Figure 32.9: The Autonomic Nervous System (ANS) Helps Control Heart Rate by Affecting Pacemaker Cells The voltage across the cell membrane of a cell is called the cell’s membrane potential. Pacemaker cells have the unusual property that, after they complete a large cell-membrane depolarization (which causes a heart contraction), their membrane potential gradually creeps upward. This upward creep ultimately reaches a threshold at which it causes another large depolarization, leading to another heart contraction. The rate of upward creep determines heart rate, because it determines how much time passes from one depolarization to the next. The creep rate is determined by the properties of ion channels in the cell membrane. Investigators reasoned that signals from the ANS could affect the ion channels and thus affect the time required for threshold to be reached—a process that would in turn affect heart rate. They studied epinephrine, which is secreted by the sympathetic division of the ANS.a