Neurotransmitter

Synapsis allows neurons to communicate with each other, transforming an electrical signal into another chemistry.

A neurotransmitter, neuromediator o second messenger is a biomolecule that allows neurotransmission, that is, the transmission of information from a neuron (a type of cell of the nervous system) to another neuron, to a muscle cell or to a gland, through the synapse that separates them. The neurotransmitter is released from the synaptic vesicles at the extremity of the presynaptic neuron, into the synapse, across the synaptic gap, and acts on specific cellular receptors on the target cell.

Definition of neurotransmitter

Neurotransmitters are biomolecules that meet the following three basic criteria:

1. The substance must be present inside the neurons. A chemical cannot be secreted from a presynaptic neuron unless it is present there.
2. The enzymes that allow the synthesis of the substance should be present in the neurons of the area where the neurotransmitter is found. Since complex biochemical pathways are needed to produce neurotransmitters, the demonstration that the enzymes and precursors necessary to synthesize the substance are present in pre-synaptic neurons provides additional evidence that the substance is used as a neurotransmitter.
3. The effect of the neurotransmitter should be reproduced if the same substance is applied exogenously. A neurotransmitter acts on its target cell (or "dian cell"), through the presence of neurotransmitter-specific receptors. The effect should be identical (action identity) to that of pre-synaptic stimulation.

Difference Between Neurotransmitter and Hormone

A neurotransmitter, when released, only communicates (through synapses) with an immediate neuron; instead, a hormone is capable of communicating with another cell far from it, using the bloodstream to do so. Although some neurotransmitters often act like hormones, they are called neurohormones.

Strictly speaking, according to Roger Guillemin's definition of hormone, a neurotransmitter would be a hormone (of paracrine secretion) released by neurons. Although due to its specific characteristics, the neurotransmitter is often considered a form of cellular communication different from hormones, the distinction between one and the other is blurred:

A hormone is any substance released by a cell acts on another cell, both nearby and distant, and regardless of the singularity or ubiquity of its origin and without taking into account the path used for its transport, be it blood circulation, axoplasmic flow or interstitial space.

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Biochemical processes associated with neurotransmission

• Synthesis of neurotransmitter by the pre-synaptic neurons. Glyal cells are involved. According to the nature of the neurotransmitter, it can be synthesized in the neuronal soma or in the nerve endings. Some neurotransmitters are synthesized directly in nerve endings thanks to enzymes that have been synthesized in the soma and have been transported to these terminations. Through the inside of the axon flows a stream of free substances or enclosed in vesicles, which can be precursors of both neurotransmitters or their enzymes, called axonic flow.

• Storage of the neurotransmitter in synaptic termination vesicles.
• Release of neurotransmitter by exocytosis, which is calcium-dependent. When a nervous impulse reaches the presynaptic neuron, it opens the calcium channels, entering the ion into the neuron and releasing the neurotransmitter into the synaptic space. Calcium also initiates exocytosis, activates the transfer of the vesicles to the places of their release with the help of plasma membrane proteins and the vesicular membrane. When calcium enters in the neuron, an enzyme called calmodulin is activated which is a proteinquinasa, responsible for phosphorylating to sinapsin I, located in the membrane of the vesicles and which binds them to the filaments of actin. When synapsine I is phosphorylated, the synaptic gallbladders take off the actin and mobilize towards the places where they should be emptied. The fusion of the vesicular membrane with the plasma membrane is a complex process in which several proteins such as synaptobrevine, sinaptotagmine, rhab-3 (in the vesicular membrane) sintaxine, SNAP-25, n-sec 1 (in the plasma membrane) and factor sensitive to N-etilmaleimide (NSF) with ATPasa activity. This set of proteins form the SNARE complex that forms a pore in the plasma membrane and allows the fusion of both membranes and the output of the substance as the vesicular content to the synaptic space.

• Activation of neurotransmitter receptor located in the plasma membrane of the post-septic neuron. The post-synaptic receptor is a protein structure that triggers an answer. Neuroreceptors can be:

ionotropic receivers: They produce a quick response by opening or closing ionic channels, which produce depolarizations, generating action potentials, exciting responses, produce hyperpolarizations or inhibitory responses. In the first case, monoionic cation channels such as sodium and potassium are performed, while in the second case, chloride channels are activated.

Metabotropic receptors: They release intracellular messengers such as cyclic AMP, calcium, and phospholipids by the signal transduction mechanism. These second messengers activate kinase proteins, which, fosforilan activating or deactivating channels inside the cell. In the case of a depolarization, are the potassium channels that close, in case of hyperpolarization, the same channels are open producing the increase of intracellular cations.

• Initiation of the actions of the second messenger.

• Inactivation of the neurotransmittereither by chemical degradation or by reabsorption in the membranes. In the synaptic space, there are specific enzymes that inactivate the neurotransmitter. In addition, presynaptic neurons have neurotransmitter receptors that have it recaptan inserting it and soothing it again in gallbladders for its later discharge.

In the nervous system there are two superfamilies of receptors for neurotransmitters, depending on the number of transmenbranal regions they have to receive information. There is a selectivity of a family of receptors for a single neurotransmitter that is only possible by binding to the appropriate membrane.

These two families are:

• The first family: Share the fact of having seven transmenbrand regions, use G protein to and make use of the second messenger (see “G-coated receiver”)

• The second family: Share the common molecular owl of each member with five transmenbrand regions and with several versions of each receiver configured around an ionic channel.

Cerebral action drugs act in one or more of these stage/s.

Classification

Neurotransmitters can be grouped into: neurotransmitters themselves, and neuromodulators. The latter are substances that act in a similar way to neurotransmitters; the difference is that they are not limited to the synaptic space, but rather diffuse through the extra-neuronal fluid, directly intervening in the postsynaptic phase of neurotransmission.

Taking into account their chemical composition, they can be classified as:

• Colinergic: acetylcholine.
• Adrenergic: that are divided in turn into Catalan, example adrenaline or epinefrine, noradrenaline or norepinephrine and dopamine; e indolamines serotonin, melatonin and histamine.
• Amino acid: GABA, taurine, ergotioneine, glycine, beta alanine, glutamate and aspartate.
• Peptideergies: endorphin, encephalin, vasopressin, oxytocin, orexine, neuropeptide Y, substance P, dinorphine A, somatostatin, cholecistoquinin, neurotensin, luteinizing hormone, gastrine and enteroglucagon.
• Free radicals: nitric oxide (NO), carbon monoxide (CO), adenosin trifosphate (ATP) and arachidonic acid.

Neurotransmitter Functioning

The neuron that releases the neurotransmitter is called the presynaptic neuron. The neuron receiving the signal is called a postsynaptic neuron. Depending on the type of receptor, postsynaptic neurons are either stimulated (excited) or de-stimulated (inhibited). Each neuron communicates with many others at the same time. Since a neuron may or may not send a stimulus, its behavior is always based on the balance of influences that excite or inhibit it at any given time. Neurons are capable of sending stimuli several times per second. When a nerve impulse arrives at the end of the axons, a neurotransmitter discharge occurs in the synaptic cleft, which is captured by specific receptors located in the membrane of the postsynaptic cell, which causes depolarization in it, and consequently, a new nerve impulse.

Main neurotransmitters

• Acetylcholine (AC). They are located in:
  • spinal cord neurons → neuromuscular union
  • Baseline Proscencephale → numerous areas of the cortex
  • Interneurons in the striated body
  • Autonomous nervous system → preganglionar neurons of the sympathetic and parasympathetic SNA, and postganglionars of the parasympathetic.

• Dopamine. They are located in:
  • Black substance → central route of the striated body, lymbic system and numerous areas of bark)
  • Circle of the hypothalamus → previous hypophysis through the portal veins

• Noradrenaline (NE). They are located in:
  • Locus coeruleus of protuberance → lipid system, hypothalamus, bark
  • Bulbo raquídeo → locus coeruleus, spinal cord
  • Postganglionar neurons of the sympathetic nervous system

• Serotonin. They are located in:
  • Nucleos of protuberancial rafe → multiple projections
  • Bulbo raquídeo/Protuberance → dorsal or posterior spinal cord

• Acid γ-aminobutyric (GABA). They are located in:
  • Main brain inhibitor neurotransmitter; highly extended cortical interneurons and long projection pathways.

• Glicina. They are located in:
  • Main spinal cord inhibitor neurotransmitter

• Glutamato. They are located in:
  • Main excitatory neurotransmitter; located throughout the CNS, even in cortical pyramidal cells.