Cardiac Muscle

Question 1

Structure of cardiac muscle

Answer 1

Has sarcomeres of actin and myosin

Surrounded by T tubules

Question 2

Ultrastructure of striated cardiac muscle

Answer 2

Has mitochondria
Sarcolemma
Tubules
Dyads instead of triads as terminal cistern are more discrete
Sarcoplasmic reticulum
Sarcomere

Question 3

Describe the ventricular myocytes

Answer 3

Brick shaped, fit together, form syncytium via intercalated disc (1 fuctional organ)

Question 4

Excitation contraction coupling of cardiac muscle

Answer 4

AP enters T tubule => VG Ltype channels open => influx of Ca

Ca binds to Ryanodine receptors on SR => Ca induced Ca release

Ca binds to troponin C => activates contraction as tropomyosin reveals myosin head binding site

Question 5

Removal of Ca from the system

Answer 5

SERCA activity reabsorbs Ca into SR
Na Ca exchange removes Ca
Na exchanged for Ca removed via NaKATPase

Question 6

Cardiac action potential in ventricular muscle

Answer 6

Depolarization due to NA influx via VGNC

Repolarization due to K efflux via VGKC

Plateau, influx of Ca, efflux of K

Repolarization, Ca channels close, K efflux

Resting potential of -90mV restored due to Na, Ca VGC closing. K leaks out

Question 7

What is the benefit of the long cardiac refractory phase

Answer 7

Prevents tetany

Protects against re entrant arrhythmias

Question 8

Mechanism by which Ca activates contraction

Answer 8

Ca2+ binds to troponin C, pulls tropomyosin out the way on actin
ATP provides energy to allow myosin head to release from actin, swing forward
Myosin heads move to next actin binding site and binds, drags myosin in sliding mechanism

Question 9

What is the relationship between calcium and tension
Compare this relationship to skeletal muscle
What 4 factors can this relationship be altered by

Answer 9

Sigmoid relationship
-at steepest point of curve, small increase in Ca => large increase in tension

Cardiac muscle more sensitive to Ca increase than skeletal muscle

Temperature
Pi
pH
Drugs

Question 10

Relationship between sarcomere length and force and why?

What happens when the length increases too much

Answer 10

Increase in length => increase in Ca sensitivity => max no of cross bridges => max tension

Troponin C’s sensitivity to Ca increases

If length increases too much, no more cross bridges form => tension decreases

Question 11

The Frank Starling Law of the heart

Answer 11

Peak systolic pressure in heart directly related to diastolic fibre length
Cardiac output directly related to filling pressure

Increased pressure => increased stretch, max cross bridge formation => increased force

Question 12

What is the force frequency relationship and why?

Answer 12

Increased frequency => increased force

• rate of Na influx and depolarisation exceeds NaCa exchange
• net increase in Ca release => increased force of contraction

Question 13

What happens in failing cardiac muscle in terms of frequency and force

Answer 13

When frequency increases, force frequency relationship becomes -ve

Down regulation of SERCA => less Ca released when Rynanodine receptors open

Upregulation of Na/Ca change, increase of intracellular Na => more Ca removed between beats

Question 14

Definition of dyad

Answer 14

Discrete terminal cistern, T tubule in the ultrastructure of cardiac muscle

Question 15

Definition of syncytium

Answer 15

Cardiomyocytes fuse together via intercalated disks so they function as 1 organ unit
Allow for synchronized ventricular contraction