Cell signaling of adjacent cells. In cardiac muscle, gap junctions between adjacent cells allows for action potential propagation from the cardiac pacemaker region of the heart to spread and coordinately cause contraction of the heart. The notch signaling mechanism is an example of juxtacrine signaling (also known as contact-dependent signaling) in which two adjacent cells must make physical contact in order to communicate. This requirement for direct contact allows for very precise control of cell differentiation during embryonic development. In the worm "Caenorhabditis elegans", two cells of the developing gonad each have an equal chance of terminally differentiating or becoming a uterine
Cardiac excitation-contraction coupling In the SAN cAMP binds to an ion channel involved in action potential initiation, speeding up the production of the action potential (see sinoatrial node for more detail). cAMP also, activates a protein called protein kinase A (PKA). PKA affects both the DHPR and RyR, increasing the rise in Ca within the contractile cells and therefore increasing rate of muscle contraction. PKA also affects the myofilaments as well as a protein called phospholamban (PLB; see sarcoplasmic reticulum for more details), speeding up the rate of Cadecline in the cell and so speeding up muscle relaxation. Parasympathetic nerves work by releasing