6. Cardiac Pacemakers

Question 1

Describe the action potential of a cardiac autorhythmic (AR) cell (needs more added to it)

Answer 1

• No constant resting membrane permeability between action potentials

1. Cyclical Decrease in passive potassium efflux due to inactivation of K+ channels
2. hyperpolarisation causes opening of If channels, causing an Na+ influx
3. Increased influx of Ca2+ moves in through transient Ca2+ channels (T-type), taking membrane to threshold

Question 2

Why does the action potential pause when it reaches the AV node?

Answer 2

• (0.1sec) Allows for completion of atrial contraction

- This overall improves the efficiency

Question 3

Describe the action potential of a cardiac myocardial cell.

Answer 3

• Resting Membrane Potential of -90 mV
• An action potential (above threshold) arrives stimulating Voltage Gated Sodium Channels to open (activation gate opens) , (why isnt it calcium?) thus increasing permeability of the cell to sodium and depolarising it to a maximum value of +20 mV.
• Sodium voltage gated channels close (inactivation gate swings shut), therefore decreasing sodium’s permeability to the cell.
• Slow calcium channels open (allows divalent cations (calcium) to enter down their concentration gradient; 10^-3 outside of cell and 10^-7 inside the cell) and the cell decreases its permeability to potassium (retention of cations it currently posesses). This plateau phase is elongating the duration of the absolute refractory period of the cardiac contractile cell to prevent tetany.
• Permeability to potassium increases allowing for a rapid efflux of potassium from the cell (repolarising the cell), whereas the permeability to calcium is decreased; basically stopping more positive shit from coming into the cell.
• Potassium leakage channels close and cell resets at resting membrane potential of -90 mV

Question 4

What is the significance of the plateau period in the cardiac action potential?

Answer 4

• <1.0msec
• Prevent Tetany
• result of the extra calcium that flows into the T-Tubules

Question 5

What is the role of the T-tubule system in contraction & relaxation in cardiac muscle?

Answer 5

• Larger than those in skelatal muscles
• Branch inside of cell
• Cardiac muscle DEPENDS upon calcium moving from ECF in to cause contraction.
• calcium diffuses directly from T-Tubules into cell cytosol
• IT IS THE EXTRA CALCIUM THAT IS LARGELY RESPONSIBLE FOR ESTABLISHING THE UNIQUE PLATEAU PHASE

Question 6

What physiological events modulate strength of contraction in the heart?

Answer 6

• Graded potentials: strength of contraction is proportional to the number of crossbridges formed in the muscle.
• More calcium leads to an increase in crossbridges resulting in more force and speed
• in the heart, contraction is modulated by the ANS and circulating adrenaline
• Phospholamban is activated by a cAMP to enhace calcium-ATPase activity in the SR which increases the ammount of calcium sequestered into the SR which, on stimulation, is then availible for release to provide a greater force of contraction

Question 7

What is the effect of the parasympathetic nervous system on heart function?

Answer 7

• slows HR, achieved by…
  
  • increasing K+ efflux
  • decreasing Ca+2 influx
• Overall effect of this is decreasing the speed at which AR cells drift through threshold

Question 8

What is the effect of the sympathetic nervous system on heart function?

Answer 8

speads heart frate by increasing calcium and “If” channel flow (decreasing the drift and fire cycle distance)