There is great diversity in the types of neurotransmitters
and their corresponding synapses. Some are rapid in action, others
are slow. Nicotinic cholinergic synapses and those involving glutamate are stimulatory in nature
and promote an action potential in the postsynaptic cell. Others,
such as those using 
-aminobenzoic acid (GABA) as a transmitter are inhibitory.
Impulses received at these synapses discourage transmission of
an action potential in the recipient neuron. Inhibition can occur
by the opening of chloride channels. Whether a neuron fires depends
on the net summation of stimulatory and inhibitory inputs.
Acetylcholine can also bind to another receptor, called the muscarinic acetylcholine receptor. This receptor can be inhibitory, but not by the chloride channel mechanism. Figure 21.36 shows a schematic representation of transmission and inhibition in a neural network.
Small peptides, such as somatostatin, neurotensin, and the enkephalins also act as neurotransmitters (Table 21.2). They may also act as neurohormones.
Some specialized synapses do not use neurotransmitters at all. These are gap junctions, which perform direct electrical-ionic conduction between neural cells.
Neurohormones affect the nervous system by modifying the way in which nerve cells respond to transmitters. Enkephalins and endorphins are small peptides that act as natural analgesics and bind to the same receptors as morphine.
Dopamine metabolism may be involved in addictive behavior. All addictive drugs have been shown to cause a surge in dopamine levels in the "reward" section of the brain, the nucleus accumbens. Glutamate receptors may also be involved in controlling addictive behavior.