 Dear students, in this topic we shall discuss the receptors involved in fast and direct neurotransmission. The membranes of post-synaptic cells have receptor proteins to which the neurotransmitters find and elicit response. Such response is produced directly. The properties of these receptors and their specificity determines the type of response. The receptors are ligand-gated ion channels. When the ligand that is the neurotransmitter binds to these receptors, the synaptic channels open. The opening of these channels allows a minute current to pass through them. When many channels open, the currents are added and the sum of these currents generates the post-synaptic potential. We shall take an example of acetylcholine receptors to explain the functioning of such receptors which are involved in fast and direct neurotransmission. The acetylcholine receptors are channel proteins. They are found at the neuromuscular junctions of vertebrates. There are two types of acetylcholine receptors in the body. First are called the nicotinic acetylcholine receptors which are also activated by nicotine in addition to the actual substrate which is acetylcholine. Second type are the muscarinic acetylcholine receptors which in addition to acetylcholine are also activated by a drug which is called muscarin. This muscarin is produced by torch tools and acts as a poison. The nicotinic acetylcholine receptors were the first receptors which were discovered and purified chemically and then studied electrically. They are bound by nicotine in addition to acetylcholine but these type of acetylcholine receptors do not bind muscarin. Nicotinic receptors are found on two types of cells or two types of synapses. First in the synapses in the autonomic ganglia of sympathetic and parasympathetic nervous system. Secondly they are present at the non-autonomic nerve endings of neuromuscular junctions. Now we shall discuss the structure of nicotinic acetylcholine receptors. Each nicotinic acetylcholine receptor consists of five subunits which associate and form a channel at the center. There are two identical alpha subunits and the other three subunits include the beta, gamma and sigma subunits. The receptor sites are on each of the two alpha subunits when ligand molecules that is acetylcholine, jacarbacol or nicotine. So these channels become activated and they open. As a result they allow the sodium and potassium ions to flow. Dear students, Second type of acetylcholine receptors, i.e. muscarinic acetylcholine receptors are activated by muscarin. They are quite different from nicotinic receptors because they are not ion channel proteins. They belong to a family of G protein coupled receptors. So when acetylcholine acts through these receptors, it produces slow, indirect neurotransmission. Dear students, Other ligand-gated ion channels that are found in cells or neurons and are involved in fast, direct neurotransmission, include those receptors like glycine and gamma amino butyric acid, i.e. GABA receptors. Every type of receptor is expressed on specific cells in the brain of mammals. Dear students, These channels which are involved in the fast, direct neurotransmission and are ligand-gated show a particular pattern in evolution. The channels for acetylcholine, glycine and GABA have certain similarities that show their homology. i.e. all have pentameric structures. All are composed of two to four types of subunits out of which one subunit binds the neurotransmitter. Dear students, Other than ligand-gated ion channels, the receptors of the second type are glutamate receptors. Glutamate receptors are related to a separate family and they have very few similarities with acetylcholine receptors. The members of this family are called glutamate receptors. At this time, scientists are very interested in glutamate receptors because glutamate is the major neurotransmitter produced by the cells of the brain which are involved in very important functions of keeping memory and learning. Therefore, there is a lot more research going on on these neurotransmitters.