathway for relaxation

Answer 12

end of depol; decrease in IC Ca through L-type Ca channels in sarcolemma; Ca uptake by SERCA pump (and Na/Ca exchangers); decrease in IC Ca; dissociation of Ca from TNC–> relaxation; also ATP binding to myosin head

Question 13

what is an ECG/EKG?

Answer 13

Graphic recording/display of the biopotentials generated by the myocardium during the cardiac cycle; shows electrical events–repol and depol, but does not show mechanical events– contraction and relaxtion

Question 14

ECG is a vector = measures direction and magnitude

Answer 14

ya

Question 15

study slide 27 of Nov 21

Answer 15

ya