Cardiac muscle cell fatigue. There are two types of cells within the heart: the cardiomyocytes and the cardiac pacemaker cells. Cardiomyocytes make up the atria (the chambers in which blood enters the heart) and the ventricles (the chambers where blood is collected and pumped out of the heart). These cells must be able to shorten and lengthen their fibers and the fibers must be flexible enough to stretch. These functions are critical to the proper form during the beating of the heart. Cardiac pacemaker cells carry the impulses that are responsible for the beating of the heart. They are distributed throughout the heart
Cardiac excitation-contraction coupling a neurotransmitter called acetylcholine (ACh) which binds to specific receptor (M2 muscarinic receptor) on the sarcolemma of both SAN cells and ventricular cells. This again activates a G-protein. However this G-protein works by inhibiting, the cAMP pathway, therefore, preventing the sympathetic nervous system from increasing heart rate. As well as this, in the SAN, the G-protein activates specific potassium channel, that opposes action potential initiation (see SAN for more details), thus slowing heart rate. Cardiac excitation-contraction coupling Cardiac excitation-contraction coupling (Cardiac EC coupling) describes the series of events, from the production of an electrical impulse (action potential) to the contraction