Most neurotransmitter receptors induce changes in postsynaptic cells by opening or closing ion channels. How they do so is the basis for grouping receptors into two general categories:
► Ionotropic receptors are themselves ion channels. Neurotransmitter binding by an ionotropic receptor causes a direct change in ion movement across the plasma membrane of the postsynaptic cell. These proteins are also called ligand-gated channels.
► Metabotropic receptors are not ion channels, but they induce changes in the postsynaptic cell that can secondarily lead to changes in ion channels.
Postsynaptic cell responses mediated by metabotropic receptors are generally slower and longer-lasting than those induced by ionotropic receptors.
The acetylcholine receptor of the motor end plate is an example of an ionotropic receptor. It consists of five subunits, each of which extends through the plasma membrane (see Figure 44.14). When assembled, the subunits create a central pore that allows ions to pass through. There are several different kinds of subunits, and only one kind has the ability to bind acetylcholine. Each functional receptor has two of the acetylcholine-binding subunits and three other subunits.
Metabotropic receptors are also transmembrane proteins, but instead of acting as ion channels, they initiate an intra-cellular signaling process that can result in the opening or closing of an ion channel. These receptors have seven transmembrane domains, and they are linked to G proteins (Figure 44.16; see also Figure 15.7). When a neurotransmitter binds to the extracellular domain of a metabotropic receptor, the intracellular domain activates a G protein. In its inactive state, the G protein has three subunits, one of which (the a subunit) is bound to a molecule of GDP. When the receptor binds its neurotransmitter, the GDP is replaced with a GTP molecule, and the a subunit separates from the other two subunits (called P and y). The a subunit moves laterally in the membrane until it binds to and opens an ion channel or binds to an effector protein that activates a second messenger cascade, which in turn opens an ion channel.
Was this article helpful?
This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.