-- on the order of 0.02 micron. Across the gap, the neurotransmitters bind to membrane receptors: large proteins anchored in the cell membrane of the post-synaptic neuron. At this location, under an electron microscope, you can observe an accumulation of opaque material which consists of the cluster of receptors and other signalling proteins that are essential for chemical neurotransmission. Any given neurotransmitter has several sub-types of receptors that are specific to it. It is the presence or absence of certain of these sub-types that causes a cascade of specific chemical reactions in the postsynaptic neuron. These reactions result in the excitation or inhibition of this neuron. A neurotransmitter's agonist is a molecule that has the same effect on the postsynaptic neuron as the neurotransmitter itself does. An antagonist is a molecule that blocks the effect that the neurotransmitter normally has on the post-synaptic neuron. It was long thought that a given neuron released only one kind of neurotransmitter. But today, many experiments show that a single neuron can produce several different neurotransmitters. Neurons that use GABA and glutamate as neurotransmitters are used by more than 80% of the neurons in the brain and constitute the most important inhibition and excitation systems, respectively, of the substantia nigra pars compacta (SNc). NEUROTRANSMITTERS This section describes a few of the best known neurotransmitters that are involved in many functions in both the central and the peripheral nervous systems. Apart from acetylcholine, they all belong to the family of amines or amino acids. Neurotransmitter