Cervical plexus

The cervical plexus (Latin: plexus cervicalis) is a network of nerve fibers formed by the ventral rami of the upper four cervical nerves (C1 - C4). It innervates certain areas of skin and muscles of the head, neck, and upper torso. The cervical plexus is found in the posterior triangle on each side of the neck.

The cervical plexus is located anterior to the middle scalene and levator scapulae muscles and posterior to the deep cervical fascia, internal jugular vein, and sternocleidomastoid muscle. Two types of nerve fibers arise from the cervical plexus and supply motor, sensory or mixed innervation to the skin and muscles.

The anterior rami divide into ascending and descending branches and form communicating loops with each other. Additionally, the anterior rami of the cervical plexus anastomose with the accessory nerve (CN XI), hypoglossal nerve (CN XII), and sympathetic trunk. The communicating loops give rise to superficial and deep branches.

Superficial branches of cervical plexus

The superficial branches of the cervical plexus provide sensory fibers to the skin of the head, neck, and torso. After leaving the plexus, the superficial branches either ascend or descend. The superficial nerves arising from the cervical plexus are:

• Lesser occipital nerve
• Great auricular nerve
• Transverse cervical nerve
• Supraclavicular nerves

Lesser occipital nerve

The lesser occipital nerve arises from the communicating loop formed by the anterior rami of the second and third cervical nerves (C2 and C3). The nerve perforates the deep cervical fascia in the middle of the posterior border of the sternocleidomastoid muscle together with the other superficial branches. Subsequently, it ascends along the muscle's posterior margin. Upon reaching the lateral area of the head, the lesser occipital nerve divides and innervates the surrounding skin and posterior part of the auricle.

Great auricular nerve

The great auricular nerve originates from the communicating loop between C2 and C3. The great auricular nerve perforates the deep cervical fascia together with the lesser occipital, supraclavicular and transverse cervical nerves and ascends. After reaching the mandibular angle, the great auricular nerve furcates and innervates the parotid gland's fascia, skin, and superficial regions around the mastoid process and outer ear.

Transverse cervical nerve

The transverse cervical nerve arises from the communicating loop of the cervical plexus C2 and C3. The transverse cervical nerve perforates the deep cervical fascia together with the other superficial branches. Next, it continues medially below the external jugular vein to the anterior cervical region. In the anterior part of the neck, the transverse cervical nerve supplies sensory innervation to the skin.

Supraclavicular nerves

The supraclavicular nerves originate from the cervical plexus's communicating loop between C3 and C4. The main stem comes out posteriorly from underneath the sternocleidomastoid in the middle of the neck with the other superficial nerves, perforates the deep cervical fascia, and descends. At the clavicle, the nerve branches and innervates the anteromedial shoulder and chest.

Deep branches of cervical plexus

The deep branches of the cervical plexus supply motor or mixed innervation to the neck and upper torso muscles. The deep branches of the cervical plexus are:

• Ansa cervicalis
• Phrenic nerve
• Accessory phrenic nerve
• Muscular branches

Ansa cervicalis

The ansa cervicalis is the biggest communicating loop of the cervical plexus. The neural loop contains motor nerve fibers from the anterior rami of C1 - C3. The ansa cervicalis is situated in front of the internal jugular vein in the carotid triangle and gives four deep muscular branches that supply motor innervation to muscles of the neck. It is formed by a superior and inferior root.

The superior root originates from motor nerve fibers coming from the hypoglossal nerve (CN XII). Motor nerve fibers from the anterior ramus of C1, and sometimes also C2, join the hypoglossal nerve. Eventually, these motor fibers branch away as the superior root of the ansa cervicalis and two additional motor branches that innervate the thyrohyoid and geniohyoid.

The inferior root is formed from the anterior rami of C2 and C3. Four deep muscular branches containing motor fibers originate from the ansa cervicalis, which then innervate the superior and inferior belly of the omohyoid muscle, the sternothyroid, and the sternohyoid. An additional branch descends into the thorax to join the cardiac nerve, phrenic nerve, or both.

Phrenic nerve

The phrenic nerve is a bilateral nerve that plays an important role in respiratory regulation. The phrenic nerve arises primarily from the anterior cervical ramus C4 with additional branches from C3 and C5. It contains a mix of motor and sensory fibers. The right and left phrenic nerves have slightly different courses within the thorax. Both phrenic nerves descend along the anterior surface of the corresponding anterior scalene muscles.

The right phrenic nerve descends into the thorax along the right side of the right brachiocephalic vein and the superior vena cava. The left phrenic nerve descends into the thorax along the left side of the left subclavian artery, passes the left side of the aortic arch, and crosses the vagus nerve (CN X). Both nerves continue downwards on their respective sides of the heart between the mediastinal part of the parietal pleura and the fibrous pericardium.

After passing lateral to the right atrium and ventricle, the right phrenic nerve continues downwards through the caval hiatus of the diaphragm and enters the abdominal cavity. The left phrenic nerve descends along the left ventricle and pierces the diaphragm at its central tendon. Once in the abdominal cavity, both nerves divide into three branches. These three branches continue in a radial pattern and innervate the corresponding areas of the diaphragm.

The sensory fibers of the phrenic nerves innervate the pericardium, diaphragm, mediastinal pleura, central part of the diaphragmatic pleura and the parietal peritoneum. The innervated section of the parietal peritoneum is the part of peritoneum that covers the anterior surface of the pancreas, the inferior surface of the diaphragm and the medial part of the liver until the gallbladder. The motor fibers supply the diaphragm and are the sole source of its motor innervation.

Accessory phrenic nerve

The accessory phrenic nerve is a common anatomical variant (not seen in all individuals). In most cases, the nerve originates from the subclavian nerve or ansa cervicalis. The accessory phrenic nerve is usually located lateroposterior to the phrenic nerve and follows the phrenic nerve down its path. It typically joins up with the phrenic nerve at the level of the first rib. There are many variations in the origin, course, and termination of the accessory phrenic nerve.

Muscular branches of cervical plexus

The muscular branches of the cervical plexus originate from the communications of the plexus and they supply muscles. To be more precise, they provide motor innervation to the following neck muscles and superficial back muscles:

• Thyrohyoid (via the hypoglossal nerve)
• Geniohyoid (via the hypoglossal nerve)
• Omohyoid
• Sternothyroid
• Sternohyoid
• Rectus capitis anterior
• Rectus capitis lateralis
• Longus capitis
• Sternocleidomastoid
• Longus colli
• Trapezius
• Levator scapulae
• Middle scalene

Motor fibers ​​from the anterior ramus C1 join the hypoglossal nerve (CN XII) and continue as part of it. Eventually, the motor fibers branch away from the hypoglossal nerve as the superior root of the ansa cervicalis and two subsequent motor branches. These motor branches innervate the thyrohyoid and the geniohyoid muscles.

From the ansa cervicalis originate four separate muscle branches. These muscle branches supply motor innervation to the superior belly and inferior belly of the omohyoid muscle and the sternothyroid and sternohyoid muscles.

From the communication between C1 and C2 originate three muscular branches that innervate rectus capitis anterior, rectus capitis lateralis, and longus capitis. Longus capitis is additionally innervated by muscular branches from C3.

The lower communications of anterior rami give rise to lateral branches. From C2 and C3 originate muscle fibers that supply the sternocleidomastoid, and fibers from C2, C3 and C4 supply longus colli. Branches from C3 and C4 supply the trapezius. And branches from the communication formed from C3 and C4 supply motor fibers to the levator scapulae and scalenus medius.

References:

1. Standring, S. (2020). Neck. In Gray's Anatomy (Forty Second edition, pp. 573–606). Elsevier Limited.
2. Hombach-Klonisch, S., Klonisch, T., & Peeler, J. (2019). Neck. In Sobotta Clinical Atlas of Human Anatomy (pp. 517–551). Fischer Gustav Verlag GmbH & Co. KG.
3. Netter, F.H. (2018). Head and Neck Study Guide. In Atlas of Human Anatomy (7th ed., pp. e1–e39). Elsevier.