The cervical plexus, formed by the anterior primary rami of the first four cervical nerves (C1–4), lies deep to the internal jugular vein and the sternocleidomastoid muscle and on the surface of scalenus medius and levator scapulae. Each of the primary rami, except C1, divides into two; C1 joins the upper branch of C2, the adjacent upper and lower branches of C2 and C3 fuse as do C3 and C4, and the lower branch of C4 joins C5 and contributes to the brachial plexus. Three loops are thus formed (Figure 1).
Cutaneous branches all gain the superficial fascia of the neck near to the midpoint of the posterior border of sternocleidomastoid.
- Lesser occipital nerve (C2) ascends the posterior border of sternocleidomastoid to supply the skin of the upper neck and the scalp behind the auricle.
- Great auricular nerve (C2, 3) ascends across sternocleidomastoid to reach the skin over the parotid gland. It supplies the skin over the parotid and the posterior auricle.
- Transverse cervical nerve (C2, 3) crosses sternocleidomastoid horizontally, deep to platysma, to supply the skin of the anterior triangle.
- Supraclavicular nerves (C3, 4) descend initially as one trunk, behind sternocleidomastoid and then divide near to the clavicle to supply the skin over the shoulder and upper pectoral region.
Muscular branches pass deeply from the plexus to supply the rhomboids, serratus anterior (C4, 5, 6), sternocleidomastoid (C2), trapezius (C3, 4), levator scapulae, and scalenus medius. Branches also supply the muscles of the suboccipital triangle. The phrenic nerve (C3, 4, 5) receives the largest contribution from C4 and conveys motor, sensory and sympathetic (see below) nerve fibres to the diaphragm. It originates at the lateral border of scalenus anterior, at the level of the upper border of the thyroid, and descends behind the internal jugular vein deep to the prevertebral fascia. It gains the thoracic cavity, passing anterior to the subclavian artery.
Communicating branches are contributed to the plexus from the sympathetic nervous system. Each of the primary rami C1–4 receives a grey ramus communicantes from the superior cervical sympathetic ganglion. The plexus gives a communicating branch (C1) to the hypoglossal nerve, which shortly after joining the nerve, leaves it to supply geniohyoid and thyrohyoid muscles. A branch joins with contributions from C2 and C3 to form the ansa hypoglossi nerve, and supplies the sternothyroid, sternohyoid and omohyoid muscles.
Cervical plexus block
Regional anaesthesia is used for surgical procedures in the neck region. An effective cervical plexus block produces anaesthesia over the neck, occipital region, shoulder region and upper pectoral region. Analgesic solution is infiltrated subcutaneously around the midpoint of the posterior border of sternocleidomastoid (Figure 2). Through this anaesthetic skin the areas lateral to the transverse processes of the second, third and fourth cervical vertebrae are infiltrated. The transverse processes are easily palpable and are reached by the needle tip about 2 cm below the skin. Precautions should be taken to avoid intrathecal or intravascular injection. Occasional inadvertent injections into the phrenic and vagus nerves occur as does a Horner’s syndrome from a cervical sympathetic block.
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