In: Anatomy and Physiology
2 ) Explain why the M2 subtype of muscarinic ACh receptors is found on the autorhythmic cells of the sinoatrial node and the M3 subtype is found on smooth muscle fibers of the bronchioles. (6 pts)
Muscarinic receptors and nicotinic receptors are the 2 types of receptors for the neurotransmitter acetycholine.
There are 5 subtypes of muscarinic receptors. M1-M5 which are widely distributed throughout the human body. Each subtype is having different physiological function according to their location.
Main functions of M1 include
M4 is mainly located in brain and location of M5 receptor is not well known.
M2 receptor
M2 subtype muscarinic ACh receptors are mainly located in the heart.
They act via Gi type receptors whiich leads to reduction in cAMP in the cell and causes inhibition of voltage-gated Ca2+ channels, and increasing efflux of K+, which will lead to inhibitory-type effects.
They are found in the atria and conducting tissue of the heart and it exerts inhibitory effects on the heart.
Thus its functions include
It helps to slow down the heart rate below the normal sinus rhythm by slowing the speed of depolarization.
M3 receptor
M3 type of muscarinic ACh receptors are located in many places in the body. It is located in the smooth muscles of bronchiioles and blood vessels.
They are bound to G proteins of class Gq which upregulate phospholipase C and use inositol trisphosphate and intracellular calcium as a signaling pathway.
As it causes increase in intracellular calcium, it causes constriction of smooth muscles.
In the bronchioles, it causes bronchoconstriction. It also increases secretion in the airways.