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Table of Contents
Cranial nerve VII is the facialnerve. It supplies motor, sensory and parasympathetic innervation to various structures of the head and neck. In this article, we discuss the embryology, structure and course of the facial nerve and the brainstem nuclei associated with it.
The facial nerve is the nerve associated with the second pharyngeal arch. The specific bones and muscles derived from the second pharyngeal arch are shown below.
Styloid process of the temporal bone
Lesser cornu of the hyoid bone
Muscles and ligaments
Muscles of facial expression
Stylohyoid muscle and ligament
Posterior belly of digastric
As you progress through this article, you may notice that each of these muscles attach to one or both of the bones associated with the second pharyngeal arch.
The facial nucleus
The facial motor nucleus is a round aggregation of motor neuron cell bodies found in the pontomedullary junction.
These are paired on the left and right side of the brainstem and are neatly divided horizontally in half.
The superior half of the nucleus representing the superior half of the face and the inferior half of the nucleus representing the inferior half of the face.
The superior half of the face derives conscious control from both the left and right primary motor cortex.
The inferior half derives conscious control from only the contralateral primary motor cortex.
Exiting the nucleus posteriorly, the fibres loop around the dorsal aspect of the abducens nerve on their way towards the ventral surface of the brainstem.
As the facial nerve motor fibres are moving through the brainstem, fibres from two important structures join together to run adjacent to the motor root of the facial nerve as a sensory root:
Somatic sensory fibres from the concha of the auricle (intermediate nerve);
Taste fibres join from the rostral solitary nucleus; and
Parasympathetic fibres from the superior salivatory nucleus.
The intracranial facial nerve
After exiting the cerebellopontine angle (see image 1), the two facial nerve roots are seen as a larger medial motor root and smaller lateral sensory root.
These two nerve roots travel ventro-laterally together to enter the internal auditory meatus on the posterior aspect of the petrous temporal bone.
The internal auditory meatus is approximately 1 cm long, and the facial nerve roots leave this meatus via the Z-shaped facial canal.
The facial canal begins at the internal auditory meatus and continues for 3 cm before opening at the stylomastoid foramen.
The facial canal is a point of convergence of the motor and sensory roots of the facial nerve.
The facial canal is divided into the labyrinthine, tympanic and mastoid segments. Two sensory and one motor branch leave the facial nerve here.
Labyrinthine part (first)
The facial nerve bends, known as the geniculum of the facial nerve, as a ganglion arises within it – the geniculate ganglion
The greater petrosal nerve exits through the pterygoid canal and enters the pterygopalatine ganglion
Tympanic part (second)
The facial nerve courses through the tympanic cavity
Mastoid part (third)
The nerve to the stapedius and chorda tympani both branch from the facial nerve
Functions of facial nerve sensory branches
Greater petrosal nerve
Function – parasympathetic fibres to the lacrimal gland
Structure – the first branch after the geniculate ganglion
Courses anteromedially in the temporal bone to open into the middle cranial fossa
It then travels over foramen lacerum to join with the deep petrosal nerve, forming the nerve of the pterygoid canal (Vidian nerve), which passes through the pterygoid canal (Vidian canal) to enter the pterygopalatine fossa and ganglion
Branches then exit the nerve to supply the lacrimal gland and mucosal glands of the mouth, nose and pharynx.
Function – taste fibres to the anterior 2/3 of the tongue, and parasympathetic fibres to the sublingual and submandibular glands
Structure – travels across the bones of the middle ear to leave the skull at the petrotympanic fissure. It then enters the infratemporal fossa and runs with the lingual nerve until reaching the anterior 2/3 of the tongue, or sublingual or submandibular glands
The facial nerve then exits the skull through the stylomastoid foramen.
The extracranial facial nerve
As the facial nerve emerges from the stylomastoid foramen, it gives rise to the posterior auricular branch, located immediately anterior to the mastoid process. It supplies motor function for:
Occipital part of occipitofrontalis
Auricularis anterior, posterior, superior and inferior
Continuing anteriorly, the facial nerve then gives off two branches:
Nerve to the posterior belly of digastricus – elevates the hyoid bone
Nerve to the stylohyoid muscle – elevates the hyoid bone
It then passes through the parotid gland. It does NOT innervate the parotid gland.
As it exits the parotid gland, it forms the parotid plexus and divides into five motor branches to supply different facial expression muscles of the head and neck (only the major muscles are listed):
Cervical – platysma (recognised as supplying the posterior belly of the digastric, stylohyoid muscle)
Clinical relevance – facial nerve pathology
Damage to the facial nerve can be divided in several ways. A common approach is to first consider whether the pathology is supranuclear (occurring above the facial nucleus) or infranuclear (occurring below the nucleus). To assess this, we must understand the origin of fibres sent from the primary motor cortex to the facial motor nucleus.
A supranuclear lesion to one side will cause paralysis of only the contralateral lower half of the face. This is because the upper half of the face is supplied bilaterally by the primary motor cortex, whereas the lower half of the face is supplied by only the contralateral primary motor cortex. An infranuclear lesion (referred to as a lower motor neuron lesion) will cause paralysis or weakness in both the upper and lower halves of the face.
After determining this, it can then be useful to think of pathologies as intracranial or extracranial. We know that the motor and sensory fibres for the facial nerve travel together intracranially. Extracranially, the facial nerve carries exclusively motor fibres.
If you determine an infranuclear or LMN lesion and there has been a loss of taste, tear production and facial expression muscle function, it is likely the pathology is intracranial:
Most commonly a middle ear tumour or infection
If idiopathic, it is called a Bell’s palsy
If there is a pure loss of motor function to the upper and lower facial fields, the pathology is likely extracranial:
Compressive or traumatic injuries
Idiopathic (Bell’s palsy)
CN VII is the facial nerve
It originates in the pontomedullary region
The facial nerve loops around the abducens nucleus
It passes through the internal auditory meatus and exits through the stylomastoid foramen
It provides many structures with innervation, as shown in the table below
LMN facial nerve lesions cause upper and lower facial paralysis
UMN facial nerve lesions cause lower facial paralysis only
General somatic afferent
Special visceral afferent
General visceral efferent
Special visceral efferent
Skin behind the ear
Taste to anterior 2/3 of the tongue
Parasympathetic to the lacrimal, sublingual and submandibular glands
Muscles of facial expression
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Moore, K. L., Dalley, A. F., & Agur, A. M. (2013). Clinically oriented anatomy. Lippincott Williams & Wilkins.
Nolte, J. (2002). The human brain: an introduction to its functional anatomy.
Snell, R. S. (2010). Clinical neuroanatomy. Lippincott Williams & Wilkins.
Patrick J. Lynch, medical illustrator [CC BY 2.5 (https://creativecommons.org/licenses/by/2.5)]. Modified by Dr Lewis Potter.