Nervous system

The nervous system consists of the central nervous system, in turn made up of the brain and spinal cord, and the peripheral nervous system, consisting of the autonomic or vegetative nervous system and the somatic nervous system.

The peripheral nervous system, through the efferent neurons, picks up the different internal and external stimuli coming from the periphery and transmits them to the central nervous system (afferent component of the SNP); here the signals are integrated and a response is processed, which will be conducted again to the periphery by means of sympathetic and parasympathetic neurons (efferent component of the SNP).

The peripheral system, otherwise called autonomous or vegetative, is able to perceive the stimuli and transmit the respective signals regardless of the state of consciousness; that is, this system regulates physiological functions that are not perceived by man: for example, a change in pressure at the level of the blood vessels is picked up by the baroreceptors at the level of the aorta and signaled by the efferent vascular neurons to the CNS, where a suitable response is processed for accommodate the degree of contraction or distension of the vascular smooth muscle and restore the pressure to physiological parameters. Generally all these actions are not perceived by man, except that they are not such pressure changes as to cause a very specific symptomatology. In summary, we will say that the autonomic peripheral nervous system regulates physiological functions not perceived by the individual's conscience, but necessary. to its survival.

The somatic nervous system, on the other hand, regulates voluntary functions; in fact it is made up of motor neurons, ie those neurons that innervate all skeletal muscles. There is also the enteric nervous system, which regulates the physiological functions of the gastro-intestinal system.
The vegetative nervous system is divided into: Parasympathetic and Orthosympathetic (otherwise known as sympathetic); generally these two systems regulate the same organ in the opposite direction, it is said that they are "opposed". Para and orthosympathetic control: the entire smooth muscle (bronchial, gastrointestinal, vascular, genitourinary tract muscles), glandular secretions, heart function and some phases of glucose and lipid metabolism.

Via effector of the para and orthosympathetic system.

The transmission of the impulse from the central nervous system to the peripheral organs occurs through the efferent neurons: the impulse, via the pre-ganglionic efferent neuron, reaches the para or ortho sympathetic ganglion (neuronal agglomerate), from which the post- ganglion that conducts the signal up to the receptors of the effector organ (organ from which the impulse originated and which expresses the physiological response to the signal processing).
The parasympathetic sector is also called the cranio-sacral sector, because its pre-ganglion fibers depart from the cranial nerves and reach the sacral area of the spinal cord; the orthosympathetic sector is instead called thoraco-lumbar sector, because its pre-ganglion fibers start from the first thoracic segment and reach the third lumbar vertebra. The pre-ganglionic fibers of the orthosympathetic are short, because their ganglia are placed in an anatomically close position: they are concatenated to each other and run parallel to the spinal cord; instead, long post-ganglionic fibers radiate from the ganglia, since they must get to all organs.

From the craniosacral section of the spinal cord the pre-ganglion fibers of the parasympathetic depart, in particular from the section of the skull they depart: the oculo-motor nerve that regulates ocular function, the facial and glosso-pharyngeal nerve that regulate the glands at the level of the skull, and the vagus nerve that controls cardiac, respiratory and gastrointestinal function; while the nerve that controls the genitourinary tract starts from the sacral area. The ganglia of the cranio-sacral sector are located in the immediate vicinity of the effector organ, if not on the organ itself, therefore the pre-ganglion fibers in this case will be very long while the post-ganglion fibers are extremely reduced.

The two systems act in the opposite direction on the different organs, thus balancing their functions: for example the ocular system is induced by the orthosympathetic system to contract the radial muscle and this causes a dilation of the pupil (mydriasis); vice versa, the parasympathetic induces the contraction of the sphincter of the iris with consequent narrowing of the pupil (miosis); moreover, the parasympathetic plays an important role in near vision, since it causes the contraction of the ciliary muscle which leads to a rearrangement of the lens. level of the respiratory system, the orthosympathetic system causes the relaxation of the bronchial muscles, while the parasympathetic system causes the contraction and increase of secretions.

The heart rate and the force of contraction of the heart are amplified by the orthosympathetic, in the opposite direction the parasympathetic acts. Thus also at the level of the gastrointestinal tract: the orthosympathetic acts by reducing motility and inducing contraction of the sphincters; on the contrary, the parasympathetic increases the motility and tone of the intestinal wall. The retention of urine is due to the relaxing action of the orthosympathetic on the extruder muscle, and contracting on the sphincter; while the parasympathetic causes relaxation of the sphincter and contraction of the extruder muscle, facilitating urination. At the level of the female genital apparatus there is a prevalence of the orthosympathetic, which determines a distension of the uterine muscles. The control over the musculature of the vessels is very varied, in general the parasympathetic determines vasodilation on the vessels of the skeletal muscles, while the orthosympathetic determines contraction on the cutaneous vessels, mucous membranes and viscera. "ejaculation by the orthosympathetic" and "erection by the parasympathetic". Sometimes some organs and / or systems are predominantly controlled by one of the two systems, such as the kidney, the adrenal medulla, the pilo-motor muscles and sweat glands, which are almost exclusively regulated by the sympathetic.