week 3

Question 1

What are the 3 types of Cardiac muscle cells

Answer 1

• Contractile Cells (>99%)
• pacemaker cells
• Conduction Fibers

Question 2

Contractile Cells

Answer 2

• Contain contractile machinery like skeletal muscle including sarcomeres, troponin and tropomyosin
• Connected by gap junctions in 2 syncytia (collection of cells with cytoplasmic continuity - atrial and ventricular)

Question 3

Pacemaker cells

Answer 3

• Specialized cells that create heart rhythm
• very few myofibrils
• different fiber shape
• doesn’t have contractile fibers

Question 4

Conduction fibers

Answer 4

• Conduct action potentials around heart in coordinated manner
• connect atrial and ventricular syncytia
• very few myofibrils
• no contractile fibers

Question 5

Which ions have high concentrations in the extracellular fluid

Answer 5

Na+ and Ca++

Question 6

Which ions have high concentrations in the intracellular fluid

Answer 6

K+

Question 7

What is the resting membrane potential of cardiac muscle and how does that vary from other cells such as neurons

Answer 7

Resting membrane potential: -80 to -90
- more negative than neurons

Question 8

What factors maintain a negative resting membrane potential

Answer 8

• Greater K+ membrane permeability compared to Na+
• the Na+/K+ ATPase (3 Na+ out/ 2K+ in)

Question 9

What is the main difference between cardiac and neuron/skeletal muscle APs causing long-lasting depolarization in cardiac fibers

Answer 9

Increased Ca++ permeability and reduced K+ permeability

Question 10

Cardiac Muscle Action Potential

Answer 10

1. Threshold depolarization opens voltage-gates Na+ channels in cell membrane
   - influx of Na+
2. Voltage- gated Na+ channels close (inactivate) and K+ channels in cell membrane close
3. Voltage-gated Ca++ channels (L-type) in cell membrane open
   - Ca++ influx
4. Ca++ channels close and K+ channels re-open
   - Ca++ influx stops, K+ efflux
5. Na+/K+ ATPase and Na+/Ca++ exchangers in cell membrance and Ca++ ATPase in sarcoplasmic reticulum help re-establish ion concentrations, resting membrane potential maintained

Question 11

Excitation-Contraction Coupling in Contractile cells

Answer 11

1. AP spreads to interior of cell via T tubules
2. Ca++ flows into cytoplasm from extracellular fluid
3. Ca++ influx stimulates opening of Ryanodine receptor channels (RyR) in the SR membrane
4. More Ca++ flows into the cytoplasm form the SR
5. Ca++ then binds to troponin, which causes a conformational change of tropomyosin that exposes myosin binding sites on actin, and cross-bridge cycling starts

Question 12

Why is there a greater effect on cardiac muscle than skeletal muscle when reducing extracellular Ca++

Answer 12

• Cardiac muscle relies more on extracellular Ca++ (more T tubules)
• Skeletal relies more on SR Ca++ (more developed SR)

Question 13

Differences in permeability seen in pacemaker cells

Answer 13

• High resting membrane Na+ permeability leads to slow depolarization (Not voltage-gated)
• Voltage-gated Ca++ channels open more slowly, so the rate of depolarization is slower than in contractile cells

Question 14

Why are many voltage-gated Na+ channels in inactivated states in pacemaker cells

Answer 14

the resting potential is never strongly negative
- many inactivation gates don’t open and many activation gates don’t close

Question 15

Which cells in the heart have pacemaker properties

Answer 15

sinoatrial node, atrioventricular node, purkinje fibers

Question 16

What sets rhythm of heart and what is the rate? What are the back up pacemaker rates

Answer 16

Heart rate set by S-A Node: 70 to 80 beats per min
A-V node: 40 to 60 beats per min
Purkinje fibers: 15-40 beats per min

Question 17

Ectopic Pacemaker

Answer 17

when somewhere other than the S-A node becomes the dominant pacemaker
- Causes abnormal sequence of contraction and weakens heart pumping

Question 18

At what rates do conduction fibers conduct APs

Answer 18

• Internodal pathways: 1m/s
• Purkinje Fibers: 1.5-4.0 m/s

Question 19

At what rate is an AP propagated between contractile cells

Answer 19

0.3-0.5 m/s

Question 20

Why do conduction fibers have faster rates of propogation

Answer 20

Large diameter and lots of gap junctions between cells

Question 21

What is the purpose of conduction fibers

Answer 21

coordinate contraction so different regions of the heart contract at appropriate times

Question 22

Sequence of excitation in the heart

Answer 22

1. An action potential is initiated in the SA node
2. AP are conducted from the SA node to the atrial muscle
3. APs spread through the atria to the AV node where conduction slows
4. APs travel rapidly through the conduction system to the apex of the heart
5. APs spread upward through the ventricular muscle
6. The entire heart returns to the resting state, remining there until another AP is generated in the SA node

Question 23

Speed of excitation through the heart

Answer 23

• Rapid through atria via internodal pathways
• delayed in the A-V node and A-V bundle due to fewer gap junctions
• fast conduction through purkinje network for rapid contraction across ventricles

Question 24

Frank-Starling Mechanism

Answer 24

The more the heart is stretched during filling, the greater the force of contraction and thus the greater stroke volume

Question 25

Why does the Frank-Starling mechanism cause an increase in contractile force

Answer 25

- Stretching increases sarcomere length such that there is a greater overlap of actin and myosin filaments increasing contractile force - Too much stretch will go past optimal zone causing too few CB to form, decreasing force of contraction

Question 26

What determines the end-diastolic pressure-volume relationship

Answer 26

The volume of blood that fills the heart during diastole - increased volume causes a gradual increase in pressure at first - at high volumes there will be a dramatic increase in pressure as the heart has to start to stretch and the pericardium puts pressure on the ventricle

Question 27

What determines end-systolic pressure-volume relationship

Answer 27

- the force of contraction at each filling volume - At lower volumes there is a gradual increase as the greater stretch causes a greater pressure produced by the myocardium - At high volumes the pressure produced dips due to decreased CB formation

Question 28

How do pressure-volume loops relate to pressure-volume curves?

Answer 28

The changes during the cardiac cycle lie between the EDPVR and ESPVR

Question 29

How do pressure-volume loops change with and without pre-load compared to a control loop?

Answer 29

WITH PRELOAD - increased end diastolic volume - greater pressure created during isovolumetric contraction - higher end systolic presure and higher end systolic volume WITHOUT PRELOAD - lower EDV - less pressure during isovolumetric pressure -lower ES pressure and volume

Question 30

What is an example of intrinsic regulation of CO

Answer 30

Frank-Starling mechanism

Question 31

What is an example of extrinsic regulation of CO

Answer 31

Sympathetic and parasympathetic nervous systems

Question 32

The role of the sympathetic nervous system on regulation of CO

Answer 32

Neurons innervate S-A and A-V nodes, conduction fibers, and contractile cells and act to increase SV and HR (Contract Harder and faster) - Increase pressure produced (ESPVR) at a given volume

Question 33

The role of the parasympathetic nervous system on regulation of CO

Answer 33

Neurons innervate the S-A node and A-V nodes and primarily act to reduce HR -Also innervate contractile cells creating a modest effect on SV by reducing ventricle filling

Question 34

How does the SNS regulate SV (physiological level)

Answer 34

- Activates beta1 receptors in contractile cells, enhancing the components of Ca++ cycling - pathway activates through G protein and cAMP activating protein kinase which upregulates Ca++ channels

Question 35

How does the SNS and PNS regulate HR

Answer 35

Change rate of spontaneous depolarization SNS - increase # of Na+ leak channels open PNS - Increase the number of K+ leak channels offsetting the effect of the Na+ leak channels

Question 36

How do the contributions of SV and HR increase change throughout higher intensity exercise

Answer 36

- SV more significant at first - SV plateaus due to limitation of pericardium - HR increases rather steadily throughout