Generality
The spinal nerves are the nerves of the peripheral nervous system that originate from the combination of the dorsal roots with the ventral roots of the spinal cord.
The spinal nerves are altogether 31 pairs; of these, 8 are born from the cervical sector of the spinal cord (cervical spinal nerves), 12 are born from the thoracic sector of the spinal cord (thoracic spinal nerves), 5 are born from the lumbar sector of the spinal cord (lumbar spinal nerves), 5 are born from the sacral sector spinal cord (sacral spinal nerves) and one arises from the coccygeal sector of the spinal cord (coccygeal spinal nerves).
A brief review of what a nerve is
Nerves are important structures of the nervous system, resulting from the combination of a group of axons and having the important task of spreading the impulses necessary for muscular movement, transporting sensitive information and controlling reflex responses.
An axon is the characteristic extension of neurons (ie the cells of the nervous system), which serves to spread nerve signals even at long distances.
The human nervous system includes 3 types of nerves:
- The efferent nerves (or motor nerves ), which carry information from the central nervous system (CNS) to the so-called periphery. These nerves control the movements of the muscles, so I am in charge of the motor sphere.
- Afferent nerves (or sensory nerves ), which carry information from the periphery to the central nervous system. These nerves have the task of picking up what happens on the skin surface, on the organs, etc. and communicate it to the SNC, waiting for a reply.
The afferent nerves are at the head of the sensitive sphere.
- Mixed nerves, which play the role of both efferent nerves and afferent nerves.
What are spinal nerves?
The spinal nerves are the nerves that, in groups of two (therefore in pairs), originate from the spinal cord .
Spinal nerves are mixed nerves, so they group together both the axon of neurons with motor function and the axon of neurons with sensory function.
Spinal Nerves and Peripheral Nervous System
The spinal nerves contribute, together with the cranial nerves, to the constitution of the so-called peripheral nervous system ( SNP ).
The peripheral nervous system is the set of nerves responsible for controlling the communication between the organs of the central nervous system (ie between brain and spinal cord ) and providing the passage of information between the central nervous system and the periphery (ie muscles, skin surface, internal organs, glands etc.).
Anatomy
Before proceeding with the anatomical description of the spinal nerves, it is necessary to review the spinal cord.
- The spinal cord is the nervous structure which, together with the brain, constitutes the central nervous system.
- The spinal cord takes place inside the spinal column, in a space called the vertebral canal, which is the result of the superposition of the vertebral holes present in the individual vertebrae .
- The spinal cord is a cord that can be ideally divided into 5 large portions (the cervical, the thoracic, the lumbar, the sacral and the p. Coccygeal), which, in turn, can be divided into short traits, called segments .
- From this meticulous subdivision of the spinal cord it appears that the latter comprises 31 total segments: 8 segments, in the cervical portion (cervical segments), 12 segments, in the thoracic portion (thoracic segments), 5 segments, in the lumbar portion (lumbar segments), another 5 segments, in the sacral portion (sacral segments), and a segment, in the coccygeal portion (coccygeal segment).
- To characterize each spinal cord segment is the presence of a couple of emergencies on the abdominal half, called ventral roots, and a pair of emergencies on the half closest to the back, called dorsal roots .
- Located one on the right and one on the left, the ventral roots are the emergencies that group the axons of the motor neurons constituting, within the spinal cord, the gray matter areas known as ventral horns .
- Placed one on the right and one on the left, the dorsal roots are, on the other hand, the emergencies that enclose the axons of sensory neurons forming, within the spinal cord, the areas of gray matter known as dorsal horns .
Each spinal nerve is the result of the convergence, in a single structure, of the axons of a ventral root with the axons of the dorsal root present on the same side of the same segment of spinal cord.
In each spinal nerve, the ventral root represents its axonal motor component, while the dorsal root represents its sensory axonal component.
Since the spinal cord segments are 31 and because for each segment there are a pair of ventral roots and a pair of dorsal roots, the spinal nerves appear to be a total of 31 pairs .
Nomenclature of Spinal Nerves
To facilitate consultation, anatomists have agreed to distinguish spinal nerves based on their correspondence with segments of the bone marrow. From this it emerged that there are:
- 8 pairs of cervical spinal nerves (because 8 are the cervical bone marrow segments);
- 12 pairs of thoracic spinal nerves (because 12 are the thoracic segments of bone marrow);
- 5 pairs of lumbar spinal nerves (because 5 are the lumbar segments of bone marrow);
- 5 pairs of sacral spinal nerves (because 5 are the sacral segments of bone marrow);
- A pair of coccygeal spinal nerves (because one is the coccygeal segment of bone marrow).
The name of each pair of spinal nerves corresponds to the initial letter, in uppercase, of the term that identifies the portion of the spinal cord to which it belongs, plus the number indicating the segment of origin.
From all this it follows that:
- The 8 pairs of cervical spinal nerves are called C1, C2, C3, C4, C5, C6, C7 and C8;
- The 12 pairs of thoracic spinal nerves are called T1, T2, T3, T4, T5, T6, T7, T8, T9, T10, T11 and T12;
- The 5 pairs of lumbar spinal nerves are called L1 L2 L3 L4 and L5;
- The 5 pairs of sacral spinal nerves are called S1, S2, S3, S4 and S5;
- The only pair of coccygeal spinal nerves is called Co1.
Did you know that ...
After the 31st pair of spinal nerves there would be another two pairs of spinal nerves, but these do not fit into the total calculation of the spinal nerves, as they are rudimentary structures, which have no function.
Spinal nerves: the beginning of their course
Shortly after their origin, all the spinal nerves, excluding the first pair of cervical nerves (C1), pass through the intervertebral holes, ie the characteristic lateral openings of the vertebral column resulting from the superposition of the vertebrae.
In such a context, the exception represented by the first pair of cervical nerves passes in a space delimited by the atlas (first cervical vertebra), inferiorly, and by the occipital bone, superiorly.
The removal from the spinal cord by the spinal nerves sees the latter passing above the corresponding vertebrae (*), exclusively with the cervical component, and inferiorly to the corresponding vertebrae, with the remaining components (therefore with the thoracic, lumbar spinal nerves, sacral and coccygeal).
* What are the vertebrae corresponding to the spinal nerves?
The vertebra corresponding to a certain pair of spinal nerves is the vertebra which, with this pair of spinal nerves, shares the identification abbreviation (ie the name) as well as a certain closeness relationship.
Except in special cases, this means that the vertebrae corresponding to the cervical spinal nerves are the cervical vertebrae, the vertebrae corresponding to the thoracic spinal nerves are the thoracic vertebrae, the vertebrae corresponding to the lumbar spinal nerves are the lumbar vertebrae, the vertebrae corresponding to the spinal nerves the vertebrae forming the sacrum are sacral and, finally, the vertebrae corresponding to the coccygeal spinal nerves are the vertebrae forming the coccyx.
In simpler words, the cervical spinal nerves pass above the corresponding vertebrae, while the thoracic, lumbar, sacral and coccygeal spinal nerves pass below the corresponding vertebrae.
As for the passage through the intervertebral holes, also in this situation it is necessary to signal an exception: the eighth pair of cervical spinal nerves .
The C8 spinal nerves are the only ones that do not have a corresponding vertebra (there is no C8 cervical vertebra), therefore they are forced to "adopt", in replacement, the first thoracic vertebra (T1).
This is why in the representations of the spinal nerves and corresponding vertebrae, the eighth pair of cervical spinal nerves locate above the thoracic vertebra T1 (it is therefore not a graphic error).
Curiosity
The last spinal nerves of each spinal cord macrosector (therefore C8, T12, L5 and S5) pass through the intervertebral holes which can be defined as promiscuous, ie resulting from the superposition of the last vertebra of a certain tract (*) of the vertebral column with the first vertebra of the immediately following spine segment.
The above is curious, as for all the previous spinal nerves (except for the spinal nerves C1) the passage through the intervertebral holes occurs between two vertebrae belonging to the same spinal tract (for example, the thoracic spinal nerves T1, T2, T3 etc. up to T11 cross the intervertebral holes belonging to the thoracic tract of the vertebral column).
* NB: the vertebral column tracts are, from the top, the cervical tract, the thoracic tract, the lumbar tract, the sacral tract and the coccygeal tract, and are delimited by the homonymous vertebrae.
Nervi Spinali: how they continue
Shortly after crossing the intervertebral holes, the spinal nerves are subdivided into branches.
Always with a mixed character (that is, at the same time endowed with sensory axons and motor axons), these branches consist of:
- The so-called ventral branch (or anterior branch ). It is the largest branch of the spinal nerves. Its job is essentially to innervate the skin and muscles of the front part of the human body, including the limbs;
- The so-called dorsal branch (or back branch ). It is the second largest branch of the spinal nerves. Its function is, essentially, to innervate the skin and muscles of the posterior part of the human body;
- The meningeal branches . They are the branches of the spinal nerves deputed to innervate the ligaments of the vertebral column, the blood vessels of the vertebrae and the spinal cord, the dura mater that protects the spinal cord, the intervertebral discs and the facet joints and the periosteum of the vertebrae.
In order to fulfill their tasks, the meningeal branches "come back", in a sense, from where the corresponding spinal nerves originated;
- The communicating branches . They are the branches of the spinal nerves that innervate the viscera, both from a sensory and a motor point of view.
Nervi Spinali and Plessi Nervosi
With the branch called ventral branch, different groups of spinal nerves give life to the so-called nervous plexuses .
In neurology, the term nerve plexus identifies a network formation of ventral branches belonging to a group of spinal nerves.
In the human body, the nerve plexuses resulting from the combination of the ventral branches of several spinal nerves are altogether 4 and are named: cervical plexus, brachial plexus, lumbar plexus and sacral plexus .
CERVICAL PLEXUS
The cervical plexus is the plexus resulting from the union of the ventral branch of the first four cervical spinal nerves.
Branches that innervate the skin (skin branches) and branches that innervate muscles (muscle branches) originate from the cervical plexus.
Among the skin branches, there are: the great auricular nerve, the transverse cervical nerve, the small occipital nerve and the supraclavicular nerves.
Among the muscular branches, instead, fall: the cervical loop, the phrenic nerve and the so-called segmental branches.
BRACHIAL PLEXUS
The brachial plexus is the plexus resulting from the combination of the ventral branch of the last four cervical spinal nerves (spinal nerves C5, C6, C7 and C8) and of the first thoracic spinal nerve (spinal nerve T1).
From the brachial plexus originate 5 important branches, with both sensory and motor functions, which are: the musculocutaneous nerve, the axillary nerve, the median nerve, the ulnar nerve and the radial nerve .
LOMBAR PLEXUS
The brachial plexus is the plexus fruit of the combination of the ventral branch of the last thoracic spinal nerve (T12) and of the first four lumbar spinal nerves (L1, L2, L3 and L4).
The lumbar plexus is subdivided into various branches with sensory and motor or just sensory function, whose names are: ileoipogastric nerve (mixed), ileoinguinal nerve (mixed), genitofemoral nerve (mixed), femoral cutaneous nerve (sensory), the obturator nerve (mixed) and the femoral nerve (mixed).
SACRAL PLEXUS
Including two minor nerve plexuses, called plexus plexus and coccygeal plexus, the sacral plexus is the plexus deriving from the combination of the ventral branch of the last two lumbar spinal nerves (L4 and L5), of the sacral spinal nerves and of the coccygeal nerve.
Numerous branches originate from the sacral plexus, including: the superior gluteal nerve (motor), the inferior gluteus nerve (motor), the posterior cutaneous femoral nerve (sensory), the nerves responsible for innervating the piriformis muscles, the internal and square obturator of the femur, sciatic nerve (mixed), pudendal nerve (mixed) and coccygeal nerve (mixed).
Spinal Nerves and Dermatomers
Each spinal nerve innervates a specific area of skin in the human body.
The circumscribed area of skin that receives innervation from a specific spinal nerve is called a dermatome .
Distinguishable in cervical, thoracic, lumbar and sacral, dermatomers are the skin areas where the spinal nerves carry out their sensitive functions.
Did you know that ...
The name of a dermatome corresponds to the name of the associated spinal nerve. For example, the dermatomere associated with the I sacral spinal nerve (spinal nerve S1) is called the dermatomer S1.
Function
As stated on more than one occasion, the spinal nerves are mixed nerves, therefore they perform sensitive functions - which means that they control, for example, skin sensitivity - and motor functions - which means, instead, that they control the movement of the muscles.
Obviously, spinal nerves have their own area of expertise, which:
- For the cervical spinal nerves, corresponds to: back of the head, neck, shoulders, outer part of the upper limbs and diaphragm muscle;
- For the thoracic spinal nerves, it corresponds to: inner part of the upper limbs, thorax, abdomen and middle part of the back;
- For the lumbar spinal nerves, it corresponds to: lower part of the back, anterior part of the lower limbs, external part of the thighs and calves, and upper surface and lower surface of the feet
- For the sacral spinal nerves, it corresponds to: genital region and anal region, posterior part of thighs and legs, and external portion of the feet.
- For the coccygeal spinal nerves, it is limited to the coccygeal region.
diseases
Among the spinal nerve pathologies, the irritative or inflammatory phenomena borne by them, deriving from compressions or actual injuries, are reported in particular.
Medical conditions related to the spinal nerves and which are surely known to most people are peripheral neuropathy, sciatica and cruralgia .
