Final answer:
Sympathetic stimulation increases HR and contraction force by increasing intracellular calcium concentration, which does not decrease during systole. This leads to an increase in the activity of the SR calcium pump.
Step-by-step explanation:
Sympathetic stimulation of the heart is known to be a positive inotropic action, which increases the heart rate (HR) and the force of contraction. This effect is mediated by the release of norepinephrine (NE) at the neuromuscular junction from cardiac nerves and the secretion of epinephrine and NE from the adrenal cortex. These compounds bind to alpha and beta receptors on the cardiac muscle cell membrane to increase metabolic rate and the force of contraction. Such stimulation increases stroke volume (SV) and decreases the end-systolic volume (ESV).
On the molecular level, sympathetic stimulation results in an increased influx of calcium ions from the extracellular fluid, which is essential for the activation of the myosin-actin cross-bridging that facilitates muscle contraction. Hence, sympathetic stimulation leads to a higher concentration of intracellular calcium ions during systole, not a decrease. One key mechanism by which calcium ions contribute to muscle contraction is by binding to troponin, which then allows for myosin and actin interaction.
Therefore, the correct answer to the student's question is a. An increase in the activity of the SR calcium pump, which promotes muscle contraction by facilitating a higher intracellular concentration of calcium during systole.