Question

Since β-Adrenergic stimulation increases cellular uptake of potassium, this will reduce the plasma concentration of potassium. Thus, a more negative membrane voltage will arise as the potassium ions flow out of the cardiac muscle cells more easily due to the more steep ionic gradient set.

This, consequently, makes it more difficult for an action potential to fire, and as a result, a decrease in the heart rate. So now since the sympathetic nervous system increases the heart rate, then why does this beta adrenergic stimulation occur? It would decrease the heart rate, so what is happening?

a. 1 cm
b. 3 cm
c. 5.8 cm
d. 10 cm

Answer 1

Despite beta-adrenergic stimulation leading to increased potassium uptake by cells, the overall effect of sympathetic nervous system activity is to increase the heart rate. This occurs through the release of norepinephrine and epinephrine, increasing the force of cardiac muscle contractions and the speed of pacemaker cell depolarization.

Step-by-step explanation:

The seeming contradiction in how beta-adrenergic stimulation complicates the understanding of heart rate control under the influence of the sympathetic nervous system. While beta-adrenergic stimulation indeed causes an increase in the uptake of potassium by cells, leading to hyperpolarization and potentially making it more difficult for an action potential to fire, the overall effect of sympathetic stimulation is to increase the heart rate. This is because sympathetic stimulation releases norepinephrine (NE) and epinephrine, which bind to adrenergic receptors on cardiac muscle cells, increasing metabolic rate and the force of contractions, both of which contribute to increasing the stroke volume (SV) and heart rate (HR).

In addition, the sympathetic nervous system speeds up the depolarization of pacemaker cells, counteracting any hyperpolarizing effects due to potassium flow. The complexity of the autonomic regulation of heart rate involves balancing these multiple effects to ensure that the heart functions according to the body's needs at any given moment. Therefore, while it may seem counterintuitive, the overall sympathetic response, supported by multiple mechanisms, leads to an increased heart rate.