Unit 3.6: Cardiac Muscle

Question 1

What are cardiac cells also called?

Answer 1

Myocardial cells

Question 2

Are cardiac cells striated?

Answer 2

Yes.

Question 3

How are cardiac cells different than skeletal cells in terms of

• size
• nucleus
• t-tubules
• how they’re connected to each other

Answer 3

Cardiac muscle cells are much smaller with single nucleus with 1/3 of cell occupied by mitochondria
T-tubules are much larger and branched and SR is smaller
adjacent cells are joined by intercalated discs with desmosomes

Question 4

What are the 2 types of cardiac muscle cells?

Answer 4

Contractile cells and autorhythmic/pacemaker cells

Question 5

What are autorhythmic/pacemaker cells?

Answer 5

They are involved in the electrical excitation of the heart

Question 6

How are autorhythmic/pacemaker cells connected to other cardiac cells?

Answer 6

Through gap junctions

Question 7

How are cardiac cells different than skeletal muscle cells in terms of contraction?

Answer 7

Ca2+ enters through Ca2+ channels on cell membrane and SR

1. Calcium enters through cell membrane
2. Calcium induced calcium release - release of calcium from SR

Question 8

How are cardiac cells different than skeletal muscle cells in terms of relaxation?

Answer 8

Cardiac cells have Na+/Ca2+ antiport in addition to Ca2+-ATPase

Question 9

True or False

Cardiac muscles cells exhibit all-or-nothing contractions.

Answer 9

False.

Cardiac muscles exhibit graded contraction

Question 10

What are graded contraction proportional to?

Answer 10

Number of active crossbridges

Question 11

What are factors influencing cardiac muscle contraction force?

Answer 11

1. Changes in concentration of Ca2+

2. Sarcomere length

Question 12

How do changes in concentration of Ca2+ influence cardiac muscle contraction force?

Answer 12

Regulated by epinephrine and norepinephrine

• binding then activates cAMP second messenger signalling pathway that leads to
  
  • phosphorylation of voltage-gated Ca2+ channels which increases their probability to open which increases concentration of Ca2+ in cytosol
  • phosphorylation of phospholambam which leads to increase SR Ca2+-ATPase activity which increases SR Ca2+

Question 13

How do contractile cardiac muscle cells contract and generate action potentials?

Answer 13

See coursenotes p. 72

Question 14

What is sustained depolarization due to in a contractile cardiac muscle cell?

Answer 14

Due to the slow opening of the voltage-gated Ca2+ channels

Question 15

Why is sustained depolarization important?

Answer 15

Prevents tetanus and allows the heart to relax between contraction

Question 16

Why don’t cardiac muscle cells undergo summation and tetanus?

Answer 16

Because of the longer refractory period, means that the cell has finished contracting before the next action potential