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Neuromodulation

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For the therapy, see Neuromodulation (medicine). For other uses, see Neuromodulation (disambiguation).

Neuromodulation is the physiological process by which a given neuron uses one or more chemicals to regulate diverse populations of neurons. Neuromodulators typically bind to metabotropic, G-protein coupled receptors (GPCRs) to initiate a second messenger signaling cascade that induces a broad, long-lasting signal. This modulation can last for hundreds of milliseconds to several minutes. Some of the effects of neuromodulators include: altering intrinsic firing activity,[1] increasing or decreasing voltage-dependent currents,[2] altering synaptic efficacy, increasing bursting activity[2] and reconfiguring synaptic connectivity.[3]

Illustration of the brain and spinal cord connecting to a muscle, illustrating the connection between the central and peripheral nervous system.

Major neuromodulators in the central nervous system include: dopamine, serotonin, acetylcholine, histamine, norepinephrine, nitric oxide, and several neuropeptides. Cannabinoids can also be powerful CNS neuromodulators.[4][5][6] Neuromodulators can be packaged into vesicles and released by neurons, secreted as hormones and delivered through the circulatory system.[7] A neuromodulator can be conceptualized as a neurotransmitter that is not reabsorbed by the pre-synaptic neuron or broken down into a metabolite. Some neuromodulators end up spending a significant amount of time in the cerebrospinal fluid (CSF), influencing (or "modulating") the activity of several other neurons in the brain.[8] When discussing neuromodulation, there are two forms of non-invasive stimulation when treating migraines. One is electrical stimulation, and some of the characterizations include transcranial alternating stimulation and transcranial direct current stimulation. The other is magnetic stimulation, which includes single pulse and repetitive transcranial stimulation. [User:Ngwinn]/Editing User:Ngwinn/Neuromodulation - Wikipedia].[9]

Neuromodulatory systems

See also: Neural pathways

The major neurotransmitter systems are the noradrenaline (norepinephrine) system, the dopamine system, the serotonin system, and the cholinergic system. Drugs targeting the neurotransmitter of such systems affect the whole system, which explains the mode of action of many drugs.

Most other neurotransmitters, on the other hand, e.g. glutamate, GABA and glycine, are used very generally throughout the central nervous system.

Neuromodulator systems
System Origin[10] Targets[10] Effects[10]
Noradrenaline system Locus coeruleus Adrenergic receptors in:
  • arousal (Arousal is a physiological and psychological state of being awake or reactive to stimuli)
  • reward system
Lateral tegmental field
Dopamine system Dopamine pathways: Dopamine receptors at pathway terminations.
Serotonin system caudal dorsal raphe nucleus Serotonin receptors in:
rostral dorsal raphe nucleus Serotonin receptors in:
Cholinergic system Pedunculopontine nucleus and dorsolateral tegmental nuclei (pontomesencephalotegmental complex) (mainly) M1 receptors in:
basal optic nucleus of Meynert (mainly) M1 receptors in:
medial septal nucleus (mainly) M1 receptors in:

Noradrenaline system

Further information: Norepinephrine § Norepinephrine system
Skeletal formulae diagram of Noradrenaline

The noradrenaline system consists of around 15,000 neurons, primarily in the locus coeruleus.[13] This is diminutive compared to the more than 100 billion neurons in the brain. As with dopaminergic neurons in the substantia nigra, neurons in the locus coeruleus tend to be melanin-pigmented. Noradrenaline is released from the neurons, and acts on adrenergic receptors. Noradrenaline is often released steadily so that it can prepare the supporting glial cells for calibrated responses. Despite containing a relatively small number of neurons, when activated, the noradrenaline system plays major roles in the brain including involvement in suppression of the neuroinflammatory response, stimulation of neuronal plasticity through LTP, regulation of glutamate uptake by astrocytes and LTD, and consolidation of memory.[14]

Dopamine system

Further information: Dopamine § Functions in the brain

The dopamine or dopaminergic system consists of several pathways, originating from the ventral tegmentum or substantia nigra as examples. It acts on dopamine receptors.[15]

Skeletal formulae diagram of Dopamine

Parkinson's disease is at least in part related to dropping out of dopaminergic cells in deep-brain nuclei, primarily the melanin-pigmented neurons in the substantia nigra but secondarily the noradrenergic neurons of the locus coeruleus. Treatments potentiating the effect of dopamine precursors have been proposed and effected, with moderate success.

Dopamine pharmacology

Serotonin system

Further information: Serotonin § Gross anatomy
Skeletal formulae of Serotonin or 5-HT

The serotonin created by the brain comprises around 10% of total body serotonin. The majority (80-90%) is found in the gastrointestinal (GI) tract.[16][17] It travels around the brain along the medial forebrain bundle and acts on serotonin receptors. In the peripheral nervous system (such as in the gut wall) serotonin regulates vascular tone.

Serotonin pharmacology

  • Selective serotonin reuptake inhibitors (SSRIs) such as fluoxetine are widely used antidepressants that specifically block the reuptake of serotonin with less effect on other transmitters.[18][19][20]
  • Tricyclic antidepressants also block reuptake of biogenic amines from the synapse, but may primarily affect serotonin or norepinephrine or both. They typically take four to six weeks to alleviate any symptoms of depression. They are considered to have immediate and long-term effects.[18][20][21]
  • Monoamine oxidase inhibitors allow reuptake of biogenic amine neurotransmitters from the synapse, but inhibit an enzyme which normally destroys (metabolizes) some of the transmitters after their reuptake. More of the neurotransmitters (especially serotonin, noradrenaline and dopamine) are available for release into synapses. MAOIs take several weeks to alleviate the symptoms of depression.[18][20][22][23]

Although changes in neurochemistry are found immediately after taking these antidepressants, symptoms may not begin to improve until several weeks after administration. Increased transmitter levels in the synapse alone does not relieve the depression or anxiety.[18][20][23]

Cholinergic system

The cholinergic system consists of projection neurons from the pedunculopontine nucleus, laterodorsal tegmental nucleus, and basal forebrain and interneurons from the striatum and nucleus accumbens. It is not yet clear whether acetylcholine as a neuromodulator acts through volume transmission or classical synaptic transmission, as there is evidence to support both theories. Acetylcholine binds to both metabotropic muscarinic receptors (mAChR) and the ionotropic nicotinic receptors (nAChR). The cholinergic system has been found to be involved in responding to cues related to the reward pathway, enhancing signal detection and sensory attention, regulating homeostasis, mediating the stress response, and encoding the formation of memories.[24][25]

GABA

GABA nomenclature example

Gamma-aminobutyric acid (GABA) has an inhibitory effect on brain and spinal cord activity.[18] GABA is an amino acid that is the primary inhibitory neurotransmitter for the central nervous system (CNS). It reduces neuronal excitability by inhibiting nerve transmission. GABA has a multitude of different functions during development and influences the migration, proliferation, and proper morphological development of neurons. It also influences the timing of critical periods and potentially primes the earliest neuronal networks. There are two main types of GABA receptors: GABAa and GABAb. GABAa receptors inhibit neurotransmitter release and/or neuronal excitability and are a ligand-gated chloride channel. GABAb receptors are slower to react due to a GCPR that acts to inhibit neurons. GABA can be the culprit for many disorders ranging from schizophrenia to major depressive disorder because of its inhibitory characteristics being dampened.[26][27][28]

Neuropeptides

Neuropeptides are small proteins used for communication in the nervous system. Neuropeptides represent the most diverse class of signaling molecules. There are 90 known genes that encode human neuropeptide precursors. In invertebrates, there are ~50 known genes encoding neuropeptide precursors.[29] Most neuropeptides bind to G-protein coupled receptors, however some neuropeptides directly gate ion channels or act through kinase receptors.

  • Opioid peptides – a large family of endogenous neuropeptides that are widely distributed throughout the central and peripheral nervous system. Opiate drugs such as heroin and morphine act at the receptors of these neurotransmitters.[30][31]
  1. Endorphins
  2. Enkephalins
  3. Dynorphins

Neuromuscular systems

Neuromodulators may alter the output of a physiological system by acting on the associated inputs (for instance, central pattern generators). However, modeling work suggests that this alone is insufficient,[32] because the neuromuscular transformation from neural input to muscular output may be tuned for particular ranges of input. Stern et al. (2007) suggest that neuromodulators must act not only on the input system but must change the transformation itself to produce the proper contractions of muscles as output.[32]

Volume transmission

Neurotransmitter systems are systems of neurons in the brain expressing certain types of neurotransmitters, and thus form distinct systems. Activation of the system causes effects in large volumes of the brain, called volume transmission.[33] Volume transmission is the diffusion of neurotransmitters through the brain extracellular fluid released at points that may be remote from the target cells with the resulting activation of extrasynaptic receptors, and with a longer time course than for transmission at a single synapse.[34] Such prolonged transmitter action is called tonic transmission, in contrast to the phasic transmission that occurs rapidly at single synapses.[35][36]

Other uses

Neuromodulation also refers to an emerging class of medical therapies that target the nervous system for restoration of function (such as in cochlear implants), relief of pain, or control of symptoms, such as tremor seen in movement disorders like Parkinson's disease. The therapies consist primarily of targeted electrical stimulation, or infusion of medications into the cerebrospinal fluid using intrathecal drug delivery, such as baclofen for spasticity. Electrical stimulation devices include deep brain stimulation systems (DBS), colloquially referred to as "brain pacemakers", spinal cord stimulators (SCS) and vagus nerve stimulators (VNS), which are implanted using minimally invasive procedures, or transcutaneous electrical nerve stimulation and scrambler therapy devices, which are fully external, among others.[37]

See also

  • 5-HT2c receptor agonist
  • Natural neuroactive substance
  • Symptom Validity Assessment definition- an examinee's validity is tested by examining their truthfulness and accuracy about a specific behavioral presentation, self-reports, and neuropsychological performance. [38] Performance Validity Testing definition- an examiner's way of measuring the overall efficacy of an evaluation method. [39] [User:Ngwinn]/Editing User:Ngwinn/Neuromodulation - Wikipedia]

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