Cardiac Muscle

Question 1

Which structure joins adjacent cardiac muscle cells together?

Answer 1

Intercalated discs

Question 2

Name the proteins that make up the myofibrils of myocardial cells.

Answer 2

Actin

Myosin

Question 3

Which protein makes up the thick filament?

Answer 3

Myosin

Question 4

Which protein makes up the thin filament?

Answer 4

Actin

Also troponin and tropomyosin

Question 5

Each globular head of myosin has 2 binding sites. What attaches to each site?

Answer 5

One for actin

One for ATP

Question 6

Name two molecules attached to the myosin head (that don’t bind to the binding site)

Answer 6

Pi and ADP

Question 7

What is the role of troponin in muscle contraction?

Answer 7

Troponin changes shape when Ca2+ binds to it. This pushes tropomyosin, exposing the underlying myosin binding site on actin, enabling contraction.

Question 8

What is within the A band of the sarcomere?

Answer 8

Thick and few overlapping thin filaments

Question 9

What is within the I band of the sarcomere?

Answer 9

Only thin filaments

Question 10

What is within the H zone of the sarcomere?

Answer 10

Only thick filaments

Question 11

What does one sarcomere consist of in relation to bands and zones?

Answer 11

2 I bands
1 A band
1 H zone
1 M line 
2 Z lines

Question 12

Is the resting potential of the cardiac sarcolemma more permeable to K+ or Na+?

Answer 12

K+ as K+ ion channels are open.

Question 13

Describe the action potential of a cardiac sarcolemma.

Answer 13

1. When the action potential arrives, Na+ voltage gated ion channels open, Na+ enters the cell and depolarises the cell (positive feedback)
2. When cell potential reaches +52mV, the voltage gated Na+ channels close and voltage gated K+ channels open - partially repolarising the cell
3. Ca2+ voltage gated channels open at the T Tubules, resulting in the inflow of Ca2+. This balances the flow of K+ ions out of the cell, keeping the membrane depolarised at the plateau level of 0mV
4. Repolarisation occurs when the Ca2+ channels close to stop inflow and K+ channels open so K+ outflows.

Question 14

Describe the steps of the cross bridge cycle.

Answer 14

1. Ca2+ binds to a binding site on the troponin protein of the actin filament
2. Troponin changes shape, displacing tropomyosin on actin and exposing the myosin binding sites
3. The myosin head binds to the actin filament via the binding site. Pi is released, whilst ADP is still attached to the head
4. Myosin head then releases the ADP to contract and pulls the actin filament over the myosin filament = power stroke, decreasing the Z lines
5. ATP binds to the myosin head, detaching the head from the actin filament, and moving it to the start position
6. ATPase in myosin head hydrolyses ATP into ADP + Pi ready for the next contraction

Question 15

Why does rigour mortis occur in dead people?

Answer 15

No ATP production, so myosin head remains attached to actin, resulting in stiffness of skeletal muscle

Question 16

Describe the contraction time difference between cardiac muscle and skeletal muscle.

Answer 16

Contraction is longer in cardiac muscle due to slow calcium channels

Question 17

What is the blood supply and venous drainage of the myocardial cells?

Answer 17

Supply: Coronary arteries
Drainage: Coronary sinus, empties into right atrium