Cardiac Function and Control (Cardiovascular II)

Question 1

What are the 2 types of cells in the myocardium?

Answer 1

• contractile cells
• autorhythmic cells

Question 2

What is required for a contractile cell to function?

Answer 2

• AP required for contraction
• excitable –> must be stimulated

Question 3

What are 2 characteristics of autorhythmic cells?

Answer 3

• modified non-contractile cells
• spontaneously generate AP (gives heart rhythm + stimulation)

Question 4

What is the purpose of gap junctions?

Answer 4

• adjacent cells connected by water-filled pores form opening connection
• allows ions + electrical activity to move freely form one cell to the next
• allows for increased speed

Question 5

What generates the electrical conduction system?

Answer 5

• pacemaker cells (autorhythmic cells)

Question 6

What are 4 characteristics of the SA Node? Where is it located?

Answer 6

• command center (determines heart contraction)
• autorhythmic (self-excitation)
• fastest paced + controlled by sympathetic nerve fibers
• imposes speed of heart contraction
• located near vena cava

Question 7

What are 3 characteristics of the AV node?

Answer 7

• autorhythmic ability
• slow pace (under control of SA node)
• allows propagation of AP by SA node from atria to ventricle

Question 8

What are the 3 roles of the electrical system?

Answer 8

• maintain appropriate heart rate
• coordinate contraction of atria + ventricles
• coordinate contraction of each chamber

Question 9

What is an arrhythmia?

Answer 9

• abnormal rhythym due to problems with conduction

Question 10

Explain the sequence of excitation.

Answer 10

1. SA node self excitation –> generates AP (depolarization)
2. AP propagates through atria (atrial contraction)
3. AV node activated by AP wave –> transmits electrical activity to bundle of His + Purkinje fibers to allow blood into ventricles + closure of AV valves (AP moves into apex)
4. electrical activity propagates throguh ventricles (ventricle contraction)

Question 11

Explain the generation of AP in the SA node under normal conditions.

Answer 11

• cells from SA node gradually depolarize
• drift in potential (unstable RMP) –> leakage of Na+ inside cell + reduced outflow of K+
• selective permeability changes + threshold met
• AP generated

Question 12

Explain the generation of AP in the SA node when controlled by the ANS?

Answer 12

• sympathetic fibers –> reduce time to reach threshold (faster pace = increased HR)
• parasympathetic fibers –> prolong time required to reach threshold (slower pace = decreased HR)
• decrease Na+ entering cell or increase outflow of K+

Question 13

What cells are responsible for muscle contraction? How does this occur?

Answer 13

• contractile cells
• AP from autorhythymic cells propagate to contractile cells

Question 14

True or False: There is no drift potential in contractile cells.

Answer 14

True: Contractile cells have stable RMP (no drift).

Question 15

True or False: Skeletal muscles have long AP + cardiac muscles have short AP.

Answer 15

False: Skeletal muscles have short AP + cardiac muscles have long AP.

Question 16

True or False: Skeletal muscles have shorter refractory period than cardiac muscles.

Answer 16

True

Question 17

What is the length of refractory period relative to?

Answer 17

• duration of the AP

Question 18

Why can a heart cramp not occur?

Answer 18

• cells are not excitable until they are relaxed

Question 19

What is phase 1 of the action potential? What position are the voltage gated channels in?

Answer 19

• resting membrane potential
• all channels closed

Question 20

What is phase 2 of the AP? What position are the voltage gated channels in?

Answer 20

• rapid depolarization –> rapid influx of Na+
• Na channels open
• K+ and Ca2+ channels closed

Question 21

What is phase 3 of the AP? What position are the voltage gated channels in?

Answer 21

• short phase repolarization –> loss of K+ from cell
• K+ channels open
• Na+ and Ca2+ channels closed

Question 22

What is phase 4 of the AP? What position are the voltage gated channels in?

Answer 22

plateau phase –> influx of Ca2+ into the cell
- cannot fully repolarize until Ca2+ channels close
- Ca2+ and K+ channels open
- Na+ channels closed

Question 23

What is phase 5 of the AP? What position are the voltage gated channels in?

Answer 23

• repolarization phase –> outward movement of K+
• K+ channels open
• Na+ and Ca2+ channels closed

Question 24

Explain the difference between skeletal muscle + heart muscle.

Answer 24

Skeletal Muscle:
- short AP + refractory period
- ability to be re-stimulated quickly + reach “tentanic conctraction”
- different fibers stimulate separately

Heart Muscle:
- longer AP + refractory period
- prevents rapid repeated contraction (no tetanic state)
- gap junction ensures all cells contract at once

Question 25

Explain the calcium-stimulated-calcium release.

Answer 25

1. depolarization
2. voltage gated Ca2+ channels open –> triggers release of lots of Ca2+ later on (responsible for Ca2+ plateau)
3. Ca2+ entry causes large amount of Ca2+ to be released from sarcoplasmic reticulum
4. large increase in intracellular Ca2+ –> triggers contraction
5. Ca2+ pumped back into pre-stimulation compartment (may require ATP)

Question 26

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