Spinal nerve

The spinal nerves or also known as spinal nerves are those that extend from the spinal cord and cross the vertebral muscles to be distributed to areas of the body.

A spinal nerve is a mixed nerve, carrying motor, sensory, and autonomic signals between the spinal cord and the body. In the human body there are 31 pairs of spinal nerves, one on each side of the vertebral column. These are grouped into the corresponding cervical, thoracic, lumbar, sacral, and coccyx regions of the spine. There are eight pairs of cervical nerves, twelve pairs of thoracic nerves, five pairs of of lumbar nerves, five pairs of sacral nerves and one pair of coccygeal nerves. Spinal nerves are part of the peripheral nervous system.

Structure

Each spinal nerve is a mixed nerve, formed by the combination of nerve fibers from its dorsal and ventral roots. The dorsal root is the afferent sensory root and carries sensory information to the brain. The ventral root is the efferent motor root and carries motor information from the brain. The spinal nerve emerges from the spinal column through an opening (intervertebral foramen) between adjacent vertebrae. This is so for all spinal nerves except for the first pair of spinal nerves (C1), which emerges between the occipital bone and the atlas (the first vertebra). Thus, the cervical nerves are numbered by the vertebra below, except for the C8 spinal nerve, which exists below the C7 vertebra and above the T1 vertebra. The thoracic, lumbar, and sacral nerves are numbered by the vertebra above. In the case of a sacralized S1 (also known as L6) lumbar vertebra or a sacralized L5 vertebra, the nerves are normally counted up to L5 and the next nerve is S1.

Outside the spinal column, the nerve divides into branches. The dorsal ramus contains nerves serving the posterior portions of the trunk that carry visceral motor, somatic motor, and somatic sensory information to and from the skin and muscles of the back (epaxial muscles). The ventral rami contains nerves that serve the remaining anterior parts of the trunk and the upper and lower extremities (hypaxial muscles) that carry visceral motor, somatic motor, and sensory information to and from the ventrolateral body surface, body wall structures. and the extremities. The meningeal branches (meningeal or recurrent sinuvertebral nerves) branch from the spinal nerve and reenter the intervertebral foramen to serve the ligaments, dura mater, blood vessels, intervertebral discs, facet joints, and periosteum of the vertebrae.. The ramus communicans contain autonomic nerves that serve visceral functions by carrying visceral motor and sensory information to and from visceral organs.

Anterior rami fuse with adjacent anterior rami to form a nerve plexus, a network of interconnected nerves. Nerves emerging from a plexus contain fibers from several spinal nerves, which are now carried together to some target location. The major plexuses include the cervical, brachial, lumbar, and sacral plexuses.

Nerve pairs

There are between 31 and 33 pairs of spinal nerves:

• 8 pairs of tiny cervical spinal nerves
• 12 pairs of dorsal or chest nerves
• 5 pairs of lumbar rachid nerves
• 5 pairs of sacred rachid nerves
• From 1 to 3 (variable) pair of coccygeal rachid nerves

It has a posterior root that enters through the posterior horn and exits through the anterior horn or motor.

The first 7 cervical nerves (C1 to C7) exit the vertebral foramen located above their respective cervical vertebra (ie, C1 exits the vertebral canal between the skull and the first cervical vertebra; C2 exits above the second, and so on). The C8 nerve exits from below the seventh cervical vertebra, and the rest of the spinal nerves (T1 to Co) exit under their respective vertebral bodies.

Formation of spinal nerves

In the spinal cord we find gray matter, covered by white matter. Two dorsal roots (one on the left, the other on the right) and two ventral roots emerge from the gray matter. (dorsal means on the back or back, ventral means front).

Fate of spinal or spinal nerves

After the dorsal and ventral roots become a spinal nerve, it exits the vertebral column, later bifurcating into its dorsal and ventral primary rami.

The dorsal ramus carries motor and sensory innervation from the skin and muscles of the back.

The ventral ramus carries motor and sensory information to the rest of the body.

The primary ventral rami also gives rise to the roots of various plexuses (eg the brachial plexus), which become the motor and sensory nerves of the upper limbs.

Before forming the plexuses, the primary ventral branch divides into two other secondary branches that lead to the sympathetic ganglion. These ganglia connect to each other, forming the sympathetic chain.

Regional nerves

Cervical Nerves

The cervical nerves are the spinal nerves of the cervical vertebrae in the cervical segment of the spinal cord. Although there are seven cervical vertebrae (C1-C7), there are eight C1-C8 cervical nerves. All cervical nerves, except C8, emerge above their corresponding vertebrae, while the C8 nerve emerges below the C7 vertebra. In other parts of the spine, the nerve emerges below the vertebra of the same name.

The posterior distribution includes the suboccipital nerve (C1), the greater occipital nerve (C2), and the third occipital nerve (C3). The anterior distribution includes the cervical plexus (C1-C4) and the brachial plexus (C5-T1).

The cervical nerves innervate the sternohyoid, sternothyroid, and omohyoid muscles.

A loop of nerves called the cervical loop is part of the cervical plexus.

Thoracic Nerves

The thoracic nerves are the twelve spinal nerves that arise from the thoracic vertebrae. Each T1-T12 thoracic nerve originates below each corresponding thoracic vertebra. Branches also exit the spine and go directly to the paravertebral ganglia of the autonomic nervous system where they are involved in the functions of the organs and glands of the head, neck, thorax, and abdomen.

• Previous divisions: The intercostal intercostal nerves come from the T1-T11 thoracic nerves, and run between the ribs. In T2 and T3, other branches form the intercostobrachial nerve. The subcostal nerve comes from the T12 nerve, and runs below the twelfth rib.
• Subsequent divisions: The middle branches (ramus medialis) of the later branches of the upper six thoracic nerves run between the dorsal semi-pedals and the multiphides, to which they supply; then they pierce the muscles Rhomboid muscles and Trapecus Trap Muscle, and reach the skin on the sides of the spinal apophysis. This sensitive branch is called medium cutaneous branches.

The medial rami of the lower six distribute mainly to the multifidus and longissimus dorsi, occasionally giving off filaments to the skin near the midline. This sensitive branch is called the posterior cutaneous ramus. The thoracic spinal nerves have two branches: external branches, which supply the skin of the thoracic region, and internal branches, which supply muscles related to the thorax.

Lumbar nerves

The lumbar nerves are the five spinal nerves that arise from the lumbar vertebrae. They are divided into posterior and anterior divisions.

Posterior divisions: The middle rami of the posterior divisions of the lumbar nerves run close to the articular processes of the vertebrae and terminate in the multifidus muscle.

The laterals supply the erector spinae muscles.

The three upper ones give off cutaneous nerves that pierce the aponeurosis of the latissimus dorsi at the lateral border of the erector spinae muscles and run down the posterior part of the iliac crest to the skin of the buttock, some of their branches reaching as far as the level of the buttocks. greater trochanter.

Anterior divisions: The anterior divisions of the lumbar nerves (rami anteriores) increase in size from top to bottom. They are joined, near their origins, by the gray ramus communicans of the lumbar ganglia of the sympathetic trunk. These rami consist of long, slender branches that accompany the lumbar arteries around the sides of the vertebral bodies, below the psoas major. Their disposition is somewhat irregular: a ganglion can give rami to two lumbar nerves, or a lumbar nerve can receive rami from two ganglia.

The first and second, and sometimes the third and fourth lumbar nerves are each connected to the lumbar part of the sympathetic trunk by a white ramus communicans.

The nerves pass obliquely outward behind the psoas major, or between its fascicles, distributing filaments to it and the quadratus lumborum.

The first three and most of the fourth are connected to each other in this situation by anastomotic ties, and form the lumbar plexus.

The smaller part of the fourth unites with the fifth to form the lumbosacral trunk, which helps form the sacral plexus. The fourth nerve is called the furcal nerve, due to the fact that it subdivides between the two plexuses.

Sacral Nerves

The sacral nerves are the five pairs of spinal nerves that exit the sacrum at the bottom end of the vertebral column. The roots of these nerves begin within the spinal column at the level of the L1 vertebra, where the cauda equina begins, and then descend to the sacrum.

There are five pairs of sacral nerves, half of which arise through the sacrum on the left side and half on the right side. Each nerve emerges in two divisions: one division through the anterior sacral foramina and the other division through the posterior sacral foramina.

The nerves are divided into branches and the branches of the different nerves join each other, some of them also join the branches of the lumbar or coccygeal nerves. These nerve anastomoses form the sacral plexus and the lumbosacral plexus. Branches from these plexuses give rise to nerves that supply much of the hip, thigh, lower leg, and foot.

The sacral nerves have afferent and efferent fibers, so they are responsible for part of the sensory perception and movements of the lower extremities of the human body. From S2, S3, and S4 arise the pudendal nerve and the parasympathetic fibers whose electrical potential supplies the colon and descending rectum, urinary bladder, and genital organs. These pathways have both afferent and efferent fibers and are thus responsible for conducting sensory information from these pelvic organs to the central nervous system (CNS) and for motor impulses from the CNS to the pelvis that control the movements of these organs. pelvics.

Coccygeal Nerve

The coccygeal nerve is the 31st^eɽ pair of spinal nerves. It arises from the conus medullaris, and its anterior root helps to form the coccygeal plexus. It does not divide into a medial and lateral branch. It is distributed to the skin over the back of the coccyx.

Importance of spinal nerves

The muscles that a particular spinal root serves are the myotomes, and the dermatomes are the sensory innervation areas in the skin that are supplied by a single sector of the spinal cord. This is so since the spinal cord is shorter than the vertebral canal, and especially at the lumbar level, the nerve roots originate at a medullary level but the spinal nerve exits through the conjunctional foramen at a lower level.

This is of great importance in the diagnosis of neurological disorders, since the lesion of one or several nerve roots will cause neurological signs or symptoms (muscular weakness, loss of sensitivity) that allow the localization of the causative lesion.

Additional images

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  Cervical vertebrae

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  Portion of the spinal cord, showing the surface of your right side. The duram has been opened to show the nerve roots.

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  Distribution of skin nerves. Ventral aspect.

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  Distribution of skin nerves. Dorsal aspect.