Brain
There are dozens of other chemical neurotransmitters that are used in more limited areas of the brain, often areas dedicated to a particular function. Serotonin, for example—the primary target of antidepressant drugs and many dietary aids—comes exclusively from a small brainstem area called the raphe nuclei. Norepinephrine, which is involved in arousal, comes exclusively from a nearby small area called the locus coeruleus. Other neurotransmitters such as acetylcholine and dopamine have multiple sources in the brain, but are not as ubiquitously distributed as glutamate and GABA.
Electroneuronography
In the human body there are twelve pairs of cranial nerves. Audiologists generally focus their expertise on the seventh and eighth nerves, which are known as the facial nerve and vestibulocochlear nerve, respectively. Electroneuronography is typically concerned with the amount of degradation in the facial nerves, each of which consists of thousands of fibers. Motor and sensory fibers are typically found in a 2:1 ratio, and it has been proposed that only half of the motor units need to be functional for normal nerve conduction to take place. The facial nerves originate in the brainstem, cross through the auditory canal, exit the skull at the stylomastoid foramen, and terminate face in 2 main branches on each side of the face. These control muscle contractions and facial expressions.