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Neurotransmitters - Anatomy & Physiology (view source)
Revision as of 15:32, 11 November 2011
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====Amino Acids====
====Amino Acids====
Amino acid '''glutamate''' is the most common '''excitatory''' SMT in the [[Nervous and Special Senses - Anatomy & Physiology#Central Nervous System (CNS)|central nervous system]] whilst '''gamma-aminobutyric acid (GABA)''' is the most common '''inhibitory''' SMT. Other amino acid SMTs include '''aspartate''' and '''glycine'''.
Amino acid '''glutamate''' is the most common '''excitatory''' SMT in the [[Nervous and Special Senses - Anatomy & Physiology#Central Nervous System (CNS)|central nervous system]] whilst '''gamma-aminobutyric acid (GABA)''' is the most common '''inhibitory''' SMT. Other amino acid SMTs include '''aspartate''' and '''glycine'''.
When '''glutamate''' is released it facilitates the opening of sodium channels within the post-synaptic membrane allowing sodium ions to enter the membrane and causing depolarisation. Therefore '''glutamate''' makes it easier for the cell to reach its depolarisation threshold and generate an action potential. Due to this, '''Glutamate''' is classified as an excitatory neurotransmitter.
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'''Glycine''' is present in the spinal cord and is crucial for limb movement; in particular the motor function associated with limb reflexes. When both '''glycine''' and '''GABA''' are released they result in the opening of chloride ion channels within the post synaptic membrane resulting in the membrane becoming hyperpolarised. The cytosolic side of the membrane becomes more negative. Therefore both neurotransmitters make it more difficult for the cell to reach its depolarisation threshold to generate an action potential.
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Image:GABA.svg|'''GABA'''
Image:GABA.svg|'''GABA'''
The biogenic amine that is derived from the amino acid '''tryptophane''' is called '''serotonin (5-HT)''' whilst the SMT derived from '''histidine''' is called '''histamine (HA)'''. Serotonergic neurons release serotonin. Whilst these SMTs are primarily of importance in the central nervous system, norepinephrine is predominantly found in the peripheral nervous system.
The biogenic amine that is derived from the amino acid '''tryptophane''' is called '''serotonin (5-HT)''' whilst the SMT derived from '''histidine''' is called '''histamine (HA)'''. Serotonergic neurons release serotonin. Whilst these SMTs are primarily of importance in the central nervous system, norepinephrine is predominantly found in the peripheral nervous system.
'''Epinephrine''' is produced by the adrenal glands. It is primarily involved in an overall activation of the sympathetic nervous system and is involved in the management of stress. '''Norepinephrine''' is also produced by adrenal glands and is involved in the initiation and maintainance of consciousness within the sympathetic nervous system. Both types of neurotransmitter utilise α or β receptors and are metabotropic. If an α<sub>1</sub> adrenergic receptor is bound this will result in depolarisation of the cell and vasoconstriction of the skin and viscera. If a β<sub>1</sub> receptor is bound this will also result in cellular depolarisation and an increase in heart rate and contractility. If a β<sub>2</sub> receptor is bound this will result in hyperpolarisation of the cell which will cause dilation of the bronchioles of the lung.
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'''Dopamine''' is involved in motivation as well as love and addiction. It is effectively a 'reward system' for the brain. Dopamine also affects the way in which the basal ganglia of the brain affect our movements and a shortage of dopamine can result in diseases such as Parkinson's. Dopamine is the primary neuroendocrine regulator of [[Lactation_-_Endocrine_Control_-_Anatomy_%26_Physiology|'''prolactin''']] from the anterior pituitary gland. Dopamine produced by the hypothalamus is secreted via the hypothalamo-hypophysal blood vessels which supply the pituitary gland. Secretion of prolactin via lactotrope cells within the pituitary is inhibited by dopamine.
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'''Serotonin''' is involved in emotions, mood, sexuality, consciousness, sleep and thermoregulation. Serotonin it utilised by the central nervous system and the gastro-intestinal system. Serotonin has also been linked to mechanisms controlling pulmonary and cerebral vascular vasoconstriction.
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'''Melatonin''' is responsible for the regulation of onset of sleep and also for seasonal changes in the body such as winter weight gain and mating seasons. '''Histamine''' release results in increased gastric secretions, dilation of capillaries, constriction of bronchial smooth muscle and decreased blood pressure.
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Although '''nitric oxide (NO)''' is a neurotransmitter, its characteristics differ from those discussed above. NO relies on calcium ion activation of the enzyme '''nitric oxide synthase (NOS)''' which is found throughout the nervous system and is the enzyme that is responsible for catalysing NO from the amino acid '''L-arginine'''. NO has a very short half-life and is highly reactive. It is able to pass easily through lipid membranes. What makes NO differ from the SMTs above is that is can be released in all directions rather than pre-synaptically as per the classical SMTs. Therefore NO is able to act as a signalling pathway for the post-synaptic neuron to affect the pre-synpatic neuron.
Although '''nitric oxide (NO)''' is a neurotransmitter, its characteristics differ from those discussed above. NO relies on calcium ion activation of the enzyme '''nitric oxide synthase (NOS)''' which is found throughout the nervous system and is the enzyme that is responsible for catalysing NO from the amino acid '''L-arginine'''. NO has a very short half-life and is highly reactive. It is able to pass easily through lipid membranes. What makes NO differ from the SMTs above is that is can be released in all directions rather than pre-synaptically as per the classical SMTs. Therefore NO is able to act as a signalling pathway for the post-synaptic neuron to affect the pre-synpatic neuron.
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The neuropeptide '''Acetylcholine (Ach)''' is primarily involved in skeletal muscle movement within the sympathetic nervous system and visceral movements as part of the parasympathetic nervous system. When binding to '''muscarinic''' receptors, ACh can have a number of different effects dependant on the type of receptor. If an '''M2''' receptor is bound this will result in hyperpolarisation of the cell and a slowing of the rate of spontaneous contraction of the heart. If an '''M3''' or an '''M5''' receptor is bound this will result in depolarisation of the cell and contraction of smooth muscle within glands.
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'''Nitrous oxide''' is involved in enlargement of the genital organs leading to erection.
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Image:Stickstoffmonoxid.png|'''Nitrous Oxide'''
Image:Stickstoffmonoxid.png|'''Nitrous Oxide'''
Endorphins and enkephalins are both examples of '''[[opioids]]''' and act within neuronal synapses to reduce the sensation of pain. In humans, it has been shown that these neuropeptides also lead to a sense of euphoria. Both of these molecules belong to the same opioid category as morphine and heroin.
Endorphins and enkephalins are both examples of '''[[opioids]]''' and act within neuronal synapses to reduce the sensation of pain. In humans, it has been shown that these neuropeptides also lead to a sense of euphoria. Both of these molecules belong to the same opioid category as morphine and heroin.
'''Vasoactive intestinal peptide (VIP)''' plays a role within the intestines and acts to greatly increase the secretion of water and electrolytes. VIP also causes dilation of the smooth muscle within the peripheral smooth muscles and inhibits [[Gut_Endocrine_Function_-_Anatomy_%26_Physiology|gastrin-stimulated]] gastric acid secretion. The neuropeptide '''vasopressin''' is responsible for metabolism and maintainance of the metabolic rate. The overall effect of VIP is to increase gastric motility.
'''Vasoactive intestinal peptide (VIP)''' plays a role within the intestines and acts to greatly increase the secretion of water and electrolytes. VIP also causes dilation of the smooth muscle within the peripheral smooth muscles and inhibits [[Gut_Endocrine_Function_-_Anatomy_%26_Physiology|gastrin-stimulated]] gastric acid secretion. The neuropeptide '''vasopressin''' is responsible for metabolism and maintainance of the metabolic rate. The overall effect of VIP is to increase gastric motility.
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'''Somatostatin''' or '''Growth hormone-inhibiting hormone (GHIH)''' is a regulatory molecule within the endocrine system but also affects neurotransmission via it's interaction with G-protein coupled somatostatin receptors. It also inhibits the production of many other secondary hormones.
'''Somatostatin''' or '''Growth hormone-inhibiting hormone (GHIH)''' is a regulatory molecule within the endocrine system but also affects neurotransmission via it's interaction with G-protein coupled somatostatin receptors. It also inhibits the production of many other secondary hormones.
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Image:Alpha-endorphin.svg|'''Alpha-Endorphin'''
Image:NeuropeptideY_1RON.png|'''Neuropeptide Y'''
Image:Oxytocin.png|'''Oxytocin'''
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===Other Types of Neurotransmitter===
In many neuronal synapses, not only do the post-synaptic membranes contain receptors for neurotransmitters, they also contain ion channels. In many cases the neurotransmitter receptors and ion channels are directly linked giving rise to '''ionotropic receptors'''. When a neurotransmitter binds its relevant receptor, this also may affect the gating of adjacent ion channels, either opening or closing the channel. Ionotrophic receptors such as this are responsible for the fastest type of synaptic transmission. An example of an ionotrophic receptor is '''zinc''' which is synaptically released via this mechanism.
Other ion channel receptors, '''metabotrophic receptors''', exist where the ion channel is less well associated with the neurotransmitter receptors. These receptors are affected indirectly via '''G-proteins''' or '''intracellular secondary messengers''' altering the status of the gate once an appropriate signal has been received. Despite relying on an intermediate messenger system, metabotrophic receptors can also propagate rapid signal transfers, although not as rapid as ionotrophic receptors.
===Other Types of Neuropeptides===
===Other Types of Neuropeptides===
As mentioned above, substances related to changes in ion channel status are closely related to the release of other neuropeptides. '''Zinc''' is therefore associated with the release of another type of neurotransmitter, neuropeptide Y.
As mentioned above, substances related to changes in ion channel status are closely related to the release of other neuropeptides. '''Zinc''' is therefore associated with the release of another type of neurotransmitter, neuropeptide Y.
==Function==
The release of excitatory neurotransmitters from the pre-synaptic membrane causes channels in the post-synaptic membrane to open and cause an increase in sodium ion concentration within the postsynaptic cell and a decrease in potassium ion concentration. This leads to a depolarisation of the postsynaptic cell, which is [[Neurons - Anatomy & Physiology#Nerve Impulse Propagation|propagated further along the axon]] by an action potential (AP). Inhibitory neurotransmitters cause hyperpolarization of the postsynaptic cell making it unable to generate an action potential.
Post-synaptic receptors determine the reaction of the neurotransmitter meaning that the same neurotransmitter may cause an excitatory effect on some membranes whilst exerting an inhibitory effect on others e.g ACH can be either excitatory to skeletal muscle cells or inhibitory to both smooth muscle and cardiac muscle.