 Dear students, in this module we shall discuss in detail the second type of synapses which are the chemical synapses. The chemical synapses are involved in chemical synaptic transmission through the use of neurotransmitters. They are most common form of synaptic transmission. There are two types of chemical synapses. That is the fast chemical synapses and slow chemical synapses. The fast chemical synapses are present in the central nervous system and at neuromuscular junctions. They produce immediate but short-lived responses. Dear students, although they are named as fast, but they are not faster than the electrical synapses. Electrical synapses speed cannot be compared to chemical synapses speed, but the terms used are compared to the chemical synapses. So in the chemical synapses, we are discussing the fast chemical synapses. The neurotransmitters which are involved in the fast chemical synapses are typically small molecules, for example acetylcholine. They are stored in small, clear synaptic vesicles in the axon terminals. Action potentials in the presynaptic neuron cause the release of neurotransmitter molecules at the axon terminals. This release occurs by exocytosis. The release occurs in the region which is called synaptic cleft through specialized sites on the membrane. Dear students, this synaptic cleft is a narrow, fluid-filled cavity which is found in two neurons and it separates pre- and post-synaptic neurons from one another. Dear students, now we shall study the mode of action of neurotransmitters involved in fast chemical transmission. The neurotransmitters bind to a specific protein receptor which are the ligand-gated ion channels present in the post-synaptic membranes. As a result of binding of the ligand-neurotransmitter molecule, the channels open and allow ionic currents to flow into the post-synaptic cell and changing its membrane potential. As a result, the signal is transmitted and action potential is initiated in the post-synaptic cell. Dear students, in contrast to the fast chemical synapses, the slow chemical synapses produce a comparatively slower response that takes about hundreds of milliseconds to start. But this response lasts much longer as compared to the response generated by fast chemical transmitters. This response may last for a few seconds up to hours. Dear students, the neurotransmitters involved in slow chemical transmission are generally large molecules. They are synthesized from amino acids. There are two major groups of neurotransmitters which are involved in slow chemical transmission. These are biogenic amines which are derived from a single amino acid, while the second group contains neuropeptides which consist of several amino acid residues. Dear students, the neurotransmitters involved in slow chemical synapses are packed in vesicles which are larger in size than those which were involved in fast chemical synapses. These neurotransmitters are synthesized in the cell body and are transported to the nerve terminals. They are released from the sites which are located far away from the sites of release of fast neurotransmitters. These sites are released of slow neurotransmitters like morphological specializations. Dear students, now we shall discuss the mechanism of action of neurotransmitters involved in slow chemical transmission. These slow response transmitters do not act through ligand-gated channels, but they act through G-protein-linked receptors. A neurotransmitter binds to its receptor, forming a neurotransmitter receptor complex that activates a G-protein. The G-protein in response activates a signal-transduction pathway that involves a second messenger. For example, C-A-M-P, Cyclic-A-M-P is the second messenger. In response, the second messenger elicits a cellular response that modifies the functions of channels and many intercellular processes. Dear students, there are some neurotransmitters which are involved in both types of transmission that is fast as well as slow, so that a single neurotransmitter can act on both the ligand-gated ion channels and G-protein-coupled receptors.