Superficial Nerves of the Face and Scalp
In gross terms, the muscles of facial expression get their motor supply from the facial nerve. That is cranial nerve 7 and arises from the pontomedullary junction, between the abducent nerve (cranial nerve 6) and vestibulocochlear nerve (cranial nerve 8). The facial nerve has 5 divisions Temporal, Zygomatic, Buccal, Mandibular and Cervical. These branches can be remembered with the mnemonic: To Zanzibar By Motor Car. If you spread your fingers and place them on the side of your face and neck, your fingers roughly correspond to the position of the branches.
The sensory innervation to the face comes from the trigeminal nerve which is the only cranial nerve that arises directly from the pons. Its three branches are the Ophthalmic (V1), Maxillary (V2) and Mandibular (V3). Each of those branches supplies the corresponding region on the face. The trigeminal also supplies the muscles of mastication (medial and lateral pterygoids, temporalis and masseter). The scalp gains innervation from branches of the cervical plexus, which will be discussed.
Nerves of the Cervical Plexus
The great auricular nerve is a branch of the cervical plexus and has nerve roots C2 and C3. Its functional role is to supply sensory innervation on the skin over the mastoid process, outer ear and parotid gland regions.
The posterior auricular nerve is a branch of the facial nerve as it emerges from the stylomastoid foramen. At this point, it receives a communicating branch from the auricular branch of the vagus nerve. This nerve then ascends between the mastoid process and the external acoustic meatus, and then divides into the occipital and auricular branches. The occipital branch is responsible for supplying the intrinsic muscles on the surface of the external ear, as well as the auricularis posterior muscle.
The occipital nerve is a longer branch which travels posteriorly along the superior nuchal line found on the occipital bone (a lateral continuation of the external occipital protuberance), and supplies the occipitalis muscle.
The greater occipital nerve is a component of the cervical plexus. It is a spinal nerve from the medial branch of the dorsal primary ramus of the second cervical nerve. It emerges between the first two cervical vertebrae along with the lesser occipital nerve. The nerve then emerges from the suboccipital triangle which is defined by the rectus capitis posterior major, the obliquus capitus inferior and the obliquus capitis superior. It also emerges from beneath the lower border muscle (the obliquus capitis inferior). At that point, the nerve pierces the trapezius muscle, and travels superiorly to innervate the skin on the posterior scalp. It has a sensory function over the ear and parotid glands as well as the scalp of the head.
The lesser occipital nerve is also known as the small occipital nerve. It arises only from the root of the second cervical nerve and it emerges from the intervertebral foramen between the second and third cervical vertebrae. The greater occipital nerve also emerges from this foramen. The function of the lesser occipital nerve is to provide sensory innervation to the lateral area of the head posterior to the ear.
Ophthalmic Division (V1) nerves
The supraorbital nerve is a terminal branch of the frontal nerve. It travels through the supraorbital foramen, and provides palpebral branches to the upper eyelid. It also supplies the conjunctiva of the eye (the transparent covering over the inside of the eyelids and the white part of the eyeball), the skin of the forehead as far back as the middle of the scalp, and the frontal sinus.
The supratrochlear nerve is one of the branches of the frontal nerve, which is a branch of the ophthalmic division of the trigeminal nerve (5th cranial nerve). The nerve travels superior to the pulley of the superior oblique muscle and then gives off a fine descending filament that joins the infratrochlear branch of the nasociliary nerve. The supratrochlear nerve has a complex course and initially travels between the supraorbital foramen and the pulley of the superior oblique. Then, it takes a gradually curving path up the forehead and passes in between two muscles of the head and face - the frontalis and the corrugator supercili. At that point, it pierces these muscles and also splits into branches that supply the skin of the upper eyelid, the skin of the forehead, near the midline (the glabella region) and also the conjunctiva.
The external nasal branches are the terminal branches of the anterior ethmoidal nerves (from the ophthalmic division of the trigeminal nerve i.e. cranial nerve 5) and function to provide sensation to the skin over the cartilaginous nasal septum and also the lower half of the nose.
The infratrochlear nerve is the terminal branch of the nasociliary nerve, which is a branch of the ophthalmic division of the trigeminal nerve. The infratrochlear nerve arises after the anterior ethmoidal nerve enters the anterior ethmoidal foramen. The nerve runs anteriorly along the superior border of the medial rectus muscle and is usually joined by a small filament of the supratrochlear nerve near the pulley of the superior oblique muscle. The nerve then travels medially to supply the skin of the bridge of the external nose and the superior eyelids, caruncle and lacrimal sac.
Maxillary Division (V2) Branches
The zygomaticofacial nerve is a branch of the maxillary nerve which is a branch of the 5th cranial nerve (trigeminal nerve). The zygomaticofacial nerve supplies the skin which lies over both the temporal bone and zygomatic bone. It achieves that by entering the orbit. This nerve is very easily confused with the zygomatic branch of the facial nerve which is a motor nerve, not a sensory nerve.
The zygomaticotemporal nerve is a small branch of the V2 (maxillary) branch of the trigeminal nerve. The nerve travels along the lateral most wall of the orbital socket, specifically in a groove in the zygomatic bone. The nerve combines with a small branch of the lacrimal nerve. It enters the temporal fossa by passing through the zygomaticotemporal foramen in the zygomatic bone. From that location, the nerve progresses in the space between the temporal bone and the temporalis muscle. It goes on to pierce the temporal fascia superior to the zygomatic arch, where it divides and goes to supply sensory innervation to the skin at the side of the forehead. As the nerve pierces the fascia, it travels between the two layers of the fascia to eventually enter at the lateral angle of the orbit. This nerve also communicates with the facial nerve and the auriculotemporal branch of the mandibular nerve.
The infraorbital nerve is a branch of the maxillary nerve as it enters the infraorbital canal. It is responsible for providing sensory innervation to the lower eyelid, upper lip, infraorbital foramen of the maxilla and the nasal vestibule.
The mandibular division (V3) of the fifth cranial nerve i.e. the trigeminal nerve gives off a branch known as the auriculotemporal nerve. Its course is similar to both the superficial temporal artery (the terminal branch of the external carotid artery) and vein. The function of the nerve is to provide sensory innervation to the areas of skin on the side of the head. The buccal nerve is another branch of the mandibular nerve and is responsible for supplying sensory innervation to the skin over the buccal membrane (internal surface) of the cheek. It is important to distinguish this nerve from the buccal branch of the facial nerve which functions to provide motor innervation to the buccinator muscle on the cheek. Sensory innervation to the external skin of the face is also done via the trigeminal nerve. The mental nerve is the terminal branch of the inferior alveolar nerve (a branch of the mandibular division of the trigeminal nerve). It exits the mandible via the mental foramen to provide sensation over the chin area.
The facial nerve is cranial nerve 7 and is responsible for providing motor innervation to the muscles of facial expression. One of its branches, the chorda tympani, also provides the sense of taste to the anterior two thirds of the tongue.
The facial nerve gives branches to the buccal area, which can be described as the area of the cheek, around the mouth and below the eyes. The most superficial branches of this nerve run beneath the skin and on the surface of the superficial muscles of the face. This nerve supplies these muscles including procerus and it also joins the medial angle with the Infratrochlear and nasociliary branches of the ophthalmic nerve (V1 branch of the trigeminal nerve). The deeper branches of the nerve supply the small nasal muscles. They travel beneath and supply both the quadratus labii superioris and zygomaticus and they go on to form an infraorbital plexus in association with the infraorbital branch of the maxillary nerve. The buccinators and orbicularis oris also get supply from the lowest of these deep branches.
The temporal branches of the facial nerve are also known as the frontal branches of the facial nerve. They cross the zygomatic arch in order to reach the temporal region. There they supply the auricularis anterior and form a connection with the auriculotemporal branch of the mandibular nerve and also the zygomaticotemporal branch of the maxillary nerve. The corrugator supercili, frontalis, orbicularis oris gain supply from the more anterior branches of this nerve, and the temporal branches serve as an efferent limb of the corneal reflex (also known as the blink reflex).
The zygomatic branches of the facial nerve are also known as the malar branches and they traverse the zygomatic bone in order to reach the lateral angle of the orbit. There the orbicularis oculi is supplied and the nerve joins some filaments of the lacrimal nerve.
The cervical branch of the facial nerve is the lowest branch of the nerve and travels forward underneath the platysma muscle. The nerve forms a distinctive series of arches as it progresses through the suprahyoid region and across the neck itself. Some branches of the nerve supply the platysma and also the depressor anguli oris. A specific branch also joins the cervical plexus branch (the cervical cutaneous nerve).
The marginal mandibular branch of the facial nerve travels forward underneath the platysma and anguli oris, and then supplies the chin, lower lip and continues to communicate with the mental branch of the inferior alveolar nerve.