The cervical plexus is a conglomeration of cervical nerves formed by the ventral rami of the first four cervical nerves (C1 – C4). These are the roots (limbs) of the cervical plexus. However, most authors include the fifth cervical nerve (i.e. the ventral ramus of C5) to the plexus owing to its contribution to the formation of one of the motor branches of the cervical plexus called the phrenic nerve. Therefore, the cervical plexus can also be defined as a network of nerves formed by the ventral rami of C1 – C5 nerves and gives off both motor (anterior) and sensory (posterior) branches.
The cervical plexus is formed in the neck, and lies deep to the sternocleidomastoid muscle, and anterolateral to the levator scapulae and middle scalene muscles. Each of the cervical nerves forming the plexus communicates with one another in a superior-inferior fashion close to their origins, thus the C2 accepts communicating fibres from C1, C3 from C2, and so on. These communicating fibres are the contributions from the sympathetic trunk (sympathetic nervous system) to the cervical plexus. Those fibres are “gray rami” communicantes (meaning blood vessel accompanied) descending from the superior cervical ganglion (which is the largest of the three cervical ganglia).
Next they each (except the first, C1) divide into an ascending branch and a descending branch. They subsequently unite with branches of the adjacent cervical nerve to form loops, for example, the loop formed between C2 and C3 that contributes branches to the “ansa cervicalis”. Those loops and the branches from them form the cervical plexus.
Branches of the cervical plexus include the motor branches supplying muscles, and the cutaneous branches innervating the skin of the anterolateral neck, the superior part of the thorax (superolateral thoracic wall) and scalp between the auricle (pinna) and the external occipital protuberance.
The sensory (posterior or cutaneous) branches of the plexus emerge around the middle of the posterior border of the sternocleidomastoid muscle. This area is clinically significant and recognized as the nerve point of the neck.
There are four sensory branches originating from the two loops formed between the ventral rami of C2 and C3, and C1 and C4. These are:
- Branches from the loop between C2 and C3
- Lesser Occipital nerve (formed by C2)
- Great Auricular nerve (formed by C2 and C3)
- Transverse Cervical nerve (formed by C2 and C3)
- Branches from the loop between C3 and C4
- Supraclavicular nerves (formed by C3 and C4)
As highlighted in the names of those four sensory branches, a mnemonic to memorize the names can be derived from the bolded letters. Thus the mnemonic for the sensory branches is “OATS”.
On the other hand, the motor branches of the cervical plexus form the ansa cervicalis, which is a nerve loop innervating the infrahyoid muscles in the anterior cervical triangle. They also form the phrenic nerve which supply the diaphragm and the pericardium of the heart. Some motor branches also reach the rhomboid muscles – nerves to the rhomboids [dorsal scapular nerve (formed by C4 and C5)] and serratus anterior muscle (long thoracic nerve – formed by fibres from C5, C6 and C7 nerves, hence formed by both the cervical and the brachial plexus mainly). Therefore the motor branches include:
- Geniohyoid nerve (C1)
- Thyrohyoid nerve (C1)
- Omohyoid nerve (C1 – C3)
- Sternohyoid nerve (C1 – C3)
- Sternothyroid nerve (C1 – C3)
- Phrenic nerve (contributed mainly by C4, with little fibres from C3 and C5)
- Nerve to the rhomboid muscles
- Nerve to serratus anterior muscle
Course And Distribution
- Lesser Occipital Nerve: The branch is formed by the second cervical nerve (C2) only, and courses to supply the skin of the neck and the scalp posterosuperior to the clavicle.
- Great Auricular Nerve: This sensory branch originates from the C2 and C3 nerves. It courses upwards in a diagonal fashion and crosses the sternocleidomastoid muscle onto the parotid gland. It then divides and innervates the skin over the parotid gland, the posterior aspect of the auricle, and an area of skin extending from the angle of the mandible of the mastoid process.
- Transverse Cervical Nerve: The transverse cervical nerve is formed by axons from the second and third cervical nerves. It supplies the skin covering the anterior triangle of the neck. This branch curves around the middle of the posterior border of the sternocleidomastoid muscle and crosses it deep to the platysma muscle.
- Supraclavicular Nerve: This branch is formed by the C3 and C4 nerves, and it emerges as a common trunk under cover of the sternocleidomastoid muscle and sends small branches to the skin of the neck. Some of those branches of this branch (supraclavicular) also cross the clavicle to supply the skin over the shoulder.
- The Ansa Cervicalis: The five motor branches of the ansa cervicalis loop listed above, originate from C1 to C3 nerves. They supply the infrahyoid muscles in the anterior cervical triangle.
- Phrenic Nerve: The phrenic nerve originates chiefly from the 4th cervical nerve (C4) but receives contributions from the 3rd and 5th cervical nerves (C3 and C5). It is formed at the superior part of the lateral border of the anterior scalene muscle, at the level of the superior border of the thyroid cartilage. The phrenic nerve contains motor, sensory, and sympathetic nerve fibres. It provides the sole motor supply to the diaphragm as well as sensation to its central part. In the thorax, the phrenic nerve innervates the mediastinal pleura and pericardium of the heart. The phrenic nerve descends obliquely across the anterior scalenus muscle, deep to the prevertebral layer of deep cervical fascia and the transverse cervical and suprascapular arteries. It runs posterior to the subclavian vein and anterior to the internal thoracic artery as it enters the thorax.
Phrenic Nerve Severance
Severance (injury) of the phrenic nerve results in paralysis of the diaphragm. Temporary paralysis of the diaphragm can also result from phrenic nerve block. In this condition, the anaesthetic is injected around the nerve where it lies on the anterior surface of the middle third of the anterior scalene muscle. A surgical phrenic nerve crush will produce a longer period of paralysis (for example, for weeks after surgical repair of a diaphragmatic hernia).
Cervical Plexus Block
Nerve block is a measure to inhibit nerve impulse conductance, usually for regional anaesthesia prior to surgical operations. In a cervical plexus block, an anaesthetic agent is injected at several points along the posterior border of the sternocleidomastoid muscle, mainly at the junction of its superior and middle thirds - the nerve point of the neck. Because the phrenic nerve supplying the diaphragm pericardium is also paralysed by cervical plexus block, this procedure is not performed on patients with pulmonary or cardiac disease.
Supraclavicular Nerve Injury
This sensory branch of the cervical plexus is very vulnerable to injury in fractures of the clavicle, especially the middle third of the clavicle. Injury to the supraclavicular nerve causes loss of lateral rotation of the humerus at the shoulder so that when relaxed, the limb rotates medially in the waiter’s tip position. The ability to initiate abduction of the limb is also affected.