General Overview of Cervical Nerves
Human anatomy distinguishes 8 pairs of cervical spinal nerves, designated from C₁ to C₈. The anterior and posterior rami of these nerves serve as the foundation for the cutaneous, muscular, and mixed nerves of the neck. The cervical plexus (plexus cervicalis) specifically arises from the anterior rami of spinal nerves C₁–C₄, distributing branches to various regions of the head, neck, and chest.
Sensory (Cutaneous) Branches
The sensory branches of the cervical plexus provide cutaneous innervation to the scalp, neck, and upper chest. Most of these emerge at the punctum nervosum (nerve point of the neck) along the posterior border of the sternocleidomastoid muscle, originating from segments C₂–C₄.
- Lesser occipital nerve (n. occipitalis minor, C₂–C₃): Supplies sensation to the skin posterior to the auricle and the lateral occipital region.
- Great auricular nerve (n. auricularis magnus, C₂–C₃): Innervates the skin over the parotid gland, the auricle, and the angle of the mandible.
- Transverse cervical nerve (n. transversus colli, C₂–C₃): Supplies sensory innervation to the skin of the anterior neck.
- Supraclavicular nerves (nn. supraclaviculares): Formed from segments C₃–C₄ and divided into three groups. The medial branches innervate the skin over the manubrium of the sternum and the medial clavicle; the intermediate branches supply the upper chest and anterior shoulder; the lateral branches provide sensation over the acromion and lateral shoulder.
Additionally, the posterior scalp receives innervation from the greater occipital nerve (n. occipitalis major, C₂, posterior ramus supplying skin up to the vertex) and the third occipital nerve (n. occipitalis tertius, C₃, posterior ramus supplying the lower occipital skin).
Motor (Muscular) Branches
The motor component of the cervical plexus controls numerous neck and head muscles.
Suboccipital nerve (n. suboccipitalis, C₁): The posterior ramus of C₁, which innervates the deep suboccipital muscles: m. rectus capitis posterior major, m. rectus capitis posterior minor, m. obliquus capitis superior, and m. obliquus capitis inferior.
Muscular rami (rr. musculares) from the anterior rami distribute as follows:
- Fibers from C₁–C₄ supply the m. longus colli, m. longus capitis, m. rectus capitis anterior, m. rectus capitis lateralis, as well as the anterior, middle, and posterior scalene muscles (m. scalenus anterior, medius, posterior).
- Fibers from C₃–C₄ innervate the m. levator scapulae.
A notable structure is the ansa cervicalis (ansa cervicalis, C₁–C₃). It innervates the infrahyoid muscles: m. sternohyoideus, m. sternothyroideus, and both bellies of m. omohyoideus. Additionally, some C₁ fibers travel temporarily within the hypoglossal nerve (n. hypoglossus) before branching off to supply m. geniohyoideus and m. thyrohyoideus.
Mixed Branches and Anastomoses
The phrenic nerve (n. phrenicus) is the most critical mixed nerve of the cervical plexus. Originating primarily from segments C₃–C₅ (with C₄ being the major contributor), it performs a dual function:
- Motor: Drives contraction of the diaphragm, which is essential for pulmonary ventilation.
- Sensory: Innervates the mediastinal pleura, fibrous pericardium, and the diaphragmatic peritoneum.
Furthermore, the cervical plexus (segments C₂–C₄) communicates with the accessory nerve (n. accessorius). These connections provide proprioceptive innervation to the m. sternocleidomastoideus and m. trapezius.