Instruct: Given a question, retrieve relevant documents that can help answer the question.
Query: How many neurotransmitters are there in the human brain?
Neurotransmitters are endogenous chemicals that enable neurotransmission. It is a type of chemical messenger which transmits signals across a chemical synapse, such as a neuromuscular junction, from one neuron (nerve cell) to another "target" neuron, muscle cell, or gland cell.[1] Neurotransmitters are released from synaptic vesicles in synapses into the synaptic cleft, where they are received by neurotransmitter receptors on the target cells. Many neurotransmitters are synthesized from simple and plentiful precursors such as amino acids, which are readily available from the diet and only require a small number of biosynthetic steps for conversion. Neurotransmitters play a major role in shaping everyday life and functions. Their exact numbers are unknown, but more than 200chemical messengers have been uniquely identified.[2][3][4]
By the end of the 20th century, it became clear that the human brain operates with more than a dozen neurotransmitters and a large number of neuropeptides and hormones. The relationships between these different chemical systems are complex as some of them suppress and some of them induce each other’s release during neuronal exchanges. This complexity of relationships devalues the old approach of assigning “inhibitory vs. excitatory” roles to neurotransmitters: the same neurotransmitters can be either inhibitory or excitatory depending on what system they interact with. It became clear that an impressive diversity of neurotransmitters and their receptors is necessary to meet a wide range of behavioural situations, but the links between temperament traits and specific neurotransmitters are still a matter of research. Several attempts were made to assign specific (single) neurotransmitters to specific (single) traits. For example, dopamine was proposed to be a neurotransmitter of the trait of Extraversion, noradrenaline was linked to anxiety, and serotonin was thought to be a neurotransmitter of an inhibition system. These assignments of neurotransmitter functions appeared to be an oversimplification when confronted by the evidence of much more diverse functionality. 
Research groups led by Petra Netter in Germany, Lars Farde in Karolinska Institute in Sweden and Trevor Robbins in Cambridge, UK had most extensive studies of the links between temperament/personality traits or dynamical properties of behavior and groups of neurotransmitters 
Inositol, or more precisely "myo"-inositol, is a carbocyclic sugar that is abundant in brain and other mammalian tissues, mediates cell signal transduction in response to a variety of hormones, neurotransmitters and growth factors and participates in osmoregulation. It is a sugar alcohol with half the sweetness of sucrose (table sugar). It is made naturally in humans from glucose. Each kidney makes 2g a day; so 4g a day total is made. Other tissues synthesize it too, and the highest concentration is in the brain where it plays an important role making other neurotransmitters, and some steroid hormones bind to their receptors.
Neuroanthropology explores how the brain gives rise to culture, how culture influences brain development, structure and function, and the pathways followed by the co-evolution of brain and culture. Moreover, neuroanthropologists consider how new findings in the brain sciences help us understand the interactive effects of culture and biology on human development and behavior. In one way or another, neuroanthropologists ground their research and explanations in how the human brain develops, how it is structured and how it functions within the genetic and cultural limits of its biology (see Biogenetic structuralism and related website).
The MAOIs work by inhibiting the monoamine oxidase enzymes that, as the name suggests, break down the monoamine neurotransmitters. This leads to increased concentrations of most of the monoamine neurotransmitters in the human brain, serotonin, norepinephrine, dopamine and melatonin. The fact that they are more efficacious than the newer generation antidepressants is what leads scientists to develop newer antidepressants that target a greater range of neurotransmitters. The problem with MAOIs is that they have many potentially dangerous side-effects such as hypotension, and there is a risk of food and drug interactions that can result in potentially fatal serotonin syndrome or a hypertensive crisis. Although selective MAOIs can reduce, if not eliminate these risks, their efficacy tends to be lower.
A monoamine releasing agent (MRA), or simply monoamine releaser, is a drug that induces the release of a monoamine neurotransmitter from the presynaptic neuron into the synapse, leading to an increase in the extracellular concentrations of the neurotransmitter. Many drugs induce their effects in the body and/or brain via the release of monoamine neurotransmitters, e.g., trace amines, many substituted amphetamines, and related compounds.
Neuroscience has a unique perspective in that it can be applied in a broad range of disciplines, and thus the fields neuroscientists work in vary. Neuroscientists may study topics from the large hemispheres of the brain to neurotransmitters and synapses occurring in neurons at a micro-level. Some fields that combine psychology and neurobiology include cognitive neuroscience, and behavioural neuroscience. Cognitive neuroscientists study human consciousness, specifically the brain, and how it can be seen through a lens of biochemical and biophysical processes.[11] Behaviorial neuroscience encompasses the whole nervous system, environment and the brain how these areas show us aspects of motivation, learning, and motor skills along with many others.[12]
Neuropharmacology is the study of how drugs affect cellular function in the nervous system, and the neural mechanisms through which they influence behavior. There are two main branches of neuropharmacology: behavioral and molecular. Behavioral neuropharmacology focuses on the study of how drugs affect human behavior (neuropsychopharmacology), including the study of how drug dependence and addiction affect the human brain. Molecular neuropharmacology involves the study of neurons and their neurochemical interactions, with the overall goal of developing drugs that have beneficial effects on neurological function. Both of these fields are closely connected, since both are concerned with the interactions of neurotransmitters, neuropeptides, neurohormones, neuromodulators, enzymes, second messengers, co-transporters, ion channels, and receptor proteins in the central and peripheral nervous systems. Studying these interactions, researchers are developing drugs to treat many different neurological disorders, including pain, neurodegenerative diseases such as Parkinson's disease and Alzheimer's disease, psychological disorders, addiction, and many others.
Dendritic spikes most commonly propagate backwards from the soma to distal dendritic branches. Backward propagation serves a number of functions in the neuron, and these functions vary based on the type of neuron. In general, backwards propagation serves to communicate output information to the post synaptic membrane. In many neurotransmitter releasing neurons, backward propagation of dendritic spikes signals the release of neurotransmitters. For example, Mitral cells seem to serve both as projection neurons and as local interneurons. If the axonal output of mitral cell is shut down by somatic inhibition, local dendritic action potentials cause the mitral cell to release neurotransmitters into the environment. Backward propagation of dendritic spikes has been demonstrated in various neuronal types in the brain but has rarely been studied outside of the brain. Other than neurons in the brain, dendritic spikes have been observed in the neurons of the spinal cord.
In neuroscience, glutamate refers to the anion of glutamic acid in its role as a neurotransmitter: a chemical that nerve cells use to send signals to other cells. It is by a wide margin the most abundant excitatory neurotransmitter in the vertebrate nervous system. It is used by every major excitatory function in the vertebrate brain, accounting in total for well over 90% of the synaptic connections in the human brain. It also serves as the primary neurotransmitter for some localized brain regions, such as cerebellum granule cells.
Neuropeptides modulate neuronal communication by acting on cell surface receptors. Many neuropeptides are co-released with other small-molecule neurotransmitters. The human genome contains about 90 genes that encode precursors of neuropeptides. At present about 100 different peptides are known to be released by different populations of neurons in the mammalian brain. Neurons use many different chemical signals to communicate information, including neurotransmitters, peptides, and gasotransmitters. Peptides are unique among these cell-cell signaling molecules in several respects. One major difference is that peptides are not recycled back into the cell once secreted, unlike many conventional neurotransmitters (glutamate, dopamine, serotonin). Another difference is that after secretion, peptides are modified by extracellular peptidases; in some cases, these extracellular cleavages inactivate the biological activity, but in other cases the extracellular cleavages increase the affinity of a peptide for a particular receptor while decreasing its affinity for another receptor. These extracellular processing events add to the complexity of neuropeptides as cell-cell signaling molecules.
Brain activity is made possible by the interconnections of neurons that are linked together to reach their targets. A neuron consists of a cell body, axon, and dendrites. Dendrites are often extensive branches that receive information in the form of signals from the axon terminals of other neurons. The signals received may cause the neuron to initiate an action potential (an electrochemical signal or nerve impulse) which is sent along its axon to the axon terminal, to connect with the dendrites or with the cell body of another neuron. An action potential is initiated at the initial segment of an axon, which contains a complex of proteins. When an action potential, reaches the axon terminal it triggers the release of a neurotransmitter at a synapse that propagates a signal that acts on the target cell. These chemical neurotransmitters include dopamine, serotonin, GABA, glutamate, and acetylcholine. GABA is the major inhibitory neurotransmitter in the brain, and glutamate is the major excitatory neurotransmitter. Neurons link at synapses to form neural pathways, neural circuits, and large elaborate network systems such as the salience network and the default mode network, and the activity between them is driven by the process of neurotransmission.
Thewissen’s current work involves counting neurons in bowhead and beluga whale brain samples, to assess brain function, in Utqiagvik—formerly Barrow—Alaska. Though the size of the brains of sperm whales and killer whales are larger than those of any other organism, including humans, a better measure of brain function is to determine how many neurons there are in the brain. Suzana Herculano-Houzel has developed a method of counting of neurons of human and other animals' brains and the relation between the cerebral cortex area and thickness and number of cortical folds. Humans and other primates pack about twice the number of neurons in a cubed inch of brain as most other mammals.
Alpha-synuclein is a protein that, in humans, is encoded by the "SNCA" gene. It is abundant in the brain while smaller amounts are found in the heart, muscles, and other tissues. In the brain, alpha-synuclein is found mainly at the tips of nerve cells (neurons) in specialized structures called presynaptic terminals. Within these structures, alpha-synuclein interacts with phospholipids and proteins. Presynaptic terminals release chemical messengers, called neurotransmitters, from compartments known as synaptic vesicles. The release of neurotransmitters relays signals between neurons and is critical for normal brain function.
Glutamate receptors are synaptic and non synaptic receptors located primarily on the membranes of neuronal and glial cells. Glutamate (the conjugate base of glutamic acid) is abundant in the human body, but particularly in the nervous system and especially prominent in the human brain where it is the body's most prominent neurotransmitter, the brain's main excitatory neurotransmitter, and also the precursor for GABA, the brain's main inhibitory neurotransmitter. Glutamate receptors are responsible for the glutamate-mediated postsynaptic excitation of neural cells, and are important for neural communication, memory formation, learning, and regulation.
Acetylcholine (ACh) is an organic chemical that functions in the brain and body of many types of animals, including humans, as a neurotransmitter—a chemical message released by nerve cells to send signals to other cells [neurons, muscle cells, and gland cells]. Its name is derived from its chemical structure: it is an ester of acetic acid and choline. Parts in the body that use or are affected by acetylcholine are referred to as cholinergic. Substances that interfere with acetylcholine activity are called anticholinergics.
Acetylcholine is the neurotransmitter used at the neuromuscular junction—in other words, it is the chemical that motor neurons of the nervous system release in order to activate muscles. This property means that drugs that affect cholinergic systems can have very dangerous effects ranging from paralysis to convulsions. Acetylcholine is also a neurotransmitter in the autonomic nervous system, both as an internal transmitter for the sympathetic nervous system and as the final product released by the parasympathetic nervous system.
Using human pluripotent stem cells to create "in vitro" cerebral organoids allows researchers to summarize current developmental mechanisms for human neural tissue as well as study the roots of 
human neurological diseases. Cerebral organoids are an investigative tool, used to understand how disease 
pathology works. These organoids can be used in experiments that current "in vitro" methods are too simple for, while also being more human applicable than rodent or other mammalian models. Historically, major breakthroughs in how the brain works have resulted from injury or disorder in human brain function, leading to understanding of how regions of the brain work. An "in vitro" human brain model would allow 
for the next wave in understanding of the human brain.
Neurotransmitters are chemicals that are released at synapses when an action potential activates them—neurotransmitters attach themselves to receptor molecules on the membrane of the synapse's target cell, and thereby alter the electrical or chemical properties of the receptor molecules.
With few exceptions, each neuron in the brain releases the same chemical neurotransmitter, or combination of neurotransmitters, at all the synaptic connections it makes with other neurons; this rule is known as Dale's principle. Thus, a neuron can be characterized by the neurotransmitters that it releases. The great majority of psychoactive drugs exert their effects by altering specific neurotransmitter systems. This applies to drugs such as cannabinoids, nicotine, heroin, cocaine, alcohol, fluoxetine, chlorpromazine, and many others.
Axons transmit signals to other neurons by means of specialized junctions called synapses. A single axon may make as many as several thousand synaptic connections with other cells. When an action potential, traveling along an axon, arrives at a synapse, it causes a chemical called a neurotransmitter to be released. The neurotransmitter binds to receptor molecules in the membrane of the target cell.
Synapses are the key functional elements of the brain. The essential function of the brain is cell-to-cell communication, and synapses are the points at which communication occurs. The human brain has been estimated to contain approximately 100 trillion synapses; even the brain of a fruit fly contains several million. The functions of these synapses are very diverse: some are excitatory (exciting the target cell); others are inhibitory; others work by activating second messenger systems that change the internal chemistry of their target cells in complex ways. A large number of synapses are dynamically modifiable; that is, they are capable of changing strength in a way that is controlled by the patterns of signals that pass through them. It is widely believed that activity-dependent modification of synapses is the brain's primary mechanism for learning and memory.
The Female Brain is a book written by the American neuropsychiatrist Louann Brizendine in 2006. The main thesis of the book is that women's behavior is different from that of men due, in large measure, to hormonal differences. Brizendine says that the human female brain is affected by the following hormones: estrogen, progesterone, testosterone, (oxytocin), neurotransmitters (dopamine, serotonin), and that there are differences in the architecture of the brain (prefrontal cortex, hypothalamus, amygdala) that regulates such hormones and neurotransmitters.
Biological, psychological, and social factors are believed to be involved in the cause of depression, although it is still not well understood. Factors like socioeconomic status, life experience, and personality tendencies play a role in the development of depression and may represent increases in risk for developing a major depressive episode.There are many theories as to how depression occurs. One interpretation is that neurotransmitters in the brain are out of balance, and this results in feelings of worthlessness and despair. Magnetic resonance imaging shows that brains of people who have depression look different than the brains of people not exhibiting signs of depression. A family history of depression increases the chance of being diagnosed.
Some mental illnesses have been linked to an imbalance of chemicals in the brain called neurotransmitters. These neurotransmitters help nerve cells in the brain communicate. If these chemicals are out of balance or are not working properly, messages may not properly transmit through the brain, leading to symptoms of mental illness. In addition, defects in or Traumatic Brain Injury (TBI) to areas of the brain have also been linked to some mental conditions.
When employed in brain research, microdialysis is commonly used to measure neurotransmitters (e.g. dopamine, serotonin, norepinephrine, acetylcholine, glutamate, GABA) and their metabolites, as well as small neuromodulators (e.g. cAMP, cGMP, NO), amino acids (e.g. glycine, cysteine, tyrosine), and energy substrates (e.g. glucose, lactate, pyruvate). Exogenous drugs to be analyzed by microdialysis include new antidepressants, antipsychotics, as well as antibotics and many other drugs that have their pharmacological effect site in the brain. The first non-metabolite to be analyzed by microdialysis in vivo in the human brain was rifampicin.
There are dozens of other chemical neurotransmitters that are used in more limited areas of the brain, often areas dedicated to a particular function. Serotonin, for example—the primary target of antidepressant drugs and many dietary aids—comes exclusively from a small brainstem area called the raphe nuclei. Norepinephrine, which is involved in arousal, comes exclusively from a nearby small area called the locus coeruleus. Other neurotransmitters such as acetylcholine and dopamine have multiple sources in the brain, but are not as ubiquitously distributed as glutamate and GABA.
Possible Worlds addresses many controversial topics regarding today’s scientific advancements and the moral issues that relate to it. A main overriding theme is human emotions and how they are constructed and what they mean. This theme is there to be questioned not to be answered. Another main theme is that of the human consciousness and how the brain is able to construct it. This play is asking whether we should be allowed to figure out exactly how the brain works, essentially pitting every emotion thought and movement to a function in the brain, essentially turning all the mystery in to the light and making the brain seem like a machine. This is a controversial idea because to many people who study religion this could debunk their religious teachings. Religions tend to heavily rely on the idea of a spirit or a soul and if the brain were demystified then there would most likely be no room for a soul and this could lead to a large rebellion against the church. By keeping the brain human and not a machine it leaves room for a “miracle” or other unexplained happening which leaves room for religion. With the advancements in science we will most likely some day know exactly how the brain works but right now we are far from the answer but get closer all the time. The biggest road block is going to be with human testing because it is seen as immoral but without it the advancements in science may not happen. Whether that is to be allowed is a question up to the moral of the human populous as a whole.