Neurotransmitters are endogenous chemicals that enable communication within the nervous system and between the nervous system and the rest of the body. Structurally, they are classified as monoamines, amino acids, and peptides. Functionally, neurotransmitters are classified as excitatory neurotransmitters, inhibitory neurotransmitters, and neuromodulators. Neurotransmitters transmit signals across a synapse at various locations. A synapse is a junction through which a neuron relays information to another neuron.