Sinoatrial node rate, and increasing heart rate. An increase in heart rate is known as positive chronotropy. The parasympathetic nerves supplying the SA node (in particular the Vagus nerves) originate in the brain. These nerves release a neurotransmitter called acetylcholine (ACh). ACh binds to a receptor called an M2 muscarinic receptor, located on the SA node membrane. Activation of this M2 receptor, then activates a protein called a G-protein (in particular G protein, i for inhibitory). Activation of this G-protein, blocks the cAMP pathway, reducing its effects, therefore inhibiting sympathetic activity and slowing action potential production. As well as this, the G-protein
Orbital node moves south through the plane. In the case of objects outside the Solar System, the ascending node is the node where the orbiting secondary passes away from the observer, and the descending node is the node where it moves towards the observer. The position of the node may be used as one of a set of parameters, called "orbital elements", which describe the orbit. This is done by specifying the longitude of the ascending node (or, sometimes, the longitude of the node.) The line of nodes is the intersection of the object's orbital plane with the plane of reference. It