 Dear students, in this module, we shall discuss the fast direct neurotransmitters in detail. There are two major types of neurotransmitters that act as fast direct neurotransmitters. These are fast excitatory synaptic transmitters and fast inhibitory transmitters. First we shall discuss the fast excitatory synaptic transmitters. These neurotransmitters act by opening the ion channels in the post-synaptic cell membrane. These neurotransmitters include acetylcholine, glutamate and aspartate. We shall take acetylcholine in detail. Acetylcholine is the most familiar fast acting neurotransmitter. In most cases, it acts as an excitatory neurotransmitter. In some cases, it also plays an inhibitory role. For example, it inhibits or slows the heart by controlling the vagus nerve at the end of the peripheral sympathetic nerve in the heart. Dear students, those neurons that produce acetylcholine as neurotransmitters are called cholinergic neurons. In this way, the neurons are found in invertebrates and vertebrates. We shall discuss the mode of action of acetylcholine. When acetylcholine is released into the synaptic cleft, it binds to the ligand-gated acetylcholine-specific receptors in the membrane of post-synaptic cell. As a result of binding of neurotransmitters, acetylcholine, the sodium and potassium ion channels open briefly. As a result, an excitatory post-synaptic potential is produced in the post-synaptic cell. Dear students, When acetylcholine is released into the synaptic cleft, it is necessary to remove it from there. For this purpose, an important enzyme in the synaptic cleft is acetylcholine esterase. It is abundantly present. This enzyme causes the termination of transmission at cholinergic synapses. To end the transmission, acetylcholine is hydrolyzed in the acetylcholine and acetate. The acetylcholine produced by acetylcholine is actively reabsorbed by the pre-synaptic membrane, where it is recycled again because it condensates acetyl with the enzyme and produces the molecule of acetylcholine again. Dear students, Acetylcholine esterase enzyme activity is extremely important for the functioning of the nervous system. Some poisons, some insecticides and nerve gases block the acetylcholine esterase activity. The result is that acetylcholine is collected in the synaptic cleft. As a result, it causes continuous stimulation of the po-synaptic cell. It results in the disruption of neuromuscular system. That can even cause death. Dear students, Acetylcholine and acetylcholine bind with receptors of acetylcholine. Acetylcholine is known as agonists. Acetylcholine has three major agonists, carbacol, nicotine and mescarine. These three agonists of acetylcholine can activate the cholinergic synapses. Now we shall discuss the antagonists. The molecules which can block the action of a neurotransmitter are called antagonists. Acetylcholine is an antagonist of acetylcholine. This molecule is found in a poison which is called South American Blodart Poison Corare. This poison blocks the transmission of the cholinergic synapses. Dear students, After acetylcholine, the second important neurotransmitter which does fast, direct action is glutamate. Where acetylcholine is the most common neurotransmitter found on neuromuscular junctions, glutamate is the most common excitatory neurotransmitter in the brain of vertebrates. In insects and crustaceans, glutamate plays a fast excitatory role in their neuromuscular junctions. The third most common fast directly acting neurotransmitter is aspartate. It acts as excitatory transmitter in the ventral nerve cart. It is produced in the mitochondria from where it is transported to the cytoplasm of the neuron and then packaged into synaptic vesicles. The reason for this is that it makes an excitatory inhibitory pair with a glycine in the spinal cord due to which different action potentials or excitations are inhibited. These neurotransmitters always work in the form of pairs. Dear students, now we shall discuss the fast inhibitory transmitters. These are glycine and gamma amino butyric acid that is GABA. Glycine is secreted mainly as an inhibitory transmitter in the spinal cart. It always acts as inhibitory neurotransmitter. The second inhibitory fast acting neurotransmitter is gamma amino butyric acid. This plays inhibitory role in the vertebrates central nervous system. It forms an excitatory inhibitory pair with glutamate in the brain. It is also released at the inhibitory motor synapses, incrustations and anilides. It produces inhibitory post-synaptic potentials by increasing permeability of chloride ions.