It is a neurological structure of great interest that is formed by the ventral branch of the four upper cervical spinal nerves.
Except for the first cervical nerve, these branches are divided into ascending and descending branches to form intricate loops that innervate the head and neck muscles together with the sensory supply.
The units also cooperate with the cranial nerves in addition to the interactions between the contributing ventral branches that form the cervical plexus.
Various branches arise from the cervical plexus and include the ansa cervical, the occipital minor, the greater auricular, the supraclavicular, the transverse, and the phrenic cervical.
Peripheral nervous systems
The peripheral nervous system is a channel for the replacement of sensory and motor impulses that occur between the central nervous system on the one hand and the superficial part of the body, the skeletal muscles, and all the internal organs on the other hand.
The peripheral nervous system comprises cranial nerves, spinal nerves, and certain parts of the autonomic nervous system.
The structures commonly known as nerves (or roots, trunks, and branches) are composed of an ordered alliance of multiple nerve cell bodies’ axonal and dendritic processes.
The ganglia are composed of a group of these cell bodies of the so-called peripheral neurons.
The nerve cells located in the ganglia can be classified based on the type of body, such as sensory or motor.
The sensory ganglia are oval-shaped bumps located in the posterior roots of the spinal nerves as well as in the origins of some cranial nerves.
These ganglia are composed of unipolar sensory neurons.
It has cellular bodies of round or slightly oval shape with concentric nuclei, which give rise to a single fiber that is subject to a bifurcation in the form of a “T.”
The brain or spinal cord originates the preganglionic fibers that enter the motor ganglia, where they form a synapse in the multipolar cell bodies.
These postganglionic cells, in turn, project their processes to the visceral structures.
The sensory reception manifests through the dorsal roots found in the spinal cord. This is displayed in the receptors of the joints, muscles, tendons, and internal organs.
Each spinal nerve is formed by the association of a ventral root and dorsal root and represents the peripheral nervous system’s basic functional and structural unit.
Structural components of the spinal nerves
There are thirty-one (31) pairs of spinal nerves; if they are named in descending order from the end of the spinal cord near the neck, eight (08) cervicals are observed, designated with the letter “C” (C1 to C8).
Twelve (12) thoracic, designated with the letter “T” (T1 to T12), five (05) lumbar, set with the letter “L” (L1 to L5), five (05) sacral, designated with the letter “S” “(S1 to S5) and one (01) coccygeal, designated with the letter” C “(Coc 1).
The first spinal nerve (C1) is the one that leaves the vertebral canal, which is located between the skull and the first cervical vertebra, so the spinal nerves C1 to C7 come out above the vertebrae correspondingly numbered from 1 to 7.
The spinal nerve C8, however, emerges between the seventh cervical vertebra and the first thoracic vertebra so that, starting with T1, all the spinal nerves appear below their corresponding vertebrae.
There are two branches located just outside the intervertebral foramen, known as gray and white communicating rami; these connect each spinal nerve with the sympathetic trunk.
In addition, there are small meningeal branches, which leave each spinal nerve and the gray branch and re-enter the vertebral canal, where they innervate the dura mater, which is the outermost part of the meninges, and the blood vessels.
The dorsal branches, except the C1, S4, S5, and Coc1, are medial and lateral branches responsible for innervating the muscles of the back and the skin that covers them.
Of the dorsal branches of the spinal nerves C2-C8, the medial and lateral branches are responsible for irrigating the skin of the neck and muscles.
The nerves from T1 to T6 are mostly skin (only the skin), while the nerves from T7 to T12 are primarily muscular.
The dorsal branches of nerves from L1 to L3 have sensory and motor fibers, while the nerves from L4 to L5 are only muscular.
The nerves from S1 to S3 innervate those deep muscles located in the lower areas of the back, as well as cutaneous expressions of the perianal site and lower buttocks.
The undivided dorsal branches of nerves S4, S5, and Coc1 also innervate cutaneous branches to the gluteal and perianal regions.
To achieve the innervation of the joints, muscles, and skin of the lateral and ventral walls of the body and extremities, the ventral branches of the spinal nerves are responsible for transporting sensory and motor fibers.
Both branches, the dorsal and the ventral, also include autonomic fibers.
The function of the spinal nerves
The spinal nerves contain fibers of sensory type (that emerge from the dorsal roots) and motor fibers (that arise from the ventral roots) and are known as mixed nerves.
When a specific function identifies the individual fibers of a spinal nerve, they can be categorized into four types:
- General somatic affections.
- General somatic efferents.
- General visceral affections.
- General visceral efferents.
The efferent fibers carry motor information to the skeletal muscle, to the autonomous ganglia, and from there to the visceral structures; they are the ones that transport their sensory information of them.
Thus the general somatic afferent receptors are those sensitive to thermal sensations, pain, touch, changes in body position, and pressure.
That sensory information that arises from tendons, muscles, or joint capsules is called proprioceptive, while sensations of temperature and pain that come from the body’s surface are called exteroceptive.
The receptors of the general visceral afferent functional components are located in the organs of the thorax, the abdomen, and the pelvis; their fibers can transmit information about the pain of the digestive tract.
In the case of general somatic efferent fibers, they originate based on the large cells of the ventral horn, which are distributed to the skeletal muscles, which are located in the body wall and the extremities.
General visceral efferent fibers also arise from the cell bodies located within the spinal cord. Still, they emerge only at the lumbar and upper thoracic levels or at the sacral levels, specifically, at T1 to L2 and S2 and S4.
The fibers of T1 to L2 pass in the sympathetic trunk, in which they descend or ascend inside the box; they can also form synaptic contacts within a ganglion or leave the chest and go to the ganglia that are closest to the target organs.
The fibers of S2 to S4 are those that desert the umbilical cord as the pelvic nerve and go to the terminal ganglia, which are found in the target organs.
The skeletal muscles commonly fuse during development, forming larger muscles innervated by nerve trunks containing axons derived from various spinal nerves.
These composite nerve trunks originate in networks with each called nerve plexus and within these are distinguished:
The cervical plexus innervates the muscles of the neck and skin of the upper chest, neck, and ears and extends into the thoracic cavity to control the diaphragm.
The brachial plexus innervates the scapular waist and the upper extremity.
Musculocutaneous nerve: flexor muscles of the arm and forearm, skin on the lateral surface of the forearm.
Median nerve: flexor muscles of the forearm and hand, skin on the lateral surface of the hand.
Radial nerve: extensor muscles of the arm, forearm, and hand and skin on the posterolateral surface of the component.
Ulnar nerve: flexor muscles of the forearm and small digital muscles, as well as the skin on the medial surface of the hand.
The lumbosacral plexus: supplies the pelvic girdle and the lower extremity. It can be subdivided into a lumbar plexus and a sacral plexus.
Lumbar plexus: femoral nerve, hip adductors, knee extensors, and skin on medial surfaces of the thigh and leg.
Sacral plexus: composed of:
- Gluteal nerve: adductors and extensors of the hip and skin on the posterior surface of the thigh.
- Sciatic nerve: flexors of the knee and ankle, flexors and extensors of the fingers, and skin on the anterior and posterior surfaces of the leg and foot.
- Saphenous nerve: skin on the medial surface of the portion.
The cervical levels C1 to C4 are the main contributors to the group of nerves called the cervical plexus; in addition, small plexus branches join C1 and C2 with the vagus nerve, C1 and C2 with the hypoglossal nerve, and C2 to C4 with the nerve accessory.
The sensory branches of the cervical plexus are the minor occipital nerve, transverse cervical nerves to the lateral and ventral surfaces of the neck, the sizeable auricular nerve to the ear, and the skin over the mastoid and parotid areas, and the supraclavicular nerves along the clavicle: the shoulder and the upper part of the thorax.
The motor branches of the cervical plexus help the muscles that give stability and flexion to the neck, the powers that provide stability to the hyoid bone, help in actions such as swallowing, and the forces that raise the upper ribs.