Cervical Plexus: Definition, Components, Spinal Nerves, Function and Innervation

It is a neurological structure of great interest that is formed by the ventral branch of the four upper cervical spinal nerves.

With the exception of the first cervical nerve, these branches are divided into ascending and descending branches to form intricate loops that innervate the muscles of the head and neck together with the sensory supply.

In addition to the interactions that exist between the contributing ventral branches that form the cervical plexus, the branches also cooperate with the cranial nerves.

A variety of branches arise from the cervical plexus and includes 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 is composed of cranial nerves, spinal nerves and certain parts of the autonomic nervous system.

The structures that are commonly known as nerves (or roots, trunks and branches) are composed of an ordered alliance of the axonal and dendritic processes of multiple nerve cell bodies.

The ganglia are composed of a group of these cell bodies of the so-called peripheral neurons.

The nerve cells that are located in the ganglia can be classified on the basis of the type of bodies, such as sensory or motor.

The sensory ganglia are oval-shaped protuberances located in the posterior roots of the spinal nerves as well as in the roots of some cranial nerves.

These ganglia are composed of sensory neurons that are unipolar.

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 shape of a “T”.

In the brain or spinal cord originate 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.

Spinal nerves

The sensory reception manifested passes through the dorsal roots that are found in the spinal cord. This is manifested 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 basic functional and structural unit of the peripheral nervous system.

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, designated with the letter “L” (L1 to L5), five (05) sacral, designated with the letter “S” “(S1 to S5) and one (01) coccygea, 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 emerge below their corresponding vertebrae.

There are two branches located just outside the intervertebral foramen, which are 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, with the exception of the C1, S4, S5 and Coc1, are medial and lateral branches, which are responsible for innervating the muscles of the back and skin that cover 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 mostly 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 area 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.

Function of the spinal nerves

The spinal nerves contain fibers of sensory type (that emerge from the dorsal roots) and motor fibers (that emerge from the ventral roots), and are known as mixed nerves.

When the individual fibers of a spinal nerve are identified by a specific function, they can be categorized into four types:

  1. General somatic affections.
  2. General somatic efferents.
  3. General visceral affections.
  4. General visceral efferents.

The efferent fibers are those that carry motor information to the skeletal muscle, to the autonomous ganglia and from there to the visceral structures, they are the ones that transport the sensorial 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 are called proprioceptive, while sensations of temperature and pain that come from the surface of the body 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 ventral horn, which are distributed to the skeletal muscles which are located in the body wall and in the extremities.

General visceral efferent fibers also arise from the cell bodies located within the spinal cord, but which emerge only at the lumbar and upper thoracic levels or at the sacral levels, specifically, at the levels of T1 to L2 and S2 a S4.

The fibers of T1 to L2 pass in the sympathetic trunk, in which they descend or ascend inside the trunk, they can also form synaptic contacts within a ganglion or leave the trunk 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.

Nervous plexus

During development, the skeletal muscles commonly fuse, 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: which 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: that 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 arm.

Ulnar nerve: flexor muscles of the forearm and small digital muscles, as well as skin on the medial surface of the hand.

The lumbosacral plexus: which 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:

  1. Gluteal nerve: adductors and extensors of the hip and skin on the posterior surface of the thigh.
  2. 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.
  3. Saphenous nerve: skin on the medial surface of the leg.

Innervation

The cervical levels C1 to C4, are the main contributors to the group of nerves called cervical plexus, in addition, small branches of the plexus 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 large 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 muscles that provide stability to the hyoid bone, help in actions such as swallowing and the muscles that raise the upper ribs.

The phrenic nerves, responsible for carrying sensory information to parts of the pleura, the lungs and the pericardium of the heart, as well as motor impulses of the diaphragm muscles, originate from C4, with small contributions of C3 and C5.