Neurotransmitters and Related Messengers

More than 100 confirmed or suspected neurotransmitters have been identified since Loewi discovered acetylcholine. Neurotransmitters fall into three major categories according to chemical composition (fig. 12.18). Some of the best-known ones are listed in table 12.3. Parts of the brain referred to in this table will become familiar to you as you study chapter 14, and you may wish to refer back to this table then to enhance your understanding of brain function.

  1. Acetylcholine is in a class by itself. It is formed from acetic acid (acetate) and choline.
  2. Monoamines (biogenic amines) are synthesized from amino acids by removal of the —COOH group. They retain the — NH2 (amino group), hence their name. The major monoamines are epinephrine, norepinephrine, dopamine, histamine, and serotonin (5-hydroxytryptamine, or 5-HT). The first three of these are in a subclass of monoamines called catecholamines (CAT-eh-COAL-uh-meens).
  3. Amino acid neurotransmitters include glycine, glutamate, aspartate, and 7-aminobutyric acid (GABA).
  4. Neuropeptides are chains of 2 to 40 amino acids. Some examples are ^-endorphin and substance P. Neuropeptides typically act at lower concentrations and have longer lasting effects than other neurotransmitters, and they are stored in secretory granules (dense-core vesicles) that are about 100 nm in diameter, twice as large as typical synaptic vesicles. Some neuropeptides also function as hormones or as neuromodulators, whose action is discussed later in this chapter. Some neuropeptides are produced not only by neurons but also by the digestive tract; thus they are known as gut-brain peptides. Some of these cause cravings for specific nutrients such as fat or sugar and may be associated with certain eating disorders.

Saladin: Anatomy & 12. Nervous Tissue Text © The McGraw-Hill

Physiology: The Unity of Companies, 2003 Form and Function, Third Edition


Amino acids




2 Aspartic acid

NH2 Glutamic acid

Biogenic amines






Phe Gly


Phe Gly


Figure 12.18 Classification of Some Neurotransmitters.

Figure 12.18 Classification of Some Neurotransmitters.

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