Physiology: cardiac muscle function

Question 1

Cardiac muscle cells size?

Answer 1

100um x 15 um

Question 2

Cardiac muscle cell has slow or fast diffusion of oxygen?

Answer 2

Fast diffusion of oxygen

Question 3

Cardiac muscle cell structure?

Answer 3

Striated (2um repeat)

Question 4

Cardiac muscle cell contains?

Answer 4

Numerous mitochondria (30% cross-sectional area) -> energy from oxidative phosphorylation

Question 5

Cardiac muscle cells are electrically coupled end-to-ed by ______.

Answer 5

Connexin 43

Question 6

Transmembrane proteins creating a barrel structure for what?

Answer 6

Ion movement for the transmission of electrical current

Question 7

End part of each cardiac muscle fibre has _______ with the next fibre to increase surface area to hold cells together and to allow ________.

Answer 7

interlaced fingers; electrical current movement

Question 8

Ventricular cells are not __________ (steep rise in membrane potential) unlike pacemaker cells and remain _____ for a while.

Answer 8

spontaneously active; depolarised

Question 9

Cardiac muscle action potential triggers _____ whose time to peak is comparable to the action potential duration.

Answer 9

Contraction

Question 10

Force of action potential starts to drop off with ______.

Answer 10

Repolarisation

Question 11

Action potential of cardiac muscle cell has ______ (300 millisecond duration (2 ms AP peak for skeletal muscle).

Answer 11

Long plateau

Question 12

Describe the sodium channel

Answer 12

• Fast
• Inward
• Causes depolarisation of action potential
• Sodium stimulates calcium channel
• Similar to nerve and skeletal muscle
• Crucial for fast AP propagation across the myocardium.

Question 13

Describe the calcium channel

Answer 13

• Slow channel
• Inwards
• Maintains long plateau of AP
• Entry of calcium contributes to excitation-contraction coupling
• Target of calcium channel blocking drugs (reduce contractility)

Question 14

Describe the potassium channel

Answer 14

• Efflux of potassium is repolarising
• voltage sensitive and sensitive to intracellular constituents e.g. ATP and calcium

Question 15

Arrhythmias are often triggered by?

Answer 15

Triggered by ischaemic events causing ectopic foci and/or re-entry circuits i.e. out of sequence contraction that can trigger abnormal rhythms.

• Other causes: hormonal adrenaline or mutations in channels (e.g. long QT syndromes)

Question 16

What are T-tubules and what do they contain

Answer 16

They are a continuation of surface membrane and contain voltage-gated calcium channels located particularly at the dyads.

Question 17

Where are T-tubules located?

Answer 17

Dyads. Where sarcoplasmic reticulum (SR) meets the T-tubule is the dyad.

Calcium is pumped into SR.

Question 18

Describe steps of contraction

Answer 18

1. calcium pumped into SR
2. Once T-tubule comes down with AP, T-tubule calcium channels open
3. Calcium enters into cytoplasm in interstitial fluid
4. Triggers activation of release channels in the dyads
5. Causes release of even more calcium from SR
6. Calcium binds to troponin and contract cardiac muscle

Question 19

Describe the steps of excitation-contraction coupling

Answer 19

1. Action potential depolarizes T-tubule membranes
2. Activates calcium channels in T-tubules resulting in a small calcium entry into ECF
3. Calcium binds to calcium release channels in sarcoplasmic reticulum (SR): calcium-induced calcium release (CICR)
4. Large calcium release from SR intracellular stores causes contraction.
5. Relaxation occurs.

Question 20

List steps of large calcium release from SR intracellular stores causing contraction

Answer 20

1. Calcium released from intracellular stores
2. Calcium in cytoplasm rises
3. Calcium binds to troponin C
4. Exposure of myosin-binding sites on the actin filaments.
5. Cross-bridge cycling begins
6. Active tension
7. z-lines pulled together

Question 21

Describe relaxation of muscle

Answer 21

1. Slow inactivation of the calcium channel in the T-tubules
2. Uptake of calcium ions into the SR by the SR calcium pump.
3. Removal of calcium ions from the cell by the sodium/calcium exchanger (calcium back into T-tubule)

Question 22

Factor of greater force of contraction?

Answer 22

Greater the intracellular calcium

Question 23

Describe steps of adrenaline/noradrenaline in nerve synapse

Answer 23

1. Binds adrenoceptor on ventricular cell surface which
2. Activates G-protein to increase cAMP which
3. Activates cAMP-dependent kinases which
4. Phosphorylates effector proteins:
   i) Voltage-gated calcium channel on the surface of T-tubules: increased inward calcium ions.
   ii) Phospholamban (regulatory protein acting on the SR calcium pump to make it act faster: increases calcium uptake by SR calcium pump so that there is more storage for next contraction, allowing for stronger contraction.
5. Both actions increase peak SR calcium release and force but shortens time-course of systole.

Question 24

Increasing peak sarcoplasmic calcium ion release and shortening systole leads to?

Answer 24

• Increase in amplitude of the AP.
• Stronger but briefer systole due to the phospholamban.
• Shorter diastole and time of contraction.

Question 25

Cardiac glycosides act on?

Answer 25

Act on Sodium-potassium pump instead

Question 26

Cardiac glycosides inhibit?

Answer 26

Inhibits the sodium pump -\> sodium inside so sodium exchange is less effective, and less calcium is pumped out by the sodium/calcium exchanger.

Question 27

Cardiac glycosides increases?

Answer 27

Increases intracellular sodium and increases cytoplasmic and SR calcium so progressively more calcium is available each cycle for contractile apparatus.

Question 28

Effect of cardiac glycosides

Answer 28

Increases in peak calcium level so increased force of contraction but no decreasing in cardiac cycle.

Question 29

Advantages of heart exclusively using aerobic method (due to high energy requirements)

Answer 29

- Very efficient in ATP production (38 ATP/glucose unit) - Large blood supply (to provide sufficient oxygen - Loaded with mitochondria - Small diameter cells - facilitates adequate oxygen diffusion - Not susceptible to fatigue - ATP production keeps pace with consumption.

Question 30

Disadvantages of heart exclusively using aerobic method (high energy requirements)

Answer 30

- Entirely dependent on aerobic metabolism (unlike skeletal muscle) - Critically dependent on blood supply - Risk of heart attacks (damage caused by block of blood supply) - Mitochondria density limits force production.