Nervous system and neurotransmitters

In the past the orthosympathetic system was called "ergo tropico"; its activation, in fact, determines a waste of energy readily made available by the degradation of glycogen into glucose, by the hydrolysis of lipids and by the acceleration of cardiac activity; in this way the organism prepares itself to react to a condition of severe stress, trauma, sudden changes in temperature or severe physical exertion ("fight or flight reaction"). This immediate response to an unfavorable condition is possible because the sympathetic person generally carries out its action in a widespread manner.

The parasympathetic system was called "trophotropic" because, unlike the orthosympathetic, it is activated in conditions of recovery or rest and digestion by the organism; therefore, this system plays a role of fundamental importance for digestive functions, for the recovery of energy reserves and for the restoration of physiological pressure and heart conditions. The response consequent to the activation of the parasympathetic is called “sectorial type”, ie it affects a "localized area" of the organism. The parasympathetic, with its trophotropic activity, is therefore responsible for maintaining the vital functions of the organism.

In physiological conditions, the ortho and parasympathetic functions are in equilibrium, and any situations of slight imbalance are physiologically corrected through "high reflex mechanisms", aimed - depending on the case - to respectively increase or decrease the "ortho action. and parasympathetic.

An example can be the common pressure drop: the vascular baroreceptors perceive this lowering and transmit the signal to the vasomotor centers at the level of the brain, where the response consisting in a reduction of the parasympathetic activity is processed (remember in fact that this system causes reduction of "cardiac activity and vasodilation) and in the strengthening of the" orthosympathetic "activity, which increases the degree of contraction of the vascular smooth muscle, bringing the pressure back to physiological values. other; the administration of certain drugs corrects this imbalance.
The transmission of the impulse in the efferent pathways is mediated by CHOLINERGIC pre-ganglionic neurons, regardless of whether they are of the ortho or parasympathetic: that is, they release the neurotransmitter Acetylcholine (Ach) at the synaptic level. The Ach interacts with the nicotinic channel receptors present on the ganglia; the receptors thus activated send the impulse to the post-ganglionic fibers, which reach the effector organ releasing: those belonging to the parasympathetic neurotransmitter acetylcholine and those belonging to the orthosympathetic Noradrenaline (Nor).
The somatic innervation, which controls all the skeletal muscles, has neuronal fibers without ganglia, originating from the spinal cord (spinal motor neurons), but also cholinergic; the latter interact with "muscle" nicotinic receptors, so called because they are located on skeletal muscles. Muscle nicotinic receptors are different from nicotinic receptors present on ganglia, therefore drugs that act on these receptors must have a selective action, otherwise there would be a risk to compromise the entire pre-ganglionic sympathetic transmission. A separate discussion must be made for the adrenal medulla, whose sympathetic innervation differs from all other organs because it lacks the post-ganglionic neuron; in other words, the pre-ganglionic neuron ganglionic release Ach directly on the nicotinic receptor present in the adrenal medulla, which will release the neurotransmitter Adrenaline directly into the blood stream, through which it reaches its active sites by interacting with the adrenergic receptors.

Tourn-over of the neurotransmitters of the ortho and parasympathetic systems

ACETYLCHOLINE: it is synthesized inside the nerve termination by the interaction of choline with acetyl-coenzyme A, stored in vesicles and released following depolarization of the cell membrane (opening of voltage-gated calcium channels) by incorporation of the vesicle with the Wall. Released in the intercellular space, acetyl-choline interacts with the post-synaptic receptors of the muscle or neuronal cell, to which it remains bound for the time necessary for the transmission of the impulse; subsequently it is untied and is again degraded by suitable esterases to choline and acetic acid. This biological path can be modified by exogenous substances, such as botulinum toxin, which blocks the release of Ach at the synaptic level, and the black widow's venom, which instead causes a continuous release.
CATECOLAMINS (adrenaline, noradrenaline and dopamine): synthesized within the orthosympathetic post-ganglionic nerve endings by transformation of the amino acid tyrosine into dopa by the tyrosine-hydroxylase enzyme, and subsequently into dopamine by the dopa-decarboxylase enzyme ; dopamine is stored in the synaptic vesicles and eventually further transformed into noradrenaline.

 

 

Dopamine itself can act as a neurotransmitter, in which case we are talking about dopaminergic neurons, which are located above all at the level of the CNS. The vesicles containing the neurotransmitter migrate to the cell membrane following depolarization and release noradrenaline at the synaptic level, where it interacts with the respective receptors. After carrying out its function, noradrenaline is taken up by the nerve endings and degraded by specific enzymes, called mono-amino-oxidase or MAO. In minimal part, at the synaptic level, noradrenaline can undergo the action of COT (catecholamin transferase).

 

Other articles on "Neurotransmitters of the parasympathetic and orthosympathetic nervous system"

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