What makes the nervous system such a fantastic device and distinguishes the brain from other organs of the body is not that it has billion neurons, but that nerve cells are capable of communicating with each other in such a highly structured manner as to form neuronal networks. Inhibitory neurotransmitters cause hyperpolarization of the postsynaptic cell; excitatory neurotransmitters cause depolarization of the postsynaptic cell.
However, proteins are long molecules while some neuropeptides are quite short. The axon of a nerve cell is responsible for transmitting information over a relatively long distance, and so most neural pathways are made up of axons.
For example, depolarization of the presynaptic membrane will always induce a depolarization in the postsynaptic membrane, and vice versa for hyperpolarization. The peak is followed by an equally rapid repolarization phase.
The neurotransmitter is either reabsorbed by the presynaptic cell and repackaged for future release, or else it is broken down metabolically. They each have their own receptors and do not interact with each other. Figure Growing axons in the developing spinal cord of a 3-day chick embryo.
In establishing the nerve connections that enable us to see, it is not only the quantity of visual stimulation that is important, but also its temporal coordination.
Glycine and GABA are considered inhibitory amino acids, again because their receptors cause hyperpolarization, making the receiving cell less likely to reach an action potential.