The force of contraction (of the heart) can be changed by:

a. beta1 adrenergic rececptors. Increasing cAMP leads to increases in PKA, which increases sarcoplasmic calcium by opening VG Ca+ channels for a longer time, opening SR Ca2+, and increasing the rate of Ca2+ removed by SERCA pumps

b. beta2 adrenergic receptors. Increasing cAMP leads to increases in PKA, which increases sarcoplasmic calcium by opening VG Ca+ channels for a longer time, opening SR Ca2+, and increasing the rate of Ca2+ removed by SERCA pumps

c. muscarinic ACh receptors. Activation of the receptors leads to decreased Ca2+ entry and increased K+ entry in ventricles

d. not by the parasympathetic NS (at least, not the ventricles)

Sinoatrial node (SA) or pacemaker or sino-atrial node: special region of the heart, which controls the heart rate. It is located near the junction between the right atrium and the superior vena cava and consists of a cluster of specialized muscle cells. The rhythmic frequency of these muscle fibers is approximately 72 contractions per minute, while the atrial muscle contracts about 60 times per minute and the ventricular muscle, about 20 times per minute. Due to the fact that the sinoatrial node has a faster rhythmic frequency in relation to the other parts of the heart, the impulses originating from it spread to the atria and ventricles, stimulating these areas so quickly, so that the rhythm of the SA node becomes the rhythm of the whole heart; that's why it's called a pacemaker.