A plexus is a web or network of spinal nerves where the fibers of the spinal nerves join each other to form the terminal branches. These terminal branches contain fibers of different spinal nerves having different root values. The terminal branches of a plexus then finally supply the peripheral body parts.
If we study the neuroanatomy of the human body, we come to know that the human body consists of four important nerve plexus. These are the cervical plexus, brachial plexus, lumbar plexus, and sacral plexus. In this article, our main focus will be the cervical plexus.
Cervical plexus is a web or network of nerve fibers present in the region of the neck. As the name indicates, it is formed by the spinal nerves originating from the cervical segment of the spinal cord. The cervical spinal nerves join together to form a plexus of nerves.
This plexus then gives rise to the terminal branches or the individual nerves supplying the structures present in the neck. The different aspects of the cervical plexus such as location, formation, distribution, clinical importance, etc. are discussed in the article below.
The cervical plexus is present in the paravertebral region of the neck. It is present in close relation to the cervical vertebrae and the muscles arising from the cervical vertebrae. It is present between the scalenus anterior and the scalenus medius muscles. The cervical plexus is covered on its anterior side by the prevertebral fascia. The internal jugular vein and the sternocleidomastoid muscle are also present anterior to the cervical plexus.
We already know that each plexus is formed by the spinal nerves. The same is the case of the cervical plexus. The cervical spinal nerves participate in the formation of the cervical plexus. It is formed by the anterior rami of the upper four cervical nerves C1 to C4. The fifth cervical nerve C5 also joins the lower part of the plexus and contributes some nerve fibers to a branch of the cervical plexus.
Unlike the brachial plexus, the cervical plexus is not divided into trunks, divisions, and cords. Rather, the four roots of the cervical plexus C1-C4 join together to form three loops. The branches of the plexus emerge from these loops and supply different structures present in the cervical region.
An important characteristic of the cervical plexus is Ansa Cervicalis.
It is a thin loop of nerve fibers that lies embedded in the anterior wall of the carotid sheath. It is formed by a superior root and an inferior root.
The superior root fibers are derived from the first cervical nerve (C1). This root is in fact continuation of the descending fibers of the hypoglossal nerve.
The inferior root fibers are derived from the second and third cervical nerves (C2-C3).
The two roots join together to form a loop called ansa cervicalis. This loop gives branches that supply the infrahyoid muscles.
Branches and Distribution
In this section of the article, we will discuss the terminal branches of the cervical plexus, their origin, and their distribution.
The branches originating from the cervical plexus are of two types. The superficial branches that provide the cutaneous innervation to the body, and the deep branches that supply the muscles and other deep structures present in the region.
The cervical plexus gives rise to four superficial or cutaneous branches. The names and distribution of these branches are as follows.
- Lesser Occipital Nerve: It consists of nerve fibers derived from the second cervical nerve (C2). It supplies the skin over the upper lateral aspect of the neck and the skin behind the ear.
- Supraclavicular Nerve: Is fibers are derived from the third and fourth cervical nerves (C3-C4). It supplies the skin over the supraclavicular region.
- Great Auricular Nerve: The fibers of this nerve are derived from the second and third cervical nerves (C2-C3). It supplies the skin over the angle of the jaw and over the region of the parotid gland.
- Transverse Cutaneous Nerve of Neck: Its fibers are also derived from the second and third cervical nerves (C2-C3). It supplies the skin at the lower margin of the lower jaw and the skin below the lower jaw.
The deep branches of the cervical plexus are of two types, communicating branches and the muscular branches.
These branches carry different nervous fibers towards and away from the plexus. The three communicating branches from the cervical plexus are as follows;
- Gray rami: The gray rami carrying the sympathetic fibers pass from the superior cervical ganglion to the cervical plexus. These rami join the roots of cervical nerves C1 to C4.
- Branch to Hypoglossal Nerve: The hypoglossal nerve supplies the muscles of the tongue. However, it also supplies some suprahyoid muscles. These muscles are supplied by a branch from the C1 root of the cervical plexus. This branch joins the hypoglossal nerve which carries the nerve fibers to the suprahyoid muscles.
- Branch to Accessory Nerve: The muscular nerves to the sternocleidomastoid and the trapezius muscles join the accessory nerve and communicate the fibers from the cervical plexus.
The deep muscular branches of the cervical plexus supply the muscles of the neck. The muscular branches of the plexus include;
- Branch to Rectus Capitis Anterior muscle from the root C1
- Branch to Rectus Capitis Lateralis from the roots C1 and C2
- Branch to Longus Capitis from roots C1 to C3
In addition, ansa cervicalis provides muscular branches to the suprahyoid muscles. The muscular branches of Ansa Cervicalis are as follows:
- Branches to the sternohyoid, sternothyroid and inferior belly of omohyoid muscles from the inferior root (C2-C3)
- Branch to superior belly of omohyoid muscle from the superior root (C1)
It is another branch of the cervical plexus. Phrenic nerve is a mixed nerve that carries both sensory and motor fibers.
Phrenic nerve originates from the third and fourth cervical nerves of the cervical plexus. It also receives contributory fibers from the fifth cervical nerve. So, the root value of the phrenic nerve is C3-C5.
Phrenic nerve travels downwards through the mediastinum to reach the diaphragm. Here, the sensory fibers of the nerve provide sensory innervation to the diaphragm, pleural membranes, pericardial membrane, and a part of the peritoneum. The motor fibers of the nerve provide innervation to the muscles of the diaphragm.
We have already studied that the branches of cervical plexus supply important muscle of the neck. Any injury to the cervical plexus will result in the paralysis of the muscles supplied by these branches.
Cervical plexus is located at a very secure place in the neck region. As a result, it is not injured readily. However, lesions of the cervical fascia or stab wounds in the neck region will result in severe injury to the cervical plexus.
The most important clinical correlate with respect to the cervical plexus is the injury of the phrenic nerve, an important branch of the cervical plexus. Phrenic nerve severance or injury results in the paralysis of one half of the diaphragm.
The paralyzed half of the diaphragm moves paradoxically with breathing movements i.e. upwards during inspiration and downwards during expiration. This paradoxical movement of the hemidiaphragm (one half of diaphragm) may produce breathing abnormalities.
Phrenic nerve injury most commonly occurs during surgical procedures. Phrenic nerve may also be blocked by injecting an anesthetic around the nerve in the neck region. This phenomenon is called phrenic nerve block.
Cervical plexus is present along the vertebrae of the cervical region.
It is present in the paravertebral region covered anteriorly by the prevertebral fascia and posteriorly by the scalene muscles.
The roots of the cervical plexus are formed by the anterior rami of the superior four cervical nerves C1 to C4.
These roots join together to form three loops.
The terminal branches of the cervical plexus originate from these loops.
A characteristic loop associated with the cervical plexus is Ansa Cervicalis.
Ansa Cervicalis is made up of a superior root from C1 and an inferior root from C2-C3.
These roots join to form a loop that lies in the anterior wall of the carotid sheet.
The suprahyoid muscles are supplied by ansa cervicalis.
Two types of branches originate from the cervical plexus:
- Superficial branches, these are the cutaneous branches supplying different areas of head and neck
- Deep branches, these include further two types;
- Muscular branches supply the muscles of the neck
- Communicating branches communicate with other nerves present in the region
Phrenic nerve is a mixed branch originating from the roots C3-C5 of the cervical plexus. This nerve provides motor supply to the diaphragm and sensory supply to pleura, pericardium, diaphragm, and part of peritoneum.
Cervical plexus lies deep, protected by the muscles of the neck. It is not easily injured except in the case of stab wounds in the neck region. Any injury to the cervical plexus results in paralysis of the muscles supplied by it.
Injury to the phrenic nerve is more common. It results in paralysis of hemidiaphragm. The paralyzed half of the diaphragm moves paradoxically during breathing.
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