Neurotransmitters

Neurotransmitters are chemical messengers produced by neurons and paraneurons which have to travel only 7-40nm through a synaptic cleft in order to reach their target cells. These agents in the nervous system aid the depolarization of postsynaptic terminals in order to facilitate the traffic of impulses across synapses and other neural junctions. There are those synaptic neurotransmitters known from classical times e.g. acetylcholine to include succinylcholine, and amines (noradrenaline, adrenaline). Other recently discovered amines include dopamine, serotonin and histamine. Other non amine transmitters are GABA (gamma aminobutyric acid) and putative neurotransmitters like amino acid glycine, glutamic acid, aspartic acid and taurine. Peptidergic synapses have recently been postulated and substances which mediate them are all polypeptides, e.g. substance P, encephalins and endorphins. Other peptides are opioid peptides, β-endorphins, met-enkephalin, leu-enkephalin, gastrin, secretin, insulin, cholecystokin, oxytocin, vasopressin, hypothalamic release factors TRF, or GnRF and somatostatin. Recently the presence of calcitonin  in various parts of the brain of the lower vertebrates suggest a calcitoningergic synapse which is probably old phylogenetically. Immunoreactivities for gastrin was shown in the pars distalis of the pituitary gland. The previous partition between neurotransmitter and hormone has broken down especially in the nervous system.

The neurotransmitters are in different categories.

  • Acetylcholine:

  • Neuropeptides: These include motilin, bombesin, insulin, calcitonin etc. Some are more of neuromodulators rather than neurotransmitters- e.g. insulin

  • Amine: These include catecolamines such epinephrine and norepinephrine, serotonin, histamine etc

  • Amino acids: GABA- gamma amino butyric acid, glycine

  • Putative neurotransmitters. These include purinergic transmitters such as ATP

Nerves are also classified in relation to their contained neurotransmitters-

  • Cholinergic nerves

  • Purinergic

  • Gabaergic

  • Aminergic

  • Adrenergic etc

But what most neuroscientists are beginning to become aware of is the fact all secretory granules of neurons contain at least one of the above substances. This include- calcium, ATP, a biogenic amine, a neuropeptide, lipid, which are packaged by the Golgi apparatus into one granule. Hence it may not necessarily hold to classify a nerve on the basis of only one of the component neurotransmitter unless it is clear that the neurotransmitter is the main component released from the synaptosome.

Synaptosomes are released directly into the synaptic cleft after they have passed the synaptic grid. They then pass through the synaptic cleft in order to impinge on the postsynaptic membrane, which they then depolarize. This then generates impulse traffic, which continues the transmission to the next axon.

 

 

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