Health & Medicine
NEUROTRANSMITTERSDr. LAKSHMI PAVANI P. (PT)
DEFINITION- Neurotransmitter is a chemical substance that acts as the mediator for the transmission of nerve impulses from one neuron to another neuron through synapse. It a messenger of neurologic information from one cell to another.HISTORY-Evidence of neurotransmitter was first discovered by an Australian scientist named Otto Loewi in 1921. He dreamt of an experiment, which he did practically and came out with this discovery.
CRITERIA FOR NEUROTANSMITTER
Nowadays, many substances are categorized as neurotransmitters. To consider a substance as a neurotransmitter, it should fulfill certain criteria as given belowThe substance must be found in a neuronIt must be produced by a neuronIt must be released by a neuronAfter release, it must act on a target area and produce some biological effect.After the action it must be inactivated
CLASSIFICATION OF NEUROTANSMITTERS
DEPENDING UPON CHEMICAL NATURE- Amino Acids- The neurotransmitters of this group are involved in fast synaptic transmission and are inhibitory and excitatory in action. Eg. GABA, glycine, glutamate and aspartate. Amines- Amines are the modified amino acids. The neurotransmitters of this group involve in slow synaptic transmission and are inhibitory and excitatory in action. Eg. Noradrenaline, adrenaline, dopamine, serotonin and histamine.Others- the one which do not fit in any of these categories. Eg. Acetyl choline and nitric oxide.
DEPENDING UPON FUNCTION
Excitatory Neurotransmitters:It is the chemical substance which is responsible for the conduction of impulses from presynaptic neurons to post synaptic neurons. The neurotransmitter released from the presynaptic axon terminal does not cause development of action potential in the post synaptic neuron. Rather, it causes some changes in the resting membrane potential- slight depolarization by the opening of sodium channels in the post synaptic membrane and the influx of sodium ions from ECF. The slight depolarization is called excitatory post synaptic potentials (EPSP).EPSP in turn causes development of action potential in the initial segment of the axon of the postsynaptic neuron. The common excitatory neurotransmitters are acetylcholine and noradrenaline.
Inhibitory Neurotransmitters: It is the chemical substance which inhibits the conduction of impulses from the presynaptic neuron to the postsynaptic neuron. When it is released from the presynaptic axon terminal due to the arrival of action potential, it causes opening of potassium channels in the postsynaptic membrane and efflux of potassium ions. This leads to hyperpolarization which is called inhibitory postsynaptic potentials (IPSP). When IPSP is developed, the action potential is not generated in the postsynaptic neuron. The common inhibitory neurotransmitters are gamma amino butyric acid (GABA) and dopamine.
TRANSPORT AND RELEASE OF NEUROTRANSMITTERS
The neurotransmitter is produced in the cell body of the neuron and is transmitted through the axon. At the axon terminal, the neurotransmitter is stored in small packets called vesicles. Under the influence of a stimuli, theses vesicles open and release the neurotransmitter into the synaptic cleft. It binds to specific receptors on the surface of the postsynaptic cell. The receptor G proteins, protein kinase or ligand- gated receptors.
INACTIVATION OF NEUROTRANSMITTERS
After the execution of the action, neurotransmitter is inactivated by four mechanisms:It diffuses out of the synaptic cleft to the area where it has no action.It is destroyed or disintegrated by specific enzymes It is engulfed and removed by astrocytesIt is removed by means of reuptake into the axon terminal.
REUPTAKE OF NEUROTRANSMIITERS
Reuptake is a process by which the neurotransmitter is taken back from synaptic cleft into the axon terminal after execution of its action. The reuptake process involves a specific carrier protein for each neurotransmitter.
AMINO ACIDSGROUPSNAME SITE OF SECRITIONACTIONAmino acidsGABACerebral cortex, cerebellum, basal ganglia, spinal cord and retinaInhibitoryAmino acidsGlycineForebrain, brainstem, spinal cord and retinaInhibitoryAmino acidsGlutamateCerebral cortex, brainstem and cerebellumExcitatoryAmino acidsAspartateCerebellum, spinal cord and retinaExcitatory
AMINESGROUPSNAMESITE OF SECRITIONACTIONAminesNoradrenalinePostganglionic adrenergic sympathetic nerve endings, cerebral cortex, hypothalamus, basal ganglia, brainstem and spinal cordExcitatory and InhibitoryAminesAdrenalineHypothalamus, thalamus and spinal cordExcitatory and InhibitoryAminesDopamineBasal ganglia, hypothalamus, limbic system, neo cortex, retina and synaptic ganglia.InhibitoryAminesSerotoninHypothalamus, limbic system, cerebellum, spinal cord, retina, GI tract, lungs and platelets.InhibitoryAminesHistamineHypothalamus, cerebral cortex, GI tract and mast cellsExcitatory
OTHERSGROUPSNAME SITE OF SECRITIONACTIOPNOthersNitric oxideMany parts of CNS, neuromuscular junction and GI tractExcitatoryOthersAcetylcholinePre ganglionic parasympathetic nerve endingsPost ganglionic parasympathetic nerve endingsPre ganglionic sympathetic nerve endingsPost ganglionic sympathetic cholinergic nerve endingsNeuromuscular junction, cerebral cortex, hypothalamus, basal ganglia, thalamus and retinaExcitatory