NEUROTRANSMITTERS [ back to Glossary Index ]
Neurotransmitters are proteins, peptides, amino acids and other substances which that chemical messages between brain cells, which can induce other cells to "fire" (excitation) or not fire (inhibition). The most common excitatory neurotransmitter is glutamate, which is involved in making memories. The most common inhibitory one is GABA. Benzodiazepines like Valium which tranquilize people activate GABA receptors. Nicotine products make people more alert by blocking the release of GABA.   To date researchers have identified about 30 neurotransmitters, but they anticipate that many more will be identified within the years to come. An excess of excitatory or inhibitory neurotransmitter  is bad, e.g. epilepsy results from over-excitation and uncontrolled nerve cell firing and obsessive-compulsive disorder is partially caused low levels of inhibitory GABA, which leaves the sufferer unable to control obsessive thoughts. Too little Dopamine causes Parkinson's Disease. Too much Dopamine is believed to contribute to schizophrenia. Dopamine is also involved in the craving associated with addiction to heroin, cocaine and other drugs.

Some neurotransmitters (e.g. acetylcholine for memory and muscle contraction) are manufactured in the axon terminal, while others (e.g. enkephalins for pain relief) are made in the cell body and transported down the axon. Once made, neurotransmitters are stored for deployment in the axon terminal in tiny packages called "pre-synaptic vessicles." When the nerve cell fires it sends a micro-electric current down the axon which signals and precipitates the release of a chemical shower of specified neurotransmitter across the gap between adjacent nerve cells known as the synaptic cleft. The neurotransmitter substance then docks or "binds" with receptor sites on the dendrites of adjacent neurons, and influences downstream neurons to fire or not fire by affecting cell membrane pore activity to permit or not permit ionic exchange with the immediate extra-cellular environment. If the membrane pores of the  downstream cell are made to allow a sufficient a influx of sodium or calcium ions (with corresponding discharge of potassium), it will fire. Once the neurotransmitter has been released and done its job, it must somehow be cleared from the synaptic gap or it will continue to cause or block nerve cell firing longer than its job required. This is done in two ways, by re-uptake of excess neurotransmitter back into the pre-synaptic vessicles of the axon terminal, or by "degradation" of the excess neurotransmitter by enzymes which break them down into smaller, inert components which have no power to influence further neurotransmission. Prozac relieves depression (a mood disorder associated with undersupply of the neurotransmitter serotonin) by blocking the re-uptake of serotonin and boosting the quantity of serotonin in the inter-cellular space). Drugs which block the enzyme Acetycholinesterase from breaking down and clearing Ach, increase the supply of acetylcholine in the brains of Alzheimer's patients and improve their memories.

The process of neurotransmission can be influenced by ingestion of drugs (such as caffeine, alcohol, cocaine, aspirin, and prescription medicines). Ingested drugs which block neurotransmission by deactivating receptor sites with an inert substance are called antagonists. Ingested drugs which promote neurotransmission by docking with receptors and activating them by imitating the body's own neurotransmitters, are called agonists. The body also manufactures substances called neuromodulators, which make neurons more susceptible or less susceptible to fire when showered with neurotransmitters. Neuromodulators attach to the cell membrane away from ion channels, insert a tail into the cytoplasm and very briefly activate G proteins which in turn can activate an intra-cellular enzyme called adenylate cyclase to synthesize many copies of cyclic AMP, a small molecule which acts as a secondary chemical messenger inside the cell body. AMP can travel to the cellular nucleus and increase or decrease expression of genes which influence mood. It can also activate a class of intra-cellular enzymes called protein kinases which add particles of phosphate to proteins inside cell membrane receptor sites, making them me or less responsive to incoming neurotransmitters.

The first neurotransmitter ever isolated was acetylcholine, which was done by Nobel Prize winner Otto Loewi who demonstrated how Ach causes contraction of the frog heart muscle . Since then neuroscientists have identified dozens of neurotransmitters and neuromodulators (which increase or decrease the strength of the chemical message). They consist of proteins, peptides (parts of protein chains) and amino acids. The most prevalent excitatory neurotransmitter is glutamate, and the most prevalent inhibitory one is GABA. Each plays a part in numerous different brain processes. For example, acetylcholine plays important roles in short term memory formation and contraction of voluntary skeletal muscles; serotonin plays important roles in blood vessel constriction and mood regulation; and dopamine plays important roles in maintaining smooth, coordinated muscle movement, in attentional processes and motivation. Imbalances of neurotransmitters from genetic defects, trauma or other causes can wreck havoc with normal neurologic functioning. A shortage of serotonin is associated with depression. A shortage of dopamine is associated with Parkinson's. A shortage of acetylcholine is associated with loss of short term memory in Alzheimer’s Disease. An excess of dopamine is associated with schizophrenia. Recent research indicates that brain trauma from a blow to the head can cause chronic imbalances of neurotransmitters and that this is responsible in part for the chronic headache, insomnia, fatigue, apathy, irritability and attentional problems involved in Post-Concussive Disorder. Medications have been developed, which can be prescribed by neurologists, to redress these imbalances to some extent. For more information on toxicity and imbalance, click here.