A&P Chapter 10
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What is the intrinsic rate of the SA node?
~ 70 bpm
What is the intrinsic rate of the AV node?
~ 40-60 bpm
What is the intrinsic rate of the purkinjie fibers?
~ 14-40 bpm
Where is the SA node located?
junction of SVC with the right atrium
superior posterolateral wall of RA (below and lateral to opening of SVC)
What is the resting membrane potential of the SA node?
-55 - - 60
What is the threshold potential of the SA node?
How long are the slow Na-Ca channels open during an SA node action potential?
What happens during phase 4 of the SA node action potential?
Na and Ca leak until threshold potential is reached
What happens during phase 0 of the SA node action potential?
Slow Na Ca channels open
What happens during phase 3 of the SA node action potential?
K channels open --> repolarization
Where do FAST sodium channels operate in the heart?
atrial and ventricular muscle and purkinjie fibers
What is responsible for the self excitation of the SA node?
leaky Na and Calcium channels
Why can't the fast sodium channels participate during SA node depolarization?
they are inactivated at -55 mV
Why is slow depolarization beneficial?
it causes a delayed conduction through the AV node which allows the atria time to empty before ventricular contraction begins
How long does it take an impulse to reach the AV node from the SA node?
Where is the AV node located?
posterior wall of the RA immediately behind the tricuspid valve
How does transmission of an AP in the purkinjie fibers compare to ventricular muscle? AV nodal fibers?
- much faster!
- 6x faster than ventricular muscle
- 150x faster than AV nodal fibers
How do the purkinjie fibers transmit the AP so quickly?
they have highly permeable gap junctions (few in number)
How does PSNS stimulation affect K and Ca channels?
- increased conductivity of K
- decreases conductivity of Ca
- = decreased rate and excitability
How does SNS stimulation affect Na and Ca ions?
increases their permeability --> increased HR
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