Sympathetic (or thoraco-lumbar) system
The sympathetic (SNS) is one of the two branches of the autonomic or vegetative nervous system (ANS), which intervenes in the control of involuntary bodily functions.
The sympathetic system has a stimulating, exciting, contracting function; as shown in the figure, it presides over the attack and flight adaptation system, preparing the organism to face the danger. A violent and unexpected noise in the dark, the cry of a friend jokingly appeared suddenly, are examples of stressful situations that lead to a massive activation of the sympathetic system. In a few moments the heart increases the force and the contractile frequency, the bronchi, the pupil and the blood vessels of the appendicular muscles and the coronary system dilate, while glycogenolysis is stimulated in the liver. , again in order to prepare the body for the imminent physical activity, the digestive processes are significantly slowed down, while the cutaneous and peripheral blood vessels are constricted and the arterial pressure increases. The bladder relaxes, while the sphincter narrows (inhibits the urination).
However, the sympathetic nervous system is not always activated in such a massive way. Normally, in fact, it contributes to the "homeostasis" of the organism by opposing the diametrically opposed actions of the parasympathetic nervous system (which promotes rest, calm and digestion).
The nerves of the sympathetic system are distributed to blood vessels, sweat glands, salivary glands, heart, lungs, intestines and numerous other organs. Unlike in the somatic (voluntary) nervous system, the impulses of the system vegetative they reach the viscera through two neurons, the first of which is located in the central nervous system and the second in the peripheral nervous system.
In particular, as regards the sympathetic system, the myelinated nerve fibers of the first neuron (called NEURON PREGANGLIARE) originate from the dorsal and lumbar tracts of the spinal cord (between T1-T12 and L1-L3, in the gray matter located between the anterior horns and the posterior horns).The axons of these preganglionic neurons come out of the medulla with the ventral roots, become part of a spinal nerve and through the white (myelinated) communicating branches lead to the ganglia of the paravertebral ganglionic chain (also known as the sympathetic trunk or chain), placed on the sides of the medulla itself. At this level they contract synapses with the cell bodies of the POSTGANGLIAR NEURONS.
Some preganglionic fibers cross without interruption the paravertebral ganglia and continue in splanchnic branches that from the sympathetic chain lead to the prevertebral ganglia (such as the mesenteric and celiac) placed in front of the column.
From the paravertebral and prevertebral ganglia, the unmyelinated nerve fibers (axons) of the second neuron (postganglionic) depart and reach the target organs.
The sympathetic fibers that innervate the adrenal medulla are an exception to this double neuron rule (in this case there are no pre and post-ganglion fibers but a single neuron).
The neurotransmitter characteristic of the preganglionic neuron of the sympathetic is acetylcholine, while the postglanglionic one exploits noradrenaline and adrenaline (produced by the adrenal medulla and stimulated by the acetylcholine released by the single neuron): for this reason the sympathetic nervous system is also known as an adrenergic system.