Neurotransmitters are chemical substances which are released from neurons at the event of an action potential, typically into microscopic gaps (synapses) between points at which neurons meet. These substances bind to specific receptors at the receiving (post-synaptic) neuron, and can affect the activity of that neuron. The effects of neurotransmitters are classified as excitatory, inhibitory and modulatory, referring to the manner in which they effect the voltage (and thereby firing rate) of the post-synaptic neuron.

A wide range of neurotransmitters exist in human and animal nervous systems, and the same neurotransmitter will often have varying effects, depending on which receptors are present at the membrane of the postsynaptic neurons.

Neurotransmitter receptors can be divided into two main types: ionotropic and metabotropic. Ionotropic receptors affect the activity of the postsynaptic neuron quickly and briefly (via so-called ion channels), while metabotropic receptors assert their effects indirectly and more slowly via cascade processes within the postsynaptic cell. Most neurotransmitters act upon both ionotropic and metabotropic receptors (1).

Although there are dozens of chemicals which have been identified as neurotransmitters, much of the synaptic communication in the brain is accomplished by glutamate, which has excitatory effects, and GABA, which has inhibitory effects (2).

Sources:

1. Brodal, P. (2010) The central nervous system (10th ed.). New York: Oxford University Press. 40-54
2. Vander et al. (2001) Human Physiology: The Mechanism of Body Function. (8th Ed.) McGraw−Hill 204-208