Skin and motor innervation of head and neck

he skin of the head and neck is innervated by branches of the trigeminal nerve (CN V) and cervical spinal nerves.

1. Trigeminal Nerve (CN V)

The trigeminal nerve provides sensory innervation to the face and has three major divisions:

• Ophthalmic nerve (V1) – Supplies the forehead, scalp, upper eyelid, and dorsum of the nose.
• Maxillary nerve (V2) – Supplies the lateral nose, upper lip, lower eyelid, cheek, and upper jaw.
• Mandibular nerve (V3) – Supplies the lower lip, chin, lower jaw, and part of the auricle of the ear.

2. Cervical Spinal Nerves

• Greater occipital nerve (C2) – Supplies the posterior scalp.
• Lesser occipital nerve (C2, C3) – Supplies the lateral side of the neck and behind the ear.
• Great auricular nerve (C2, C3) – Supplies the lower auricle and parotid region.
• Transverse cervical nerve (C2, C3) – Supplies the anterior neck.
• Supraclavicular nerves (C3, C4) – Supply the lower neck and upper chest.

II. Motor Innervation of the Head and Neck

Motor innervation in the head and neck is provided by cranial nerves and cervical spinal nerves.

1. Facial Nerve (CN VII)

• Innervates the muscles of facial expression.
• Also supplies the posterior belly of the digastric muscle, stylohyoid muscle, and stapedius muscle.

2. Trigeminal Nerve (CN V) – Mandibular Division (V3)

• Provides motor innervation to the muscles of mastication (masseter, temporalis, medial, and lateral pterygoid muscles).
• Also supplies the mylohyoid, anterior belly of digastric, tensor tympani, and tensor veli palatini muscles.

3. Glossopharyngeal Nerve (CN IX)

• Innervates the stylopharyngeus muscle (important in swallowing).

4. Vagus Nerve (CN X)

• Provides motor innervation to most muscles of the pharynx, larynx, and soft palate (except stylopharyngeus and tensor veli palatini).

5. Accessory Nerve (CN XI)

• Innervates the sternocleidomastoid and trapezius muscles.

6. Hypoglossal Nerve (CN XII)

• Innervates all intrinsic and extrinsic muscles of the tongue (except palatoglossus, which is innervated by CN X).

7. Cervical Spinal Nerves

• Ansa cervicalis (C1-C3) – Supplies the infrahyoid muscles (sternohyoid, sternothyroid, and omohyoid).
• C1 via hypoglossal nerve – Supplies the thyrohyoid and geniohyoid muscles.
• Cervical plexus (C1-C4) – Supplies motor branches to the neck muscles.
• Brachial plexus (C5-T1) – Contributes to motor innervation of some neck and upper limb muscles.

Clinical Correlations

• Trigeminal neuralgia – Severe facial pain due to trigeminal nerve irritation.
• Bell's palsy – Facial muscle paralysis due to damage to the facial nerve (CN VII).
• Accessory nerve palsy – Weakness in the trapezius and sternocleidomastoid muscles.
• Hypoglossal nerve palsy – Tongue deviation toward the affected side.