CBSE Class 10 Answered

 In the upper part of the right atrium of the heart is a specialized bundle of neurons known as the sinoatrial node (SA node). It is a group of cells positioned on the wall of the right atrium, near the entrance of the superior vena cava. These cells are modified cardiac myocytes. Though they possess some contractile filaments, they do not contract.

The SA node fibres vaguely resemble cardiac myocytes; however, they are measurably thinner, more tortuous and stain less intensely than cardiac myocytes. The SA node is richly innervated by parasympathetic nervous system fibers and by sympathetic nervous system fibers. This unique anatomical arrangement confers the SA node susceptible to distinctly paired and opposed autonomic influences. In the majority of individuals, the SA node receives blood from the right coronary artery.

 The position of SA node can be seen in the following site.

http://www.mda.org/publications/images/q12-3_final_elect.jpg