Cardiac Muscle

Question 1

What is cardiac muscle?

Answer 1

-Made up of cardiac myocytes (myocardial muscle cells) which are shorter, branched cells and usually contain a single nucleus

Interconnected by intercalated disks:
-desmosomes link mechanically
-Gap junctions link them electrically

-Striated and has sarcomeres (similar array to skeletal muscle
-Less abundant but larger T-tubules
-Smaller amounts of SR (cardiac muscle requires the entry of extracellular Ca2+)
-An abundance of mitochondria (1/3 of cell volume) for oxidative metabolism for ATP production

Question 2

How is contraction initiated in cardiac muscle?

Answer 2

Question 3

Describe APs in autorhytmic myocardial cells

Answer 3

Question 4

Describe APs in contractile myocardial cells

Answer 4

Question 5

What are the effects of long APs in cardiac muscle?

Answer 5

Long action potential duration prevents summation of contraction (refractory period during muscle tension)

Question 6

Explain E-C coupling in cardiac muscle

Answer 6

Question 7

Explain SERCA pump in cardiac muscle

Answer 7

-SERCA pump is regulated by phospholamban
-When phosphorylated: Ca2+ pump inhibition is removed, enhancing relaxation rates and contractility
-Rate and amount of Ca2+ uptake is increased, causing quicker relaxation and larger store of Ca2+ for subsequent contractions

Question 8

How can contractile force be enhanced in cardiac muscle?

Answer 8

Question 9

What neuronal input modifies conduction/contraction in the heart?

Answer 9

The heart is innervated by the autonomic nervous system:
-Sympathetic increases heart rate/conduction and contractility (autorhythmic and contractile)
-Parasympathetic decreases heart rate/conduction (autorhythmic)

Question 10

What is the sympathetic modulation of contraction

Answer 10

Question 11

Describe the cardiac length tension relationship

Answer 11

A slightly stretched sarcomere increases the Ca2+ sensitivity of the myofilaments:
-decreased diameter may reduce the distance that Ca2+ needs to diffuse increasing probability of cross bridge cycling

A slightly stretched sarcomere puts additional tension on stress-activated Ca2+ channels, increasing Ca2+ induced Ca2+ release

Question 12

Explain tonic control of heart rate

Answer 12

Question 13

What is parasympathetic modulation of pacemaker activity and what channels are affected?

Answer 13

-HCN channels
-K+ (GIRK) channels
-T-type Ca2+ channels

Question 14

Explain sympathetic modulation of pacemaker activity

Answer 14