19.6b Electrical Events at the SA Node: Initiation of the Action Potential.

SA nodal cells exhibit ___. Which allows SA nodal cells to serve as the pacemaker of the heart.

autorhythmicity

audtorhythmicity/automaticity(in SA Cells means?)

That they are capable of depolarizing and initiating an action potential spontaneously without any external influence.

SA nodal cells exhibit autorhymicity because…

they do not have stable resting membrane potential (RMP)

The membrane potential gradually increases in SA nodal cells by…

the reopening of voltage-gated cation channels, an event triggered by depolarization.

Pacemaker potential

The ability to reach the threshold without external stimulation.

The 3 primary events that occur within SA nodal cells as they function as the pacemaker for the heart.

• Reaching threshold. Voltage-gated cation channels open (due to repolarization from the previous cycle). The Nat flows into the SA nodal cells (along with the opening of T-type Calt channels and the influx of Calt; not shown in L figure 19.16). The influx of positively charged ions changes the membrane potential from -60 mV to - 40 mV, which is the threshold value. The voltage-gated cation channels close.

• Depolarization. Changing of the membrane potential to the threshold triggers the opening of voltage-gated Calt channels (L-type), and Caft entry into the SA nodal cell causes a change in the membrane potential from -40 mV to a slightly positive membrane potential (just above 0 mV).
  This reversal of polarity is called depolarization.

• Repolarization. Calcium channels close and voltage-gated Kt channels open; Kt flows out of the cell to change the membrane potential from a positive value to -60 mV, which is the RMP. The process of reestablishing the RMP is called repolarization. Repolarization triggers the reopening of voltage-gated cation channels, and the process begins again.

Depolarization

The reversal of polarity

Repolarization

The process of reestablishing the RMP.

Depolarization in the nodal cells is due to what?

inflow of Ca2+

depolarization in the nodal cells is due to what?

the outflow of K+

The process of initiating the action potential in the SA Node takes how long?

0.8 second and continuously repeats

Formula to determine how many times the initation of the action potential in the SA node happens:

one cycle/0.8 seconds X 60 seconds/1 minute=60/0.8=75 75 cycles in a minute.

The inherent rhythm at which SA nodal cells spontaneously depolarize is?

100 times per minute.

Average resting heart rate

75 beats per minute

What determines the average resting heart rate?

The continuous parasympathetic stimulation of the SA node by the vagus nerve.

Vagal tone

The slowing of the heart rate that keeps the average resting heart rate of 75 beats per minute.