Cardiac pacemaker Cardiac pacemaker The contraction of cardiac muscle (heart muscle) in all animals is initiated by electrical impulses known as action potentials. The rate at which these impulses fire controls the rate of cardiac contraction, that is, the heart rate. The cells that create these rhythmic impulses, setting the pace for blood pumping, are called pacemaker cells, and they directly control the heart rate. They make up the cardiac pacemaker, that is, the natural pacemaker of the heart. In most humans, the concentration of pacemaker cells in the sinoatrial (SA) node is the natural pacemaker, and the resultant rhythm is a
Cardiac excitation-contraction coupling Ca Uniport system, which pumps Ca into the mitochondia. Heart rate is affected by nerves. Sympathetic nerves, coming from the spinal cord, increase heart rate, whereas parasympathetic nerves (for example the vagus nerves) work to decrease it. Sympathetic nerves work by releasing a protein (neurotransmitter) called noradrenaline which binds to a specific receptor (beta 1 adrenoceptor) located in the sarcolemma and the t-tubule membrane of cardiac cells. This activates a protein, called a G-protein and results in a series of reactions (known as a cyclic AMP pathway) that leads to the production of a molecule called cAMP (cyclic adenosine monophosphate).