por What are spinal nerves?
The spinal or spinal nerves They are those that are born in the spinal cord and reach different areas through the vertebral muscles. They belong to the peripheral nervous system, and include a total of 31 pairs of nerves that innervate the entire body, except for the head and some parts of the neck.
Of the 31 pairs that make up the set of spinal nerves, there are 8 cervical, 12 dorsal, 5 lumbar, 5 sacral, and one coccygeal pair. They all have a mixed function, that is, they are sensitive and motor, carrying information both from the spinal cord and towards it.
The spinal nerves are numbered from top to bottom, naming them according to the region of the body in which they are located. The two roots of each one of them have their origin in the spinal cord, having a posterior sensory one and an anterior motor one. Both unite to form the trunk of the spinal nerve, which passes through an intervertebral foramen.
pairs of nerves
Spinal nerves are divided into five groups. Each of them is related to an area of the spine, and their names have their origin in the vertebrae from which they arise.
cervical nerves
Cervical nerves are spinal nerves whose origin is in the cervical segment of the vertebral column. Although there are only seven cervical vertebrae (C1-C7), there are eight such nerves (C1-C8).
All of them, except C8, arise above their corresponding vertebra, the latter emerging below C7.
This makes them nerves different from the rest, since the others emerge below the vertebrae that give them their names. The posterior distribution includes the suboccipital nerve (C1), the greater occipital nerve (C2), and the third occipital nerve (C3).
On the other hand, the anterior distribution includes the cervical plexus (C1-C4) and the brachial plexus (C5-T1). The cervical nerves, on the other hand, innervate muscles such as the sternohyoid, sternothyroid, and omohyoid.
thoracic nerves
The thoracic nerves are the twelve spinal nerves that originate from the thoracic vertebrae. Each of them comes out below its corresponding vertebra: thus, the T1 has its origin below that of the same name. Its branches go directly to the paravertebral ganglia, which are part of the autonomic nervous system.
The thoracic nerves are involved in the functions of organs and glands in the head, neck, thorax, and abdomen. On the other hand, there are several divisions that it is important to take into account when studying them.
In the anterior divisions, the intercostal nerves come from nerves T1 through T11, and pass between the ribs. At T2 and T3, other branches form the intercostobrachial nerve. The subcostal nerve arises from T12, and passes below the twelfth rib.
As for the posterior divisions, the medial branches of the posterior rami of the six superior thoracic nerves pass between the dorsum semispinatus and the multifidus. Then they reach the rhomboid and trapezium, reaching the skin on the sides of the spinous process. This sensory branch is known as the medial cutaneous branch.
The medial branches of the six lower thoracic nerves are distributed mainly to the multifidus and longissimus dorsi, although occasionally some of their filaments extend to the skin. This sensory branch is known as the posterior cutaneous branch.
lumbar nerves
later divisions
The medial branches of the posterior divisions of the lumbar nerves pass close to the articular processes of the vertebrae and terminate in the multifidus muscle. The laterals work in conjunction with the erector spinae muscles.
The three upper lumbar nerves send cutaneous nerves to the latissimus dorsi at the lateral border of the erector spinae muscles. They then travel down the posterior part of the iliac crest, until they reach the skin of the buttocks. Some of its ramifications extend to the level of the great trochanter.
previous divisions
The anterior divisions of the lumbar nerves and their ramifications increase in size the further down the body they are found. They join, near their origins, with the gray communicating rami of the lumbar ganglia and the sympathetic trunk.
These rami are formed by long, thin branches that accompany the lumbar arteries around the sides of the vertebral bodies, below the psoas major. This arrangement is somewhat irregular, in the sense that one ganglion can give rami to two lumbar nerves, or one of these nerves can receive rami from two ganglia.
The first four lumbar nerves are connected to the lumbar part of the sympathetic trunk by a white ramus communicans. The nerves pass obliquely outward under the psoas major, or between its fascicles, distributing filaments both to it and to the quadratus lumborum.
The first three nerves and a large part of the second are connected at this location by anastomotic loops, thus forming the lumbar plexus. The smaller part of the fourth unites with the fifth to form the lumbosacral trunk, which aids in the formation of the sacral plexus.
Thus, the L4 nerve is known as the furcal nerve, because it is divided between the two plexuses.
sacral nerves
The sacral nerves are five pairs of spinal nerves that exit the sacrum at the bottom of the spine. Its roots begin within the spine at the level of the L1 vertebra, where the cauda equina begins, and later descend to the level of the sacrum.
There are five pairs of sacral nerves, with half emerging from the sacrum on the left side and half on the right. Each of them emerges in two distinct divisions: one through the anterior sacral foramina, and the other through the posterior sacral foramina.
These nerves are divided into different branches, and those of each of them join with those of the others, in addition to the branches of the lumbar and coccygeal nerves. These nerve anastomoses form the sacral and lumbosacral plexuses. The branches of these plexuses are those that work in areas such as the hips, calves, legs or feet.
The sacral nerves have both afferent and efferent fibers, and are therefore responsible for most of the sensory perception and movements of the lower extremities of the human body.
From the S2, S3, and S4 nerves arise the pudendal nerve and parasympathetic fibers, whose electrical potentials work with the descending colon, rectum, bladder, and genital organs. These pathways also have both afferent and efferent fibers, and therefore carry sensory information to the CNS and motor commands to these organs.
coccygeal nerve
Finally, the coccygeal nerve is number 31 within the spinal nerves. It arises from the conus medullaris, and its anterior root helps to form the coccygeal plexus.
Unlike the previous ones, it does not divide into a medial and a lateral branch. Its ramifications reach mainly the skin on the back of the coccyx.
Origin and destination
Spinal nerves travel from the Central Nervous System (CNS) to virtually every corner of the human body. With the exception of some areas of the head and neck, which are controlled by the cranial nerves, all the organs, muscles, and glands of the body transmit and receive their information through these nerves.
Thus, a single nerve can transmit and collect information from several different organs, from the skin, or from different glands. Through the ramifications into which they are divided, each of them can perform multiple functions, forming a complex system that connects all parts of the body with the central nervous system.
functions
As we have already seen, the spinal nerves are both afferent and efferent. This means that each one of them fulfills a double function, both fundamental for the correct functioning of the human body.
On the one hand, the spinal nerves collect information from the organs, glands or muscles with which they are connected and transmit it to the central nervous system through the spinal cord. In this way, the brain can process all this data and develop an appropriate response to a certain situation.
On the other hand, the same spinal nerves are in charge of taking the response elaborated by the CNS to the effector organs, in such a way that we can react and function correctly in our environment.
References
Spinal nerves. Retrieved from healthpages.org.