Cervical plexus

The cervical plexus is a network of nerves formed by the anterior rami of the upper four cervical spinal nerves (C1–C4). It is located deep in the neck, on the lateral side of the upper cervical vertebrae. The plexus lies beneath the sternocleidomastoid muscle and adjacent to the levator scapulae and scalenus medius muscles. It extends from the level of the first cervical vertebra (C1) to the fourth (C4), forming both motor and sensory branches that supply various structures in the neck, head, and shoulders. The cervical plexus is closely associated with the brachial plexus, which lies lower in the neck and shoulder region.

Location

The cervical plexus is located in the deep lateral aspect of the neck, lying beneath the sternocleidomastoid muscle. It is found within the prevertebral fascia, which surrounds the muscles of the spine. The cervical plexus lies between the transverse processes of the cervical vertebrae and the muscles of the lateral neck, such as the levator scapulae and scalenus medius.

Anatomy

The cervical plexus is a complex network of nerves that provides innervation to various structures in the head, neck, and upper thorax. Below is a detailed description of its formation, roots, branches, course, and anatomical relationships.

Formation

The cervical plexus is formed by the anterior rami (ventral branches) of the first four cervical spinal nerves (C1, C2, C3, and C4). These rami emerge from the cervical spinal cord and interconnect, creating a nerve network. The plexus primarily supplies motor and sensory innervation to the neck and portions of the head and shoulders.

The plexus is composed of both superficial sensory branches and deep motor branches, which contribute to different functional roles in the region.

Roots of the Cervical Plexus

The cervical plexus is formed by contributions from the ventral rami of the first four cervical nerves (C1 to C4). Each of these rami contributes to the plexus as follows:

• C1 Nerve: The C1 root is the highest contributor to the cervical plexus and often travels with the hypoglossal nerve (cranial nerve XII) for a portion of its course.
• C2 Nerve: The C2 root contributes sensory and motor fibers to the plexus and forms connections with the spinal accessory nerve (cranial nerve XI) in some individuals.
• C3 and C4 Nerves: The C3 and C4 roots contribute significant motor and sensory fibers to the plexus, forming important branches such as the phrenic nerve (C3, C4, C5) and several cutaneous nerves.

Branches of the Cervical Plexus

The cervical plexus gives rise to both superficial (sensory) and deep (motor) branches. These branches serve different functions and supply different regions of the neck and head.

• Superficial (Sensory) Branches:
• The sensory branches of the cervical plexus emerge from the posterior border of the sternocleidomastoid muscle and supply sensation to the skin of the neck, head, and upper chest. These branches include:
• Lesser Occipital Nerve (C2):
• This nerve ascends along the posterior border of the sternocleidomastoid muscle and innervates the skin of the lateral occipital region of the scalp.
• Great Auricular Nerve (C2, C3):
• This nerve crosses the sternocleidomastoid muscle and supplies sensation to the skin over the parotid gland, auricle, and angle of the mandible.
• Transverse Cervical Nerve (C2, C3):
• The transverse cervical nerve courses across the neck and provides sensory innervation to the skin of the anterior and lateral neck.
• Supraclavicular Nerves (C3, C4):
• These nerves descend toward the clavicle and provide sensation to the skin over the shoulder, upper chest, and clavicle.

Deep (Motor) Branches:

The deep branches of the cervical plexus primarily provide motor innervation to the muscles of the neck, including the infrahyoid muscles and the diaphragm (through the phrenic nerve). These branches include:

• Ansa Cervicalis (C1-C3): The ansa cervicalis is a loop formed by fibers from C1, C2, and C3. It innervates the infrahyoid muscles, which play a role in swallowing and positioning the larynx.
• Phrenic Nerve (C3, C4, C5): The phrenic nerve arises from C3, C4, and C5 and descends into the thorax to innervate the diaphragm, playing a crucial role in respiration.
• Motor Branches to Prevertebral Muscles: These branches supply the muscles located deep in the neck, including the longus capitis and longus colli muscles, which are responsible for neck flexion.
• Motor Branches to Sternocleidomastoid and Trapezius: The cervical plexus provides additional motor fibers that contribute to the innervation of the sternocleidomastoid and trapezius muscles, although these muscles are primarily innervated by the spinal accessory nerve (cranial nerve XI).

Course of the Cervical Plexus

The cervical plexus forms lateral to the cervical vertebrae and extends between the deep neck muscles, sternocleidomastoid muscle, and prevertebral fascia. The sensory branches of the plexus exit from the posterior border of the sternocleidomastoid muscle, while the motor branches travel deeper, along with other important structures in the neck, such as the carotid sheath and cervical lymph nodes.

Relationships with Surrounding Structures

The cervical plexus lies in close relation to several important anatomical structures in the neck:

• Sternocleidomastoid Muscle: The cervical plexus is located deep to this muscle, and its sensory branches emerge from its posterior border.
• Carotid Sheath: The carotid sheath, containing the common carotid artery, internal jugular vein, and vagus nerve, is closely related to the cervical plexus, especially the deep motor branches.
• Cervical Sympathetic Chain: The cervical sympathetic chain lies medially and posteriorly to the cervical plexus, influencing autonomic functions in the head and neck.

Variations

The anatomy of the cervical plexus may vary between individuals. Variations may occur in the number and distribution of branches, the connections with cranial nerves (especially the hypoglossal and accessory nerves), and the contribution to the phrenic nerve. These anatomical differences can influence surgical approaches and procedures in the neck.

Function

The cervical plexus has both motor and sensory functions, supplying various structures in the head, neck, and upper chest. Its primary role is to provide sensory innervation to the skin and motor innervation to muscles in these regions. Below is a detailed description of the functions of the cervical plexus.

Sensory Innervation

The cervical plexus has several sensory branches that provide sensation to specific regions of the neck, head, and shoulders. These branches help detect touch, pain, temperature, and other sensory stimuli from the skin and tissues.

• Lesser Occipital Nerve (C2): Provides sensory innervation to the skin of the lateral scalp, particularly behind the ear and toward the occipital region. It detects touch and temperature on the lateral aspect of the posterior head.
• Great Auricular Nerve (C2, C3): Supplies sensory innervation to the skin over the parotid gland, the auricle (external ear), and the area around the angle of the mandible. This allows for the perception of touch, pain, and temperature in these regions.
• Transverse Cervical Nerve (C2, C3): Innervates the skin of the anterior and lateral neck, allowing for sensation in these regions. It plays an important role in detecting stimuli like light touch and temperature changes across the neck.
• Supraclavicular Nerves (C3, C4):These nerves provide sensation to the skin over the shoulder, clavicle, and upper chest. The supraclavicular nerves are responsible for sensory input in areas where the shoulder and chest meet, helping detect pain, pressure, and temperature changes.

Motor Innervation

The cervical plexus also plays a key role in motor innervation, particularly to the muscles in the neck. These muscles are involved in functions such as swallowing, breathing, head movements, and stabilizing the hyoid bone and larynx.

Infrahyoid Muscles (via Ansa Cervicalis):

The ansa cervicalis, a loop of nerves derived from the cervical plexus (C1–C3), provides motor innervation to the infrahyoid muscles. These muscles are responsible for moving and stabilizing the hyoid bone and larynx, which are important for swallowing and speech.

Sternohyoid Muscle:

The sternohyoid muscle, innervated by the ansa cervicalis, depresses the hyoid bone after it has been elevated during swallowing.

Omohyoid Muscle:

The omohyoid muscle, also innervated by the ansa cervicalis, depresses and stabilizes the hyoid bone. It consists of two muscle bellies connected by an intermediate tendon.

Sternothyroid Muscle:

This muscle lowers the thyroid cartilage (Adam’s apple) after it has been elevated, contributing to the movement of the larynx during swallowing.

Thyrohyoid Muscle (via C1 Fibers):

Although not directly innervated by the ansa cervicalis loop, the thyrohyoid muscle is innervated by fibers from the C1 nerve that travel along the hypoglossal nerve (cranial nerve XII). It raises the larynx and depresses the hyoid bone during swallowing.

Diaphragm (via Phrenic Nerve):

The phrenic nerve, derived from the C3, C4, and C5 nerves of the cervical plexus, is crucial for respiration. It provides motor innervation to the diaphragm, the primary muscle involved in breathing.

• Diaphragmatic Movement: The phrenic nerve controls the contraction of the diaphragm during inspiration, which allows the lungs to expand and draw air in. Damage to this nerve can severely impair breathing.

Motor Control of the Sternocleidomastoid and Trapezius Muscles:

While the sternocleidomastoid and trapezius muscles are primarily innervated by the spinal accessory nerve (cranial nerve XI), the cervical plexus provides additional motor fibers that contribute to their function.

• Sternocleidomastoid Muscle: The cervical plexus assists in controlling this muscle, which is involved in head rotation and flexion. The sternocleidomastoid muscle allows the head to turn from side to side and flex forward.
• Trapezius Muscle: The cervical plexus contributes to the innervation of the trapezius muscle, which helps move and stabilize the shoulder blade (scapula). It plays a role in shrugging the shoulders and extending the neck.

Prevertebral and Lateral Neck Muscles:

The deep motor branches of the cervical plexus supply the prevertebral muscles of the neck, including the longus capitis and longus colli, which are important for neck flexion and maintaining neck posture.

• Neck Flexion and Stabilization: The prevertebral muscles, innervated by the cervical plexus, help flex the neck forward and maintain its stability. These muscles are essential for head movements and postural control of the neck.

Role in Reflexes

The cervical plexus plays a role in certain reflexes related to the neck and diaphragm:

• Cervical Plexus Contribution to the Gag Reflex: Although the primary reflex arc for the gag reflex involves cranial nerves, the cervical plexus contributes by innervating some muscles involved in the reflex response.
• Hiccups: Irritation of the phrenic nerve (derived from the cervical plexus) can lead to hiccups, which are involuntary contractions of the diaphragm.

Clinical Significance

The cervical plexus is clinically significant due to its involvement in various motor and sensory functions in the neck, head, and diaphragm. Here are key aspects of its clinical relevance:

• Nerve Blocks: The cervical plexus is often targeted for regional anesthesia during surgeries involving the neck, such as thyroidectomy, carotid endarterectomy, or lymph node dissections. A cervical plexus block can provide effective pain relief and reduce the need for general anesthesia.
• Phrenic Nerve Injury: Since the phrenic nerve arises from the cervical plexus (C3, C4, C5), injury to the plexus can affect diaphragm function, leading to respiratory issues. Phrenic nerve damage can result in diaphragmatic paralysis, causing difficulty breathing, especially if the injury is bilateral.
• Trauma or Surgery-Related Injuries: Damage to the cervical plexus during trauma, neck surgery, or improper positioning during medical procedures can lead to sensory loss, pain, or motor deficits, such as difficulty swallowing or impaired neck movements.
• Cervical Plexus Compression: Conditions such as tumors, cervical spine injuries, or muscle inflammation can compress the plexus, resulting in sensory disturbances or motor dysfunction in the areas innervated by the plexus.