Summary of Neurotransmitters

Neurotransmitters are molecules responsible for the carrying of messages (signals) between neurons. It involves various areas, including biology, psychology, and chemistry, making it an especially crucial topic. Today, we will discuss the major neurotransmitters used by the human body and also their significances.

Acetylcholine

Acetylcholine is a transmitter used by all motor axons, autonomic preganglionic neurons, postganglionic parasympathetic fibers, and some cells of the motor cortex and basal ganglia. It is the chief neurotransmitter of the parasympathetic nervous system, which contracts smooth muscles, dilates blood vessels, increases bodily secretions, and slows the heart rate (vasodilator).

*preganglionic neurons connect the CNS to the ganglia (groups of neuron cell bodies in the peripheral nervous system), and postganglionic neurons connect the ganglia to the effector organ. The terms autonomic, parasympathetic, motor cortex (the region of the cerebral cortex involved with voluntary movements), and basal ganglia (a group of subcortical nuclei in the brains of vertebrates) will be discussed in later articles.

In the central nervous system, acetylcholine appears to have multiple roles, such as thought, memory, and learning; it is in abnormally short supply in the brains of persons with Alzheimer's disease.

Biogenic Amines

Biogenic amines include epinephrine, norepinephrine, dopamine, histamine, and serotonin. The first three belong to the category catecholamine (neurotransmitters containing the catechol, benzene with two side-by-side hydroxyl groups). Histamine and serotonin have otherwise composition.

The catecholamines are all derived from the amino acid tyrosine that is catalyzed by the enzyme tyrosine hydroxylase into DOPA. Information about this detailed process will be explained in another article regarding adrenergic transmission [click to understand more].

Dopamine is produced by the action of the enzyme DOPA carboxylase on DOPA. It is present in several brain regions. Dopamine coming from the substantia nigra into the corpus striatum represents the major dopamine activity, and it plays an essential role in the coordination of body movements. Dopamine is also believed to be involved in motivation, reward, and reinforcement.

In Parkinson's disease, the dopaminergic neurons of the substantia nigra degenerate, leading to characteristic motor dysfunction.

Norepinephrine is synthesized from dopamine by the enzyme dopamine β-hydroxylase. It is the primary transmitter for postganglionic sympathetic neurons, which is released into the blood from the adrenal medulla. In the brain, norepinephrine is synthesized by the locus coeruleus, a nucleus (cluster of neurons in the CNS) in the pon, where it influences sleep and wakefulness, attention, and feeding behavior.

Epinephrine is produced from norepinephrine by the enzyme phenylethanolamine-N-methyltransferase (PNMT). It is released by chromaffin cells (neuroendocrine cells) of the adrenal medulla and the rostral ventromedial medulla (RVM), a group of neurons in the medulla oblongata. Epinephrine plays an important role in the fight-or-flight response, as well as regulating visceral functions (e.g. respiration)

Histamine is present in neurons of the hypothalamus. In the brain, histamine mediates arousal and attention, similar to that of ACh and norepinephrine. It is also released from mast cells in response to allergic reactions or tissue damage.

Serotonin is found in high concentrations in the raphe region of the pons and upper brainstem and has a wide projection to the forebrain. It involves a wide range of behaviors, including sleep and wakefulness, cognition, the feeling of happiness, etc. (unnecessary to really know its specific effect)

Reuptake by the presynaptic membrane is a major factor in terminating transmitter action of the biogenic amines.

Amino Acids

Amino acids include glycine,y-aminobutyric acid (GABA), glutamate, and aspartate.

Glycine is an inhibitory transmitter in spinal interneurons. GABA is an inhibitory transmitter of the central nervous system. Both of them generate IPSPs via ligand-gated Cl- channels. Glycine is produced by the mitochondria, whereas GABA is synthesized from glutamate.

Glutamate and aspartate are excitatory transmitters of the CNS that generate EPSPs, and are products of the Krebs cycle. Both glutamate and aspartate are produced in the mitochondria

While non-peptide transmitters are synthesized in nerve terminals, peptide transmitters are synthesized in the neuron cell body, packaged in vesicles, and then transported to nerve terminals [click to understand more]. Both glutamate and aspartate are produced in the mitochondria

Reuptake by presynaptic membranes is a major factor terminating the transmitter action of the amino acids.

Nitric Oxide (NO)

Unlike other transmitters, NO is neither packaged in vesicles nor released by exocytosis. As a gas, it readily diffuses across cell membranes to adjacent target tissue once being synthesized. NO is an inhibitory transmitter in the central and enteric nervous systems. In smooth muscles, NO is responsible for vasodilation and increasing blood flow (endothelial-derived relaxing factor).

List of neurotransmitters and their locations.

Reference:

Robert B. Dunn. 2002. USMLE Step 1: Physiology Notes.

Purves D, Augustine GJ, Fitzpatrick D, et al., editors. Neuroscience. 2^nd ed., 2001.

The Editors of Encyclopaedia Britannica. “Acetylcholine.” Encyclopædia Britannica, Encyclopædia Britannica, Inc., 26 Dec. 2019, www.britannica.com/science/acetylcholine.