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Biogenic amines are synthesised from only several types of amino acids and dependant on which amino acids are used in their formation depends on their classification. Biogenic amines that are derived from the amino acid tyrosine are classified as '''catecholamines''' and include the SMTs '''norepinephrine (NE)''' (noradrenaline), '''epinephrine (E)''' (adrenaline), '''melatonin''' and '''dopamine (DA)'''. Adrenergic neurons release norepinephrine. The biogenic amine that is dervied 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.   

Biogenic amines are synthesised from only several types of amino acids and dependant on which amino acids are used in their formation depends on their classification. Biogenic amines that are derived from the amino acid tyrosine are classified as '''catecholamines''' and include the SMTs '''norepinephrine (NE)''' (noradrenaline), '''epinephrine (E)''' (adrenaline), '''melatonin''' and '''dopamine (DA)'''. Adrenergic neurons release norepinephrine. The biogenic amine that is dervied 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.   

====Other SMTs====

====Other SMTs====

Other common SMTs include '''acetylcholine (ACh)''', '''ATP''' and '''nitric oxide (NO)'''. Ach is the most common excitatory neurotransmitter in the peripheral nervous system. Cholinergic neurons release ACh and for example, are found in the [[Muscles_-_Anatomy_%26_Physiology#Neurogenic_Contraction|neuromuscular junction]]. '''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 it's own right if the post-synpatic 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.

Other common SMTs include '''acetylcholine (ACh)''', '''ATP''' and '''nitric oxide (NO)'''. Ach is the most common excitatory neurotransmitter in the peripheral nervous system. Cholinergic neurons release ACh and for example, are found in the [[Muscles_-_Anatomy_%26_Physiology#Neurogenic_Contraction|neuromuscular junction]]. When '''ACh''' 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 '''ACh''' makes it easier for the cell to reach it's depolarisation threshold and generate an action potential.

'''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 it's own right if the post-synpatic 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.

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Although NO is a neurotransmitter, it's 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 NO is a neurotransmitter, it's 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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===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''', '''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 of a neuropeptide that acts as both a neuropeptide but also as a hormone.

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''', '''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 of a neuropeptide that acts as both a neuropeptide but also as a hormone.