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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.  

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.  

 

'''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, thus classed as inhibitory.

'''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, thus classed as inhibitory.

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.  

'''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 α₁ adrenergic receptor is bound this will result in depolarisation of the cell and vasoconstriction of the skin and viscera.  

:If an α₁ adrenergic receptor is bound this will result in depolarisation of the cell and vasoconstriction of the skin and viscera.  

:If a β₁ receptor is bound this will also result in cellular depolarisation and an increase in heart rate and contractility.  

:If a β₁ receptor is bound this will also result in cellular depolarisation and an increase in heart rate and contractility.  

:If a β₂ receptor is bound this will result in hyperpolarisation of the cell which will cause dilation of the bronchioles of the lung.  

:If a β₂ receptor is bound this will result in hyperpolarisation of the cell which will cause dilation of the bronchioles of the lung.  

 

'''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 & Physiology|'''prolactin''']] from the anterior [[Pituitary Gland - Anatomy & Physiology|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.  

'''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 & Physiology|'''prolactin''']] from the anterior [[Pituitary Gland - Anatomy & Physiology|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.  

 

'''Serotonin''' is involved in emotions, mood, sexuality, consciousness, sleep and thermoregulation. Serotonin is utilised by the central nervous system and the gastro-intestinal system. Serotonin has also been linked to mechanisms controlling pulmonary and cerebral vascular vasoconstriction.

'''Serotonin''' is involved in emotions, mood, sexuality, consciousness, sleep and thermoregulation. Serotonin is utilised by the central nervous system and the gastro-intestinal system. Serotonin has also been linked to mechanisms controlling pulmonary and cerebral vascular vasoconstriction.

 

'''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.  

'''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.

'''Histamine''' release results in increased gastric secretions, dilation of capillaries, constriction of bronchial smooth muscle and decreased blood pressure.

: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 '''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.  

:If an '''M3''' or an '''M5''' receptor is bound this will result in depolarisation of the cell and contraction of smooth muscle within glands.  

 

'''Adenosine triphosphate (ATP)''', as well as having many important '''intracellular functions''', is an important neurotransmitter and also has an '''autocrine''' and '''paracrine''' function. ATP belongs to the '''purines''' SMT group. All synaptic vesicles released by the terminal membrane of a nerve contain ATP as well as other neurotransmitters, although ATP can only function as a neurotransmitter in its own right if the post-synaptic terminal membrane contains ATP receptors. These ATP receptors are referred to as '''purinergic receptors'''. A pre-synaptic nerve terminal or terminal membrane never releases multiple types of SMT in addition to ATP, although it is common that neuropeptides are released in addition to ATP and SMTs. Other SMTs within the purine group include '''Guanosine triphosphate (GTP)''' and their derivatives.

'''Adenosine triphosphate (ATP)''', as well as having many important '''intracellular functions''', is an important neurotransmitter and also has an '''autocrine''' and '''paracrine''' function. ATP belongs to the '''purines''' SMT group. All synaptic vesicles released by the terminal membrane of a nerve contain ATP as well as other neurotransmitters, although ATP can only function as a neurotransmitter in its own right if the post-synaptic terminal membrane contains ATP receptors. These ATP receptors are referred to as '''purinergic receptors'''. A pre-synaptic nerve terminal or terminal membrane never releases multiple types of SMT in addition to ATP, although it is common that neuropeptides are released in addition to ATP and SMTs. Other SMTs within the purine group include '''Guanosine triphosphate (GTP)''' and their derivatives.

 

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. Nitrous oxide is involved in enlargement of the genital organs leading to erection.

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. Nitrous oxide is involved in enlargement of the genital organs leading to erection.

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===Neuropeptides===

==Neuropeptides==

The neuropeptide group of neurotransmitters contain a wide range of molecules of which only the major transmitters are included below. These include; '''enkephalin''', '''subtance P''', '''LHRH''', '''[[vasopressin]]''', '''cholecystokinin/CKK''', '''vasoactive intesinal peptide (VIP)''', '''endorphin''', '''neurotensin''', '''TRH''', '''angiotensin-II''', '''somatostatin''' and '''oxytocin'''. These neuropeptides have a wide range of effects throughout the nervous system. Many of these neuropeptides are released from nerve terminals but also as hormones from endocrine cells, cholecystokinin is an example.

The neuropeptide group of neurotransmitters contain a wide range of molecules of which only the major transmitters are included below. These include; '''enkephalin''', '''subtance P''', '''LHRH''', '''[[vasopressin]]''', '''cholecystokinin/CKK''', '''vasoactive intesinal peptide (VIP)''', '''endorphin''', '''neurotensin''', '''TRH''', '''angiotensin-II''', '''somatostatin''' and '''oxytocin'''. These neuropeptides have a wide range of effects throughout the nervous system. Many of these neuropeptides are released from nerve terminals but also as hormones from endocrine cells, cholecystokinin is an example.

 

'''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 & Physiology|gastrin-stimulated]] gastric acid secretion. 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.

The neuropeptide '''vasopressin''' is responsible for metabolism and maintainance of the metabolic rate.  

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'''Substance P''' is involved in the transmission of pain from peripheral receptors to the central nervous system. It acts to increase the sensation, and therefore the consciousness, of pain and is released when [[Pain|nociceptors]] are activated. '''Enkaphalin''' acts to inhibit the release of substance P therefore acting to diminish the sensation of pain.

'''Substance P''' is involved in the transmission of pain from peripheral receptors to the central nervous system. It acts to increase the sensation, and therefore the consciousness, of pain and is released when [[Pain|nociceptors]] are activated. '''Enkaphalin''' acts to inhibit the release of substance P therefore acting to diminish the sensation of pain.

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Endorphins and enkephalins are both examples of '''[[opioids]]''' and act within neuronal synapses to reduce the sensation of pain acting as natural pain killers. 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. They are produced by the pituitary gland and the hypothalamus and they chemically resemble opiates in their ability to produce analgesia and a sense of well-being. Endorphin has also been shown to stimulate dopaminergic neurones. In addition to this, endorphin can act to inhibit the release of '''substance P''' and therefore decrease the conscious perception of pain.

'''Endorphin''' is part of a family of endogenous opioids which act as natural pain killers. They are produced by the pituitary gland and the hypothalamus and they chemically resemble opiates in their ability to produce analgesia and a sense of well-being. Endorphin has also been shown to stimulate dopaminergic neurones. In addition to this, endorphin can act to inhibit the release of '''substance P''' and therefore decrease the conscious perception of pain.

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'''Cholecystokinin''' or '''CKK''' is secreted as a hormone and is involved in [[Gut Endocrine Function - Anatomy & Physiology|gastric enzyme secretion]]. CKK affects the secretion of pancreatic enzymes but also promotes feelings of satiety within the cortex of the brain following a meal. It is also involved in smooth muscle contraction within the small intestine.  

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'''Cholecystokinin''' or '''CKK''' is secreted as a hormone and is involved in [[Gut_Endocrine_Function_-_Anatomy_%26_Physiology|gastric enzyme secretion]]. CKK affects the secretion of pancreatic enzymes but also promotes feelings of satiety within the cortex of the brain following a meal. It is also involved in smooth muscle contraction within the small intestine.  

'''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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'''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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===Other Types of Neurotransmitter===

==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.

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. Zinc is associated with the release of another type of neurotransmitter, neuropeptide Y.

 

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 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.

==Function==

==Function==

 

[[Category:To Do - Review]]

[[Category:A&P Done]]

[[Category:Nervous System - Anatomy & Physiology]]

[[Category:Nervous System - Anatomy & Physiology]]