sympathetic nervous system increase Beta 1 adrnergic receptors on contractile cell 

calcium can combine to troponin tropomyosin which mean that we can get stronger contraction or contractile cell increase contraction 

less calcium flow in  ( concentration gradient distrupt ) so that not true 

hyperkalemia tends to block depolarization ( pacemaker cell ) K want to leave the cell 

not contract contractile cell not depolarize