10. Chapter 14- Cardiac Muscle

Question 1

What is cardiac muscle?

Answer 1

Contracts to create a pressure gradient to move blood throughout body
Made up of cardiac myocytes (shorter branched cells and usually have ONE nucleus)
Interconnected by intercalated disks:
Desmosomes link mechanically
Gap junctions link electrically
Slide 20 oct 29

Question 2

How do cardiac and skeletal muscle compare?

Answer 2

Both are striated and have sarcomeres containing similar array of thick and thin filaments

Less abundant but larger T-tubules in cardiac compares to muscles

Cardiac has smaller amounts of sarcoplamsic reticulum

Cardiac has an abundance of mitochondria for oxidative metabolism

Question 3

What are autorhythmic cells?

Answer 3

Pacemakers that generate action potentials spontaneously, the depolarization begins in sinoatrial node (main pacemaker)

1% of myocardial cells are these
Slide 22 oct 29

Question 4

What is the membrane potential characteristics of autorythmic myocardial cells?

Answer 4

Unstable resting membrane potential of -60mV (never truly rest tho)
HCN Channels opens from -60 to -40 and allow Na+ which leads to depolarization
Just before -40 HCN close and let T type Ca2+ in to further depolarize and bring to -40 threshold
Once reach threshold L type Ca is let in which causes a depolarize spike of action potential
At the end K+ channels open out of cell causing repolarization
Slide 5 Oct 31

Question 5

How are action potentials conducted in cardiac contractile (myocardial) cells?

Answer 5

Na+ channels open
Na+ channels close at end of depolarization
L type Ca channels opened fast K channels close
Ca channels close, slow K channels open
Resting membrane potential
Slide 6-8 Oct 31

Question 6

What do long action potential durations prevent in muscle cells?

Answer 6

Prevent Summation of contraction

Question 7

What is excitation-contraction coupling in cardiac cells?

Answer 7

L type Ca channels are not mechanically coupled to ryanodine receptors so Ca entry is not necessary for contraction
1. AP enters from adjacent cell
2. Ca channels open, Ca enters cell
3. Ca induces Ca release through RyR (ryanodine receptor channels)
4. local release causes Ca spark
5. Summed Ca sparks create Ca signal
6. Ca ions bind to troponin (initiates contraction)
Slide 10 Oct 31

Question 8

How does an increase in Ca2+ trigger contraction?

Answer 8

By removing inhibition of cross bridge cycling
Relaxed state has myosin head cocked, Tropomyosin partially blocks binding site on actin, myosin is weakly bound to actin
Initiation of contraction then starts by a calcium signal
Two pictures on slide 11 Oct 31

Question 9

How is a power stroke created?

Answer 9

1. ATP binds to myosin
2. Myosin hydrolyzes ATP- energy from ATP rotates myosin head to cocked position
3. Power stroke begins when Pi is released
4. Myosin releases ADP at the end of power stroke
   Slide 12 oct 31

Question 10

What is primary mechanism is cardiac muscle?

Answer 10

Reuptake into the Sarcoplamsic reticulum

Question 11

How is the SERCA pump regulated in cardiac muscle?

Answer 11

Regulated by phospholamban

Question 12

what are the 2 ways cardiac force in cardiac muscle is enhanced?

Answer 12

1. Increase in intracellular Ca2+ in cardiac myocytes enhances contractile force- more Ca equals additional troponin complexes activated and increased cross bridge formation leading to increase force
2. Length tension relationship- generates greater force when slightly stretched

Question 13

What neuronal input does the heart have that modifies conduction/contraction? (Parasympathetic and sympathetic)
What do they each do?

Answer 13

Heart is innervated by autonomic nervous system:
- sympathetic increases heart rate/conduction and contractility (autorhythmic and contractile)
- parasympathetic decreases heart rate/conduction (autorhythmic) this is dominant at resting heart rate of 70-72 BPM
Because of these heart rate is under tonic control

Question 14

What are the 4 steps of the sympathetic modulation of contraction?

Answer 14

1. Phosphorylation of Ca channels increase calcium conductane during AP
2. Phosphorylation of ryanodine receptors enhances sensitivity to Ca, increasing release of Ca, from SR
3. Increases rate of myosin ATPase
4. Phosphorylation of SERCA (PLN) increases speed of Ca reuptake which increases Ca storage
   Slide 17-18 oct 31

Question 15

How is cardiac length and tension relationship explained through overlap between thick and thin filaments? (2 points)

Answer 15

1. A slightly stretched sarcomere increases Ca sensitivity of myofilaments
   - stretches sarcomere has decreased diameter which reduces distance that Ca needs to diffuse
2. Slightly stretched sarcomere puts additional tension on stress activated Ca channels, increasing Ca entry from extracellular space and increasing Ca induced Ca release

Question 16

What do parasympathetic neurons containing ACh innervate?

Answer 16

They mainly innervate the SA and AV nose influencing autorhythmic myocardial cells which decreases frequency of action potentials (decreasing heart rate)

Question 17

What are Beta1 adrenergic receptors?

What does increased Na conductance through HCN channels and Ca through T type channels cause?

Answer 17

They can be activated by Ne released from sympathetic neurons or epinephrine from adrenal medulla

This causes cells to reach threshold more rapidly and a decreased level of repolarization