19.6b Electrical Events at the SA Node: Initiation of the Action Potential
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23 Terms
What do SA node cells exhibit?
SA node cells exhibit autorhythmicity (spontaneous firing)
What does autorhythmicity allow SA node cells to do?
They spontaneously depolarize and generate action potentials
What is step 1 in SA node action potential?
Reaching threshold
What happens to channels during threshold?
Slow voltage-gated Na+ channels open
What happens to Na+ during threshold?
Na+ flows in
How does membrane potential change during threshold?
Membrane potential changes from -60 mV to -40 mV (threshold value)
What is step 2 in SA node action potential?
Depolarization
What happens to channels during depolarization?
Fast voltage-gated Ca2+ channels open
What happens to Ca2+ during depolarization?
Ca2+ flows in
How does membrane potential change during depolarization?
Membrane potential changes from -40 mV to just above 0 mV
What is step 3 in SA node action potential?
Repolarization
What happens to calcium channels during repolarization?
Calcium channels close
What happens to potassium channels during repolarization?
Voltage-gated K+ channels open
What happens to K+ during repolarization?
K+ flows out
What happens to membrane potential during repolarization?
Membrane potential goes back to rest value (RMP = -60 mV)
What happens after membrane returns to -60 mV?
Voltage-gated Na+ channels open at -60 mV and process begins again
What is the resting membrane potential of SA node cells?
RMP is -60 mV
How often does the SA node generate an action potential at rest?
One SA node action potential starts about 0.8 sec after the last
What heart rate does this correspond to?
This translates to 75 heart beats per minute
What is the inherent firing rate of the SA node?
Inherently, SA node would fire faster at 100 per minute
Where is this intrinsic firing rate observed?
This is seen in heart tissue culture
Why is the actual heart rate lower in the body?
In the body, vagal tone keeps resting heart rate slower
What is vagal tone?
Parasympathetic activity relayed by the vagus nerve
