SA node is a group of modified cardiac cells that have an intrinsic beat...the major pathways throughout the heart that distribute the actin potential for the contractions of the heart is via modified cardiac cells - Purkinje fibers - all of this is under the modification of the ANS, the autonomic nervous system, and hormones such as adrenalin.
previous answer to one of your questions is added below -
http://www.medhelp.org/posts/Heart-Disease/potassium-and-heart/show/1489642
Here is a video that explains it - http://search.yahoo.com/search;_ylt=Aiw9j_BR1GNT0kxRzgNLa1JG2vAI?p=cardiac+action+potential+animation&fr=my-myy-s&toggle=1&cop=&ei=UTF-8
Na/K pass in and ut of cells to generate a change in the membrane potential...