Topic 11: Cardiovascular System - APs of SA and AV nodes + Myocardium

Question 1

What type of cells are is the SA and AV nodes? (2)

Answer 1

• non-contractile AUTORHYTHMIC cardiac muscle cells
• self excitable

Question 2

What is the threshold of these APs?

Answer 2

-40mV

Question 3

What are the 4 phases of Pacemaker activity?

Answer 3

1. Pacemaker potential
2. AP depolarization
3. AP repolarization
4. Na+ channels open at -50mv

Question 4

What occurs in pacemaker potential? (3)

Answer 4

• low K+ permeability (K+ voltage gates closed_
• slow inward leak of Na+ (voltage gates open)
• causes slow depolarization toward threshold (-40mV)

Question 5

What occurs in AP depolarization in SA and AV nodes? (5)

Answer 5

• at threshold -> AP
• calcium voltage gates open
• calcium moves in to get depolarization
• Na+ voltage gates close at threshold (NOT involved in AP)
• Calcium voltage gates close at peak

Question 6

What occurs in AP repolarization in SA and AP nodes? (3)

Answer 6

• K+ voltage gates open at peak
• K+ out to repolarize
• K+ gates close below threshold

Question 7

What occurs when Na+ channels open at -50mV in SA and AV nodes? (2)

Answer 7

• starts pacemaker potential (phase 1) again
• hence continuous cycle

Question 8

Does the APs of SA and AV nodes have a resting membrane potential?

Answer 8

no

Question 9

For APS in VENTRICULAR MYOCARDIUM, what type of cells are involved?

Answer 9

• contractile

Question 10

What is the pathway of APS in ventricular myocardium?

Answer 10

1. Purkinje fibers AP
2. Ventricular (contractile) myocardial AP (spread to cell to cell by gap junctions)

Question 11

What is RMP of ventricular myocardium?

Answer 11

-90mV

Question 12

What are the 3 phases of APs in Ventricular myocardium?

Answer 12

1. Depolarization
2. Plateau
3. repolarization

Question 13

What occurs in depolarization of Ventricular myocardium? (2)

Answer 13

• Na+ voltage gates open FAST, same gates as neuron, skel. muscle
• MP to +30mV

Question 14

What occurs in the plateau phase of AP for ventricular myocardium? (3)

Answer 14

• Na+ channels close and inactivate (slight drop in MP)
• calcium SLOW voltage gates are open
• calcium influx maintains depolarization

Question 15

What occurs in repolarization of APs in ventricular myocardium? (4)

Answer 15

• calcium channels close
• K+ voltage gated channels open
• increased K+ outflux
• MP decreased to resting

Question 16

What occurs in the absolute refractory period of ventricular myocardial APs? (2)

Answer 16

• LONG
• sodium channels inactivated until MP is close to -70mV

Question 17

What occurs in excitation-contractile coupling in myocardial cells? (6 steps, 8 points)

Answer 17

1) AP on sarcolemma of contractile cells triggers…
2) voltage-gated calcium channels open (plateau)
- small increase in cytosolic calcium from ECF
- not enough to trigger contraction BUT …

3) opens CHEMICALLY gated calcium channels on SR
4) increases cytosolic calcium
5) binds to troponin, etc., and leads to contraction
6) Contraction

Question 18

What occurs in contraction of myocardial cells? ie. what mechanism and when does it begin? (2)

Answer 18

• sliding filament mechanism
• begins a few msec after AP begins

Question 19

How long does AP in contraction of myocardial cells last? (3)

Answer 19

• around 250 msec (fairly long)
• duration of twitch is around 300 msec
• so contraction almost over when AP ends

Question 20

What is the result in contraction of myocardial cells? (3)

Answer 20

• NO summation
• therefore no tetanus
• get alternation of contraction/relaxation