Lecture 18

Question 1

explain the cardiac muscle cells (myocytes and pacemaker)

Answer 1

• 99% of cells are force producing
  -> myocardial cells (myocytes)
  -> striated muscle fibers
• generate APs when depolarized
• source of contraction force of the heart muscle
• atrium cell
  -1% are autorhythmic (pacemaker) cells
  -> generate spontaneous rhythmic APs
• signal for myocyte contraction
• do not contribute to the contractile force
  -> very few contractile fibers
  -> no organized sarcomeres
• spindle cell
• both types of cells can generate action potentials

Question 2

explain cardiac myocytes in more detail

Answer 2

• irregularly shaped striated muscle fibers with sarcomeres
  -> contain actin and myosin
  -> cross bridge cycling mirrors that in skeletal muscle
• Connected in series by intercalated disks which contain
  -> desmosomes = physical coupling allow force to be transferred to neighbors
  -> gap junctions = electrical coupling (when one cells depolarizes it sends it to the neighboring cell) - connexins

Question 3

what are the differences in cardiac fibers compared to skeletal muscle fibers

Answer 3

• smaller and have 1-2 nuclei per fiber
• irregular branching cells connected by intercalated disks
• T tubules are larger
• sarcoplasmic reticulum is smaller
• mitochondria occupy 1/3 of cell volume (more)

Question 4

what are the steps of excitation contraction coupling in cardiac myocytes

Answer 4

1. action potential enters from adjacent cell (upstream group or pacemaker cells)
2. voltage gated L type Ca2+ channels open, Ca2+ enters the cell
3. Ca2+ induces Ca2+ release through ryanodine receptor channels (RyR) by binding to SR RyR and triggers channel opening (CICR)
4. local release causes Ca2+ spark
5. summed Ca2+ sparks from RyR create Ca2+ signal)
   -> Ca2+ signal is 90% from SR and 10% from ECF
6. Ca2+ ions bins to troponin to initiate contraction
7. relaxation occurs when Ca2+ unbinds from troponin
8. Ca2+ is pumped back into the SR for storage (SERCA)
9. Ca2+ is exchanged with Na+ by the NCX antiporter
10. Na+ gradient is maintained by the Na+K+ ATPase

Question 5

compare skeletal and cardiac muscle
(appearance under light microscope)
(fiber arrangement)
(location)
(tissue morphology)
(internal structure)
(fiber proteins)
(control)
(contraction force of single fiber twitch)

Answer 5

appearance under light microscope
- both striated

fiber arrangement
- both sarcomeres

location
skeletal = attached to bones, a few sphincters close off hollow organs
cardiac = heart muscle

tissue morphology
skeletal = multinucleate, large cylindrical fibers
cardiac = uninucleate, shorter branching fibers

internal strucure
- both t tubule and SR

fiber proteins
- both actin, myosin, troponin and tropomyosin

control
skeletal = Ca2+ and troponin, fibers independent of one another
cardiac = Ca2+ and troponin, fibers electrically linked via gap junctions

contraction force of single fiber twitch
skeletal = not graded
cardiac = graded

contraction speed
skeletal = fastest
cardiac = intermediate

Question 6

what is the role of the cardiovascular system (transport)

Answer 6

• the arteries take blood away from the heart
• the veins bring blood back to the heart
• pulmonary system = to the lungs
• systemic system = to the tissues

Question 7

what is the location of the heart

Answer 7

• ventral side of the thoracic cavity sandwiched between the lungs
• base of heart at the top
• apex of heart at the bottom

Question 8

what cells is the heart composed of and why is the left wall thicker

Answer 8

• bulk of the heart is composed of myocardium (cardiac muscle cells)
• myocardial cells of the atria and the ventricles
• the spiral arrangement of ventricular muscle allows ventricular contraction to squeeze the blood upward from the apex of the heart
• wall of left ventricle is thicker because it pumps blood to the whole body = higher pressure but same volume of blood

Question 9

what are the main structures of the heart

Answer 9

4 major blood vessels
1. vena cava (superior and inferior)
2. pulmonary arteries
3. pulmonary veins
4. aorta

4 chambers
- right atria and ventricle
- left atria and ventricle

4 valves
- Atrioventricular valves between atria and ventricles
- semilunar valves between ventricles and arteries
1. tricuspid valves (right atria and ventricle)
2. pulmonary valve (right ventricle and pulmonary artery)
3. bicuspid/mitral valve (left atrium to left ventricle)
4. aortic valve (left ventricle to aorta)

Question 10

explain how valves have unidirectional (one way) flow (during ventricular contraction)

Answer 10

• AV valves close to prevent blood flow backward into the atria
• semilunar vales open
• blood flows from veins-> aorta-> ventricles-> arteries

Question 11

explain how valves have unidirectional (one way) flow (during ventricular releaxation)

Answer 11

• AV valves open
• semilunar valves closed, prevent blood that has entered the arteries from flowing back into the ventricles during ventricular relaxation
• AV valve flaps have chordae tendineae (attached to papillary muscle) to provide stability and prevent backflow
• cup shaped semilunar valve leaflets don’t require tendons to prevent backflow