Cervical Plexus
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Direct answer
Anterior rami of C1 to C4, joined in most people by a contribution from C5, form the cervical plexus deep to the sternocleidomastoid on the scalenus medius, and from it emerge the skin and strap muscles of the front and side of the neck. Four cutaneous nerves — lesser occipital (C2), great auricular (C2–3), transverse cervical (C2–3) and supraclavicular (C3–4) — all surface at a single point on the posterior border of sternocleidomastoid, Erb's point or punctum nervosum. The deep branches include the ansa cervicalis to the infrahyoid muscles, branches to the prevertebral muscles, and the phrenic nerve (C3, 4, 5) that descends on the scalenus anterior to the diaphragm — "C3, 4, 5 keeps the diaphragm alive."
What you must remember
- Formation: loops between the anterior rami of C1–C4 (plus C5 in many), lying on the levator scapulae and scalenus medius, deep to the prevertebral fascia and the sternocleidomastoid.
- Punctum nervosum (Erb's point): the junction of the upper and middle thirds of the posterior border of sternocleidomastoid, where all four cutaneous nerves emerge and where the spinal accessory nerve also enters the posterior triangle — a surgical no-crossing zone.
- Lesser occipital (C2): ascends along the posterior border of sternocleidomastoid to the upper ear and mastoid region behind the auricle.
- Great auricular (C2–3): the largest cutaneous branch; winds around the posterior border, ascends vertically across sternocleidomastoid to the parotid region, ear lobule and angle of mandible — at risk in parotidectomy and face-lift surgery.
- Transverse cervical (cutaneous nerve of the neck, C2–3): curves around the border and runs forward across the muscle to supply the anterior triangle skin.
- Supraclavicular (C3–4): emerges just below the others and divides into medial, intermediate and lateral groups to the lower neck, clavicle, shoulder and upper chest as far as the second rib.
- Ansa cervicalis: superior root from C1 (travelling briefly with the hypoglossal nerve), inferior root from C2–3, looped over the internal jugular vein; supplies sternohyoid, sternothyroid, omohyoid — thyrohyoid and geniohyoid get direct C1 twigs.
- Phrenic nerve (C3–5): crosses scalenus anterior from lateral to medial behind the prevertebral fascia, passes behind the subclavian vein, through the mediastinum anterior to the root of the lung and on the pericardium, to the diaphragm; motor to the muscle, sensory to the central diaphragmatic peritoneum, pleura and pericardium.
- Referral rule: diaphragmatic irritation is felt at the shoulder tip (C4 via the supraclavicular nerves) — Kehr's sign in splenic rupture, and referred shoulder-tip pain after laparoscopic gas.
Reasoning from shoulder-tip pain to nerve roots
A young man collapses after a bicycle fall; hours later his left shoulder tip aches although nothing struck it — the cervical plexus explains the paradox. The central part of the diaphragmatic peritoneum is supplied by the phrenic nerve (C3–5), and the skin over the shoulder tip and clavicle by the supraclavicular nerves (C3–4) of the same plexus. Blood irritating the left side of the diaphragm under the spleen therefore fires C3–4 segments, and the brain attributes the message to the skin it knows from those roots: referred pain, not local pain, and a classic surgical sign of intraperitoneal bleeding. The same logic explains the shoulder pain after laparoscopic insufflation and the phrenic irritation of basal pneumonia or pericarditis. Trace the nerve onward and the plexus's surgical geography unfolds: the phrenic nerve is the only structure crossing scalenus anterior from its lateral to medial border — the landmark that distinguishes it from the accessory nerve in the posterior triangle — and its anterior position on the pericardium makes it vulnerable during central venous catheter placement and cardiac surgery, where intraoperative identification is routine. Above it, the great auricular nerve's vertical ascent across sternocleidomastoid makes it the first identifiable landmark in parotid surgery, and its sacrifice leaves a numb ear lobule.
Where students slip
Students label the roots "C1–C4 spinal nerves" — the plexus is formed of anterior (ventral) rami, and saying "roots" without "rami" loses the mark in strict anatomy examinations. The second slip is attributing strap-muscle paralysis to a hypoglossal lesion: the ansa cervicalis supplies the infrahyoids, the hypoglossal only borrows C1 briefly. Third, the phrenic nerve lies on scalenus anterior under the prevertebral fascia, while the accessory nerve crosses the posterior triangle above it — confusing the two confuses two different operations and two different examinations.
Frequently asked questions
Which nerves surface at Erb's point?
The lesser occipital, great auricular, transverse cervical and supraclavicular nerves, plus the entrance of the spinal accessory nerve into the posterior triangle. The point lies at the junction of the upper and middle thirds of the posterior border of sternocleidomastoid.
What is the root value and course of the phrenic nerve?
C3, C4 and C5. The nerve forms at the upper lateral border of the scalenus anterior, crosses that muscle from its lateral to its medial border, descends through the thorax anterior to the root of the lung on the pericardium, and reaches and pierces the diaphragm.
Why is diaphragmatic irritation felt at the shoulder tip?
The phrenic nerve (C3–5) carries sensation from the central diaphragmatic peritoneum and pleura, and the supraclavicular nerves (C3–4) carry sensation from the shoulder tip skin. Shared segmental innervation redirects the pain to the shoulder — Kehr's sign in splenic rupture.
What does the ansa cervicalis supply, and how is it formed?
A loop between the superior root from C1 (joining the hypoglossal nerve briefly) and the inferior root from C2–3, lying on or looped around the internal jugular vein. It supplies sternohyoid, sternothyroid and omohyoid; C1 twigs directly supply thyrohyoid and geniohyoid.
Which cervical plexus branch is the landmark in parotid surgery?
The great auricular nerve, which ascends vertically across the sternocleidomastoid to the parotid gland, ear and angle of the mandible. Its identification protects it, or its sacrifice leaves numbness of the ear lobule and preauricular skin.