 Dear students, in this module we shall discuss the diversity and classification of neurotransmitters. By the mid 1960s, there were only three neurotransmitters that had been discovered and identified. These were acetylcholine, norepinephrine, and gamma amino butyric acid commonly known as GABA. But now, more than 100 neurotransmitter molecules have been identified. These neurotransmitters vary in their size, in their molecular weights, and in their chemical structures. Based on their chemical structure, these neurotransmitters are classified into two groups. The first group includes small low molecular weight neurotransmitters, while the second group contains larger sized high molecular weight neurotransmitters which are derived from amino acids. First, we shall discuss the small low molecular weight neurotransmitters. They include acetylcholine, some amino acid neurotransmitters, for example GABA, glycine, glutamate, and aspartate. They also include biogenic amines. That include norepinephrine, epinephrine, dopamine, serotonin, and histamine. Some gases are also small low molecular weight neurotransmitters. These are carbon monoxide and nitric oxide. The large high molecular weight neurotransmitters are the neuropeptides. These neuropeptides are large sized molecules which are derived from amino acids. More than 40 neuropeptide transmitters are present in the central nervous system of mammals. These neuropeptides include many hypothalamic and pituitary peptide hormones. They include the substance P, that is a substance that is involved in the reduction of pain. Moreover, endorphins and encephalins are also neuropeptides. Many amino acid derivatives in addition to these substances are also neuropeptides. Dear students, another pattern of classification is based on the mode of action of neurotransmitters. According to mode of action the neurotransmitters are classified into two groups. Fast direct neurotransmitters and slow indirect neurotransmitters. The fast direct neurotransmitters act directly on the ion channels and change the conductance of post-synaptic membrane for various ions. These include acetylcholine and amino acids glutamate, aspartate, glycine and GABA. Dear students, the slow indirect neurotransmitters work through indirect biochemical pathways that involve G-proteins. These neurotransmitters act through a second messenger that changes the conductance of channel proteins. There are two groups of neurotransmitters in slow indirect neurotransmitters, these are not biogenic amines or neuropeptides. Many of the neurotransmitters involved in slow indirect neurotransmission also act as neuromodulators. Neuromodulators affect the neighboring neurons and modify their behavior at once.