There are over 50 neurotransmitters.
Most common and clinically relevant are 6:
Neurotransmitter binds to receptors and activates enzymatic process in the effector tissue by directly opening ion channels or activation of a secondary messenger system.
Include:
Binding of the neurotransmitter
-> Opening of ion channels
-> Changes in membrane potential
OR
-> Changes ionic concentration
Include:
Binding of the neurotransmitter
-> activation of adenyl cyclase
-> catalyze ATP ---> cAMP
-> cAMP leads to protein phosphorylation
-> intracellular effect
Include:
Binding of the neurotransmitter
-> release of diacylglycerol and inositol triphosphate
-> protein phosphorylation
and increased intracellular [Ca2+]
-> intracellular effect
Two main neurotransmitter for ANS: acetylcholine (ACh) and norepinephrine
Autonomic neurons that secretes acetylcholine
-> cholinergic
Autonomic neurons that secretes norepinephrine or epinephrine
-> adrenerigc
Cholinergic receptors are of two types:
All ANS preganglionic neurons (sympathetic and parasympathetic)
-> cholinergic (synapse with nicotinic receptors)
All postganglionic parasympathetic neurons
-> cholinergic (synapse with muscarinic receptors)
All postganglionic sympathetic neurons
-> adrenergic
Exception:
Postganglionic sympathetic neurons in sweat glands, piloerector muscles, and some blood vessels are cholinergic (synapsing with muscarinic receptors).
Neuromuscular junction (somatic nervous system)
* neurotransmitter - acetylcholine
* receptor - nicotinic
Acetylcholine and norepinephrine - both synthesized in the terminal nerve ending.
Synthesis - combination of acetyl-CoA with choline (in terminal nerve ending)
Storage - stored in vesicles (where it is protected from degradation)
Release - into the synaptic cleft
Binding - postsynaptic receptor activated when bound
Degradation - hydrolysed by cholinerase in the synaptic cleft
Recycle - choline (not acetylcholine) is taken up by neuron
Synthesis - tyrosine -> DOPA -> dopamine
Storage - stored in vesicles (dopamine -> norepinephrine)
-> In adrenal medulla, 80% of norepinephrine is converted into epinephrine
Release - Influx of calcium causes vesicles to fuse with membrane
Binding - postsynaptic receptor activated when bound
Removal - Norepinephrine is rapidly taken up into neuron
Metabolism - excess norepinephrine is
-> methylated by catechol-O-methyltransferase (COMT) in the cleft, OR
-> oxidised by monoamine oxidase (MAO) in neuronal mitochondria
-> inactive products are excreted in urine as:
* vanillylmandelic acid (VMA)
* metanephrine
* normetanephrine
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Created | 20040512 |