Cardiac Muscle Mechanisms

Question 1

how is cardiac muscle structurally similar to skeletal muscles?

Answer 1

sarcomere organization (and therefore striation), and multinucleation (although fewer than skeletal muscle)

Question 2

what structural differences exist between cardiac and smooth muscle?

Answer 2

intercalacted discs forming a functional syncytium and branching of the cells

Question 3

what connections is an intercalated disc comprised of? what are their functions?

Answer 3

desmosomes structurally stabilize adjacent cells

gap junctions stabilize structure but also allow electrical connection between two muscle cells

Question 4

where are T tubules in cardiac muscle cells and how do they differ from their role in skeletal muscle?

Answer 4

they invaginate at the Z line and they carry action potentials but are not coupled with the SR as a triad

Question 5

what type of metabolism does cardiac muscle employ? what implication does this have for the cell?

Answer 5

aerobic metabolism

abundant myoglobin, glycogen, lipids and mitochondria

Question 6

what does the action potential cause in a cardiac muscle ceell?

Answer 6

extracellular and SR calcium influx

Question 7

how is the ryanodine receptor opening different in cardiac muscle than in skeletal muscle?

Answer 7

it is not attached to the L type calcium channel but uses calcium induced calcium release to open the receptor

Question 8

what are the phases of cardiac action potential?

Answer 8

phase 0: Na current; phase 1: transient outward current; phase 2: Ca current in K current out; phase 3: K current out; phase 4: no net current/rest

Question 9

what are the important differences between the cardiac and skeletal action potentials?

Answer 9

the cardiac potential is much longer

the action potential is calcium dependant

Question 10

what causes phase 1, the transient outward current/repolarization of the cardiac muscle cell?

Answer 10

closing of NA channels and the brief activation of a transient outward current

Question 11

what causes the extension of the refractory period in the cardiomyocyte and what impact does it have on contraction?

Answer 11

it is caused by the slow influx of Ca ions that prevents the cell from repolarizing quickly
it prevents summation of muscle twitches

Question 12

how do cardiac action potentials propagate?

Answer 12

through gap junctions between cells

positive charge is displaced through the gap junction to depolarize the next cell

Question 13

how is excitation of cardiomyocytes coupled to calcium release?

Answer 13

during the plateau phase of the action potential, Ca enters from L type Ca channels. These bind to high affinity sites of the ryanodine receptors in the SR and opens them

Question 14

what percentage of intracellular Ca is contributed from the SR and what percentage from extracellularly in cardiac muscle?

Answer 14

20% extracellular

80% SR

Question 15

what mechanism is Ca released from the SR in cardiomyocytes?

Answer 15

calcium induced calcium release

Question 16

what does the sodium calcium exchanger (NCX) do?

Answer 16

exchanges 3 sodium for 1 calcium. can work in either direction into or out of the cell during contraction or relaxation

Question 17

what are two calcium influx related differences exist between skeletal and cardiac muscle?

Answer 17

cardiac muscle does not couple L type Ca channels and Ryanodine receptors
external calcium contributes to contraction unlike in skeletal muscle

Question 18

what inactivates ryanodine receptors?

Answer 18

when calcium is at very high concentrations in the cell, it binds to the low affinity site on the ryanodine receptor causing it to close

Question 19

what are the calcium clearance systems?

Answer 19

SERCA pumps Ca into the SR
NCX exchanges 3Na in and 1Ca out
the plasma membrane Ca ATPase pumps Ca out of the cell

Question 20

which calcium clearance systems contribute most to calcium removal?

Answer 20

SERCA and NCX

Question 21

what are the implications of the contribution of extracellular calcium to the cardiac contraction?

Answer 21

cardiac muscle is much more sensitive than skeletal muscle to extracellular calcium levels and is much more sensitive to L type Ca channel blockers

Question 22

how is calcium released along the SR in cardiomyocytes if it is not coupled with the T tubule?

Answer 22

at the location of the T tubule, there is voltage gated Ca influx into the cell. this is the trigger that opens the ryanodine receptors close to it. the influx of Ca triggers the neighboring ryanodine receptors

Question 23

once there is Ca influx into the cardiomyocyte, how does this contribute to contraction?

Answer 23

it associates with troponin C and therefore removes tropomyosin from actin’s myosin binding sites

Question 24

what is the refractory period for a cardiomyocyte action potential?

Answer 24

300 msec

Question 25

how is contraction of the ventricles and atria separated?

Answer 25

there are two functional syncytia one for the atria and one for the ventricles

Question 26

what mechanism that is used to increase the force of contraction in skeletal muscle cannot be used in cardiac muscle?

Answer 26

recruitment- all muscle cells already contract because of the gap junctions

Question 27

what two changes can affect the tension in cardiac muscle?

Answer 27

if the CA is altered or the calcium sensitivity of the myofilaments is altered (changing initial length)

Question 28

what molecular effect does the ANS have on the heart rate?

Answer 28

changes K conductances in the SA node

Question 29

what does the frank-starling law of the heart explain?

Answer 29

the increase in cardiac muscle tension without the increase in intracellular calcium levels

Question 30

what is the frank-starling law?

Answer 30

increased stretching of the ventricle during diastole creates in an increase of contractile force of the chamber

Question 31

what is the cause for the increase in contractile force when there is already a maximum overlap of myosin and actin in the sarcomere during the frank- sterling law?

Answer 31

sensitivity to Ca is increased because the affinity of troponin C to Ca is increased

Question 32

what length of myocytes produces the maximal force in the heart?

Answer 32

2.2 micrometers

Question 33

what are the effects of reater initial length on cardiac muscle function?

Answer 33

increased velocity of contraction, increased shortening, increased work and increased power and increase in maximal tension

Question 34

what are the two reasons for tension to decrease in muscle cells?

Answer 34

overlapping of actin filaments and stretching of the muscle too far for myosin and actin to have many interactions

Question 35

how does norepinephrine increase tension development during cardiac twitch?

Answer 35

increases the amount of calcium by g- coupled increase of cAMP and PKA
increases contractility

Question 36

what changes immediately with administration of NE and what is a more delayed effect?

Answer 36

action potentials are increased immediately and increase in contractility
tension increases eventually with an increase in intracellular calcium concentration that takes longer to occur (in 8 beats)

Question 37

how does norepinephrine change a cardiac muscle twitch? why does this occur?

Answer 37

it increases tension and decreases duration

pumps blood harder and increases the availability for increased heart rate

Question 38

what are the two effects of increased cAMP brought about by NE stimulation of beta adrenergic receptors in cardiomyocytes?

Answer 38

increases Ca release by the SR

activation of a protein kinase that phosphorylates phoshpholamban

Question 39

what is the function of phospholamban?

Answer 39

a regulatory protein that increases the sensitivity of the SERCA and therefore reduces the duration of muscle contraction

Question 40

what does a positive inotropic agent do to the length-tension curve?

Answer 40

causes an upward shift to increase tension at a particular length (at each length)

Question 41

what happens to isotonic contractions with inotropic agents?

Answer 41

it increases the length of contraction for a particular tension (lifts a weight farther)