 Dear students, in this module we shall discuss the receptors which are involved in slow and indirect neurotransmission. The neurotransmitters that produce slow post-synaptic response bind to the receptors which are linked to G-proteins. These receptors act by activating the G-protein that uses GTP as an energy source. This G-protein is linked to the effector proteins and results in a cascade of enzyme reactions within the cell that generates the response. The G-protein are a large family of proteins that consist of about twenty different types of proteins which are composed of three subunits alpha, beta and gamma. The alpha subunit of inactive molecule binds to a GTP. When this GTP is converted into GTP, the protein becomes active. The receptor molecule is a large molecule that spans the membrane. The receptor binds at the extracellular phase of this receptor molecule. The cytoplasmic phase of this receptor activates the G-protein that is linked to it. The activated G-protein regulates the activity of effector protein which may be an enzyme. This enzyme controls the active concentration of second messenger. The second messengers are responsible for cellular response. We shall take an example of acetylcholine as acting as slow indirect transmitter. Acetylcholine acts as slow neurotransmitter on the atrial cells of heart that is the cells of atrium of the heart where muscarinic acetylcholine receptors are present. As a result it activates a G-protein. G-protein in response activates and results in the opening of potassium channels. When potassium channels open the hyperpolarization phase of action potential is prolonged. As a result action potential duration is increased and it results in decreasing the number of action potentials produced by the atrial cells of the heart. As a result cardiac activity is controlled or it is regulated. This is very important in regulating the cardiac cycle.