Action Potential
The major difference between most myocytes and those that make up the pacemaker regions, is that cells in the pacemaker regions do not have a stable resting membrane potential. Instead they gradually depolarise until they spontaneously reach the threshold to fire an action potential. The textbooks argue about why this is, but you can be sure it is because some set of ion channels (e.g. K+ channels) is gradually closing, and/or another set (e.g. Na+ channels) is gradually opening. You should view this as one of your valuable opportunities to cope with uncertainty!
