Neurotransmitters are chemical messengers or substances within the brain that transmit signals from one neuron to another target neuron such as muscle or gland cell. The chemical nature of neurotransmitters varies from amino acids like glutamate, glycine, gases like nitric oxide, amines like dopamine, serotonin, hormones like glucagon, insulin, thyroid hormones etc. Whenever an action potential reaches the pre synaptic membrane, depolarization occurs allowing opening of calcium channels and causes release of neurotransmitters from the pre synaptic vesicles to the target neurons. Depending upon the receptor, these neurotransmitters can be of two types Excitatory or Inhibitory neurotransmitter. Excitatory neurotransmitter triggers depolarization, of post synaptic membrane resulting in generation of nerve impulse such as muscle contraction. E.g.- acetylcholine. Inhibitory neurotransmitters reduce or block the activity of post synaptic cell by causing hyperpolarization. E.g.- glycine, GABA.