Introduction (Psychology and Mental Health)
In the late 1880’s and the early 1900’s, Santiago Ramón y Cajal and Charles Sherrington, respectively, demonstrated that there is a gap separating one neuron (nerve cells that transmit and store information) from another, which Sherrington called the synapse. Their work was extended by Otto Loewi, who proved in 1920 that neurons send messages across the synaptic gap using chemicals. By the 1950’s, the principle that neurons communicate with one another through chemicals was well established, stimulating both a search for new neurotransmitters and new drugs for psychiatric and other medicinal uses.
The early view was that a presynaptic (sending) neuron discharges only one kind of neurotransmitter across the synaptic cleft to a postsynaptic (receiving) neuron. However, later researchers found that not only are multiple neurotransmitters released in most synapses, but neurotransmitters can be discharged from nonsynaptic neuronal membranes and postsynaptic neurons can send chemical messages to presynaptic cells. Furthermore, the idea that thoughts, feelings, and behavior can be reduced to specific neuronal chemicals is overly simplistic. Neurotransmitter effects depend on a combination of the type of neurotransmitter, what kind of receptor (a transmitter-activated protein molecule) picks up the neurotransmitter, and where in the nervous system the chemicals are released. For example, drug addiction has been linked with high...
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Small Molecule Neurotransmitters (Psychology and Mental Health)
The first neurotransmitter discovered was acetylcholine. Acetylcholine is the primary neurotransmitter for stimulating muscles (atropine and botulism block its effects) and conveying information in the parasympathetic nervous system. In the central nervous system, acetylcholine promotes rapid eye movement (REM) sleep and is critical for learning: Foods high in choline—a precursor of acetylcholine—boost learning; low acetylcholine levels inhibit learning, such as in Alzheimer’s disease.
Several small molecule transmitters are amines(substances containing NH2, an amino group). Four amines tend to have arousing effects. Dopamine is essential for experiencing pleasure and has been implicated in almost any kind of addiction. Low levels of dopamine are associated with depression, restless leg disorder, and Parkinson’s disorder; very high levels are associated with schizophrenia and impulsiveness. Norepinephrine plays a primary role in arousal, vigilance, active emotions, and emotional memories. Stimulant drugs, such as amphetamines, activate norepinephrine pathways. Most norepinephrine receptors accept epinephrine, which is the primary neurotransmitter in the sympathetic nervous system. Histamine conducts itching sensations, and its release by mast cells causes the red flaring typical in allergic reactions. Unlike the other amines, higher levels of serotonin tend to induce a calming effect, reducing...
(The entire section is 365 words.)
Peptides (Psychology and Mental Health)
The largest group of neurotransmitters—several dozen—are peptides (amino acid chains). Two peptides have a complementary effect: Substance P is the main carrier of pain; beta endorphins decrease pain. Beta endorphins and enkephalins belong to a family of opiate chemicals produced by the brain that typically increase pleasure but may inhibit learning.
Neuropetides play a role in many basic drives, including drinking (vasopressin), eating (neuropeptide Y), and sexuality (oxytocin). Oxytocin, the hormone that is involved in uterine contractions and lactation, also serves as the bonding neurotransmitter. Higher levels of oxytocin stimulate and help to maintain pair bonding, parenting, and other prosocial behaviors.
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Lipids, Nucleotides, and Gases (Psychology and Mental Health)
The brain not only produces opioids but also cannabis-like lipids called endocannabinoids. One of these chemicals, anandamide, helps to regulate the release of several small molecule transmitters. Endocannabinoids appear to interact with opioids to produce pleasurable effects, and, as with the opioids, overrelease may interfere with learning.
Adenosine, a nucleotide involved in sleep production, is also released by the nervous system’s other main cell type: glia. Caffeine has excitatory effects because it blocks adenosine receptors.
Two water-soluble gases, nitric acid and carbon monoxide, are neurotransmitters that can be released from any neuronal area, unlike all other neurotransmitters. Both neurotransmitters modulate the activity of other neurotransmitters and play a role in metabolic processes. Nitric oxide also dilates blood vessels: Erectile dysfunction drugs enhance the activity of nitric oxide.
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Sources for Further Study (Psychology and Mental Health)
Bohlen und Halbach, Oliver, and Rolf Dermietzel. Neurotransmitters and Neuromodulators: Handbook of Receptors and Biological Effects. Hoboken, N.J.: Wiley, 2006. A comprehensive overview of neurochemicals is presented, including the synthesis, locations, and effects of all the major neurotransmitters.
Carlson, Neil R. Foundations of Physiological Psychology. 7th ed. Boston: Allyn & Bacon, 2008. Chapter 4 of this popular textbook contains a good introduction to neurotransmitters and psychopharmacology.
Ingersoll, R. Elliott, and Carl F. Rak. Psychopharmacology for Helping Professionals: An Integral Perspective. Belmont, Calif.: Thomson Brooks/Cole, 2006. The authors discuss numerous aspects of psychotropic drugs and their effects on neurotransmitters in this accessible introduction to psychopharmacology.
Julien, Robert M., Claire D. Advokat, and Joseph E. Comaty. A Primer of Drug Action. 11th ed. New York: Worth Publishers, 2008. The impact of drugs on neurotransmitters is thoroughly covered.
Lajtha, Abel, and E. Sylvester Vizi, eds. Handbook of Neurochemistry and Molecular Neurobiology: Neurotransmitter Systems. 3d ed. New York: Springer, 2008. Various scientists present recent discoveries about neuronal communication.
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Neurotransmitters (Encyclopedia of Neurological Disorders)
Neurotransmitters are chemicals that allow the movement of information from one neuron across the gap between it and the adjacent neuron. The release of neurotransmitters from one area of a neuron and the recognition of the chemicals by a receptor site on the adjacent neuron causes an electrical reaction that facilitates the release of the neurotransmitter and its movement across the gap.
The transmission of information from one neuron to another depends on the ability of the information to traverse the gap (also known as the synapse) between the terminal end of one neuron and the receptor end of an adjacent neuron. The transfer is accomplished by neurotransmitters.
In 1921, an Austrian scientist named Otto Loewi discovered the first neurotransmitter. He named the compound "vagusstoff," as he was experimenting with the vagus nerve of frog hearts. Now, this compound is known as acetylcholine.
Neurotransmitters are manufactured in a region of a neuron known as the cell body. From there, they are transported to the terminal end of the neuron, where they are enclosed in small membrane-bound bags called vesicles (the sole exception is nitric oxide, which is not contained inside a vesicle, but is released from the neuron soon after being made). In response to an action...
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Neurotransmitters (Encyclopedia of Mental Disorders)
Neurotransmitters are chemicals located and released in the brain to allow an impulse from one nerve cell to pass to another nerve cell.
There are approximately 50 neurotransmitters identified. There are billions of nerve cells located in the brain, which do not directly touch each other. Nerve cells communicate messages by secreting neurotransmitters. Neurotransmitters can excite or inhibit neurons (nerve cells). Some common neurotransmitters are acetylcholine, norepinephrine, dopamine, serotonin and gamma aminobutyric acid (GABA). Acetylcholine and norepinephrine are excitatory neurotransmitters while dopamine, serotonin, and GABA are inhibitory. Each neurotransmitter can directly or indirectly influence neurons in a specific portion of the brain, thereby affecting behavior.
Mechanism of impulse transmission
A nerve impulse travels through a nerve in a long, slender cellular structure called an axon, and it eventually reaches a structure called the presynaptic membrane, which contains neurotransmitters to be released in a free space called the synaptic cleft. Freely flowing neurotransmitter molecules are picked up by receptors...
(The entire section is 1042 words.)
Neurotransmitter (Encyclopedia of Psychology)
Chemical substances or molecules which aid in message transmission between neurons.
Communication at the synapses between neurons relies on chemicals called neurotransmitters. Secreted from a part of one neuron (the axon) into the synaptic gap between two others, neurotransmitters diffuse across this space and combine with specific proteins on the surface of the receiving cell, triggering an electrochemical response in the target cell. Afterward, neurotransmitters are either destroyed or reabsorbed back into the neuron for storage and reuse. The release of neurotransmitters by a neuron has three main functions: 1) exciting a second neuron, thus causing it to depolarize; 2) inhibiting a second neuron, which prevents it from depolarizing; and 3) stimulating a muscle fiber to contract.
More than 50 different neurotransmitters have been identified, and more are constantly being discovered. Researchers have proposed that almost all drugs work through interaction with neurotransmitters. Important neurotransmitters include acetylcholine (ACh), which is used by motor neurons in the spinal cord; the catecholamines (including norepinephrine and dopamine), which are important in the arousal of the sympathetic nervous system; serotonin, which affects body temperature, sensory perception, and the onset of sleep; and a...
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Neurotransmitters (World of Forensic Science)
The forensic investigation of an accident or death is not always aided by the presence of physically obvious signs, such as a stab wound or gunshot wound. Injury or death inflicted by toxic agents may have less subtle physical effects. Toxins can interfere with the normal physiological functions of the body. Then, their presence is forensically evident by a physiological change in the norm. One example is agents that disrupt the action of neurotransmitters.
Neurotransmitters are chemicals released in minute amounts from the terminals of nerve cells in response to the arrival of an action potential. There are now more than 300 known neurotransmitters and they act either locally in point-to-point signal transmission (e.g., the motor nerve of a neuromuscular junction) or at a distal site (e.g., the hypothalamic releasing hormones acting on the anterior pituitary). Locally acting neurotransmitters relay the electrical signal traveling along a neuron as chemical information across the neuronal junction, or synapse, that separates one neuron from another neuron or a muscle. Neurons communicate with peripheral tissues, such as muscles, glands etc., or with each other, largely by this chemical means rather than by direct electrical transmission.
Neurotransmitters are stored in the bulbous end of the nerve cell's axon. When an electrical impulse traveling along an axon reaches the junction, the neurotransmitter is released and diffuses across the synaptic gap, a distance of as little as 25 nanometers (nm) or as great as 100 micrometers (mm). The interaction of the neurotransmitter with the postsynaptic receptor of the target cell generates either an excitatory postsynaptic potential (EPSP) or an inhibitory postsynaptic potential (IPSP). Transmitters that lead to EPSPs appear to open large, non-specific membrane channels, permitting the simultaneous movement of Na+, K+ and Cl-. IPSPs are caused by Cl- flux only.
Neurotransmitters include such diverse molecules as acetylcholine, noradrenalin, serotonin, dopamine, γ-aminobutyric acid, glutamate, glycine and numerous other small monoamines and amino acids. There are also small peptides, which appear to act as chemical messengers in the nervous system. They include substance P, vasopressin, oxytocin, endorphins, angiotensin, and many others. A rather unusual but interesting neurotransmitter is the gas nitric oxide. This diverse range of chemical neurotransmitters may suggest that chemical coding could play as important a part in communication between neurons as do the strict point-to-point connections of neural circuitry.
Acetylcholine is one of the neurotransmitters functioning in the peripheral nervous system. It is released by all motor nerves to control skeletal muscles and also by autonomic nerves controlling the activity of smooth muscle and glandular functions in many parts of the body. Norepinephrine is released by sympathetic nerves controlling smooth muscle, cardiac muscle, and glandular tissues. In these tissues acetylcholine and norepinephrine often exert diametrically opposed actions.
The neurotransmitters used by the majority of fast, point-to-point neural circuits in the central nervous system (CNS) are amino acids. Of these, the inhibitory substance γ-aminobutyric acid (GABA) is well characterized and it is present in all regions of the brain and spinal cord. GABA rapidly inhibits virtually all CNS neurons when applied locally by increasing cell permeability to chloride ions, thus stabilizing resting membrane potential near the chloride equilibrium level. Although GABAergic (GABA-producing) neurons also exist in the spinal cord, another inhibitory amino acid, glycine, predominates in this region of the CNS. Glycine is present in small inhibitory interneurons in the spinal cord gray matter and mediates the inhibition of most spinal neurons. The amino acids L-glutamate and L-asparagine depolarize neurons by activating membrane sodium channels and are ubiquitously distributed, appearing as the most common excitatory transmitters for interneurons in the CNS.
In contrast to the point-to-point signaling in which amino acids are involved, the monoamines are mainly associated with the more diffuse neural pathways in the CNS. The monoamines are present in small groups of neurons, primarily located in the brain stem, with elongated and highly branched axons. These diffuse ascending and descending monoaminergic innervations impinge on very large terminal fields and there is evidence that the monoamines may be released from many points along the varicose terminal networks of monoaminergic neurons. Most monoamines released in this way occur at nonsynaptic sites and a very large number of target cells may be affected by the diffuse release of these substances, which are therefore thought to perform modulatory functions of various types.
One of the most remarkable developments was the realization that most peptide hormones of the endocrine and neuroendocrine systems also exist in neurons. These are by far the largest group of potential chemical messengers. For example, the opioid peptides (endorphins) have attracted enormous interest because of their morphine-like properties. They are consequently of considerable interest in the understanding of pain. Endorphins represent a family of chemical messengers found in all regions of the CNS including the pituitary (e.g., beta-endorphin and dynorphin) and the peripheral enteric nervous system. Their presence in regions such as the basal ganglia and the eye's retina, where it is unlikely that they have any connection with pain pathways, suggests that they may also have other diverse functions. There is still much to be learned about the possible functions of neuropeptides in the CNS. In all cases so far examined the peptides seem to be capable of being released by a specialized secretory mechanism from stimulated CNS neurons. They can exert powerful effects on the CNS. For example, the direct administration of small amounts of peptide to the brain can elicit a variety of behavioral responses, including locomotor activity (substance P), analgesia (endorphins), drinking behavior (angiotensisn II), female sexual behavior (LHRH), and improved retention of learned tasks (vasopressin).
An interesting and novel neurotransmitter identified in the 1980s is nitric oxide (NO). This is a highly reactive naturally occurring gas generated in the body from arginine and has the alternative name "epithelium-derived-relaxing factor." Synthesis of NO in blood vessel epithelia occurs in response to the distortion of blood vessels by blood flow. The gas then rapidly diffuses into the surrounding muscle layers, causing them to relax. It, therefore, has vasodilatory (dilation of blood vessels) properties and as a neurotransmitter occurs in a number of nerve networks. For example, it is known to be active in the dilation of arteries supporting the penis and in the relaxation of muscles of the corpora cavernosa (the two chambers filled with spongy tissue which run the length of the penis). NO released from stomach nerves causes the stomach to relax in order to accommodate food. Intestinal nerves also induce the relaxation of the intestinal muscle by releasing NO. In addition, nervous activity in the cerebellum is increased by NO and it appears that NO is an important neurotransmitter associated with memory. Despite its usefulness, nitric oxide can have a toxic effect on body cells and has been implicated in Huntington's disease and Alzheimer's disease.
SEE ALSO Death, cause of; Nervous system overview; Toxicology.
Neurotransmitters (Encyclopedia of Drugs, Alcohol, and Addictive Behavior)
A neurotransmitter is any chemical substance (the first recognized was ACETYLCHOLINE) that NEURONS (nerve cells) secrete to communicate with their target cells (glands, muscles, and other neurons). Neurotransmitters diffuse from their sites of releaserom the presynaptic nerve terminalcross the synaptic cleft, to bind to receptors on the external surface of the postsynaptic cell. Activation of these receptors allows for the transmission of commands (excitation, inhibition, and other more complex forms of regulation) from the presynaptic neuron to the postsynaptic cell.
A neurotransmitter is released from a nerve ending, interacts with specific RECEPTORS, and is then either transported back into the presynaptic neuron or destroyed by metabolic enzymes in the synaptic cleft.
Chemically, neurotransmitters are amino acids, amines, or peptides. Peptide transmitters commonly coexist and may be cosecreted with amino acid or amine transmitters.
(SEE ALSO: Dopamine; Endorphins; Neurotransmission; Norepinephrine; Serotonin)
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