Characteristics Of Neurotransmitters
The number of known neurotransmitters has increased tremendously over the past several years. One of the earliest ones studied was acetylcholine, the most common neurotransmitter found in both invertebrates and vertebrates. It is the stimulating agent for skeletal muscle cells, but is the inhibiting agent for heart muscle cells, which demonstrates that the action of a neurotransmitter is influenced by the target receptor cells. Norepinephrine, a catecholamine, is an example of a biogenic amine, which is derived from the amino acid tyrosine. It often works in an opposite way from acetylcholine in the autonomic nervous system and is also found in the cerebrum, cerebellum and spinal cord. Another catecholamine found in the brain is dopamine. It appears to be important in movement and regulating emotional responses. A widely distributed neurotransmitter, serotonin, is found in blood platelets, the lining of the digestive tract, and the brain. It causes very powerful contractions of smooth muscle, and is associated with mood, attention, emotions, and sleep. LSD and mescaline, psychoactive drugs, mimic the structure and function of serotonin and other biogenic drugs to change the mental state of the user. Some new anti-depression drugs, for example, allow lengthened serotonin activity. One very common drug, Prozac (Fluoxetine), is a selective serotonin re-uptake inhibitor (SSRI). As the name implies, the drug inhibits the re-uptake of serotonin neurotransmitter from synaptic gaps, thus increasing neurotransmitter action. In the brain, then, the increased serotonin activity alleviates depressive symptoms. One of the other amino acid derivatives, GABA (gamma-aminobutyric acid) is a major inhibitory transmitter in the central nervous system and it seems to play a role in Huntington's disease. In addition to the well known neurotransmitter, there are peptides, such as the opioids involved in killing pain and causing sleepiness, that act as neurotransmitters. Recently, certain gases, nitric oxide and carbon monoxide, have been discovered to be released from neurons. Nitric oxide has been found to diffuse to the nerves of the digestive system and govern the relaxation required for the normal movements of digestion.
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