Cervical plexus
The cervical plexus is a network of nerve fibers originating from the anterior (ventral) rami of the first four cervical spinal nerves (C1–C4), forming a complex neurologic structure that provides both sensory and motor innervation primarily to the neck, upper anterior torso, and certain muscles of the head and shoulders.[1][2] It arises within the upper neck, lying deep to the sternocleidomastoid muscle and internal jugular vein, and anterior to the scalenus medius and levator scapulae muscles, with each ramus receiving sympathetic input from the superior cervical ganglion via a grey ramus communicans.[1][2][3] The plexus divides into superficial and deep components: the superficial portion emerges laterally from the posterior border of the sternocleidomastoid muscle to supply cutaneous sensation, while the deep portion remains intramuscular to innervate deeper structures.[1][2] It communicates with nearby cranial nerves, including the facial, hypoglossal, spinal accessory, and vagus nerves, as well as the sympathetic trunk, facilitating integrated innervation of the region.[3] Key sensory branches include the lesser occipital nerve (from C2), which innervates the scalp posterior to the auricle; the great auricular nerve (C2–C3), supplying skin over the parotid gland, mastoid process, and angle of the mandible; the transverse cervical nerve (C2–C3), providing sensation to the anterior and lateral neck; and the supraclavicular nerves (C3–C4), which cover the skin of the upper chest, clavicle, and acromion.[1][2][3] Motor branches encompass the ansa cervicalis (C1–C3), which innervates the infrahyoid muscles involved in swallowing and head movement; direct branches to muscles such as the geniohyoid, thyrohyoid, sternocleidomastoid, trapezius, levator scapulae, and scalenus medius; and the phrenic nerve (primarily C4 with contributions from C3–C5), which is crucial for diaphragmatic contraction and respiration.[1][2][3][4] Clinically, the cervical plexus is significant for procedures like regional anesthesia blocks, which target its superficial branches for surgeries involving the neck, thyroid, or carotid artery, often guided by ultrasound to minimize risks such as phrenic nerve palsy or vascular injury.[1][2] Injuries or neuropathies affecting the plexus can lead to sensory deficits in the neck and shoulder or motor impairments in neck flexion and diaphragmatic function, underscoring its role in both daily mobility and critical respiratory support.[2][3]Overview
Definition and Formation
The cervical plexus is a plexiform network of nerve fibers formed by the anterior (ventral) rami of the first four cervical spinal nerves (C1–C4).[1] This structure arises as the ventral roots from the spinal cord's basal plate extend laterally to form the anterior rami, which then interconnect to create the plexus.[5] The formation involves the interlacing of these rami within the prevertebral layer of the deep cervical fascia that serve as the foundation for subsequent branching.[4] Specifically, the C1 ramus joins the hypoglossal nerve (cranial nerve XII) and travels with it before detaching as the descendens hypoglossi, carrying cervical fibers to innervate certain infrahyoid muscles.[6] Each ramus also receives sympathetic input from the superior cervical ganglion via gray rami communicantes, integrating autonomic components into the plexus.[6] Embryologically, the cervical plexus develops from the ventral rami of spinal nerves in the cervical region as part of the peripheral nervous system formation following neural tube development.[5] This foundational anatomy was detailed in seminal texts such as Gray's Anatomy: The Anatomical Basis of Clinical Practice (42nd edition, 2020), with no substantive updates to its core formation and structure reported since.Location and General Structure
The cervical plexus is situated deep to the sternocleidomastoid muscle and lies lateral to the transverse processes of the C1-C4 vertebrae. It is positioned within the deep cervical fascia, specifically between the prevertebral fascia anteriorly and the scalene muscles posteriorly. This arrangement places the plexus anterior to the scalene muscles and posterior to the carotid sheath, deep to the investing layer of the deep cervical fascia.[6] Formed from the anterior rami of the C1-C4 spinal nerves, the cervical plexus exhibits a general structure divided into superficial and deep divisions. The superficial division primarily provides cutaneous innervation, whereas the deep division supplies motor innervation to neck muscles. These divisions arise from interconnecting loops of the ventral rami, contributing to the plexus's organized layout in the upper neck region.[6][7] The plexus emerges at the "nerve point of the neck," located in the posterior triangle of the neck approximately 2-3 cm superior to the clavicle, near the midpoint of the posterior border of the sternocleidomastoid muscle. Surrounding landmarks include the internal jugular vein and sternocleidomastoid muscle anteriorly, as well as the scalenus medius and levator scapulae muscles posteriorly.[6][7] In adults, the cervical plexus extends from the base of the skull to the clavicle. This vertical extent underscores its role in bridging upper cervical structures with supraclavicular regions.[6]Anatomy
Components and Loops
The cervical plexus exhibits an internal organization characterized by a three-loop system, formed by the anterior rami of the first four cervical spinal nerves (C1-C4). The superior loop arises from the union of C1 and C2 rami, the middle loop from C2 and C3, and the inferior loop from C3 and C4; these loops interconnect via rami communicantes, facilitating fiber exchange between levels and contributing to the plexus's integrated neural architecture.[8][9] The plexus divides into superficial and deep components, each serving distinct roles in innervation. The superficial division comprises primarily cutaneous branches originating from C2 and C3, emerging laterally from the posterior border of the sternocleidomastoid muscle to supply sensory fibers to the skin. In contrast, the deep division encompasses motor branches from C1 to C4, including a key contribution to the phrenic nerve (primarily C3-C5, with C4 input from the cervical plexus), which descends to innervate the diaphragm.[1][8] Internally, nerve fibers within the loops intermingle extensively before segregating into specific pathways, enabling a mosaic distribution of sensory and motor contributions across branches; this arrangement occurs deep to the sternocleidomastoid and anterior to the scalene muscles. The C1 ramus frequently lacks a dorsal root, consisting almost entirely of motor fibers that join the hypoglossal nerve to form part of the ansa cervicalis loop.[9][10] At the microscopic level, the cervical plexus consists of bundles of myelinated axons—both A-alpha fibers for motor transmission and A-delta/beta fibers for sensory conduction—ensheathed by Schwann cells, which myelinate peripheral nerves and support axonal regeneration following injury.[1]Branches
The cervical plexus gives rise to superficial and deep branches. The superficial branches emerge from the posterior border of the sternocleidomastoid muscle at its midpoint, known as the nerve point of the neck. These branches form through the interconnected loops of the anterior rami of C1-C4 spinal nerves. Except for the phrenic nerve, which arises higher from the superior aspect of the plexus, the superficial branches exit via this common point.[4][1]Superficial Branches
The superficial branches, also termed cutaneous branches, provide coverage to the skin of the neck, scalp, and upper thorax. They include the lesser occipital, great auricular, transverse cervical, and supraclavicular nerves.- The lesser occipital nerve arises from C2, with occasional contributions from C3. It emerges at the nerve point and ascends along the posterior border of the sternocleidomastoid muscle, curving around the accessory nerve to reach the posterosuperior aspect of the scalp near the auricle.[4][11]
- The great auricular nerve originates from C2 and C3. Emerging at the nerve point, it courses superiorly and anteriorly across the sternocleidomastoid muscle, ascending diagonally to the parotid gland and the posterior aspect of the auricle, extending from the angle of the mandible to the mastoid process.[4][1]
- The transverse cervical nerve derives from C2 and C3. It emerges at the nerve point, wraps horizontally around the posterior border of the sternocleidomastoid muscle, and pierces the deep cervical fascia deep to the platysma to reach the anterior triangle of the neck.[4][11]
- The supraclavicular nerves stem from C3 and C4. They emerge as a single trunk at the nerve point behind the sternocleidomastoid muscle, descend inferiorly, and divide near the clavicle into medial, intermediate, and lateral branches that cross over the clavicle to the shoulder region.[4][1]
Deep Branches
The deep branches, primarily muscular, extend to various neck muscles and form structures such as loops for distribution. They include segmental nerves to specific suprahyoid and infrahyoid muscles, the ansa cervicalis, nerves to the levator scapulae and trapezius, and the phrenic nerve.- Segmental branches from C1 travel with the hypoglossal nerve: one to the geniohyoid muscle and another to the thyrohyoid muscle. Additional segmental branches from C1-C3 supply the omohyoid, sternohyoid, and sternothyroid muscles via the ansa cervicalis loop. The ansa cervicalis itself forms from a superior root (C1, sometimes with C2) descending superficial to the internal jugular vein and an inferior root (C2-C3) joining it in the anterior cervical triangle to create a U-shaped loop.[3][4]
- The nerve to the levator scapulae arises from C3 and C4, emerging within the deep aspect of the plexus to reach the muscle directly. Branches to the trapezius originate from C2-C4 and join the spinal accessory nerve (CN XI) before distributing to the muscle.[4][3]
- The phrenic nerve forms from C3, C4, and C5 (primarily C4), arising from the superior part of the plexus at the lateral border of the anterior scalene muscle. It descends obliquely across the anterior surface of the scalene muscle, posterior to the subclavian vein, and enters the thorax anterior to the lung root.[4][11]