The facial nerve (CN VII) is the seventh paired cranial nerve and is associated with the derivatives of the second pharyngeal arch. In this article, we shall look at the anatomy of the facial nerve – its anatomical course, functions and clinical correlations.
The facial nerve has associated motor, sensory and special sensory functions. Motor-wise, the facial nerve is responsible for innervating the muscles of facial expression, posterior belly of the digastric, stylohyoid and stapedius muscles. Sensory-wise, the facial nerve innervates a small area around the concha of the external ear, and special sensory-wise, the facial nerve provides special taste sensation to the anterior two-thirds of the tongue via the Chorda Tympani and also provides parasympathetic innervation to many of the glands of the head and neck, including the submandibular and sublingual salivary glands, nasal, palatine and pharyngeal mucous glands and the lacrimal glands.
The course of the facial nerve is very complex; there are many branches, which transmit a combination of sensory, motor, and parasympathetic fibres. The anatomical course of the facial nerve can be divided into two parts- intracranial and extracranial and the two parts will now be discussed in turn.
The facial nerve arises in the pons, an area of the brainstem. It begins as two roots; a large motor root, and a small sensory root (the part of the facial nerve that arises from the sensory root is sometimes known as the intermediate nerve). The two roots travel through the internal acoustic meatus, a 1cm long opening in the petrous part of the temporal bone. Here, they are in very close proximity to the inner ear. Still within the temporal bone, the roots leave the internal acoustic meatus, and enter into the facial canal; a ‘Z’ shaped structure. Within the facial canal, three important events occur- Firstly the two roots fuse to form the facial nerve. Next, the nerve forms the geniculate ganglion (a ganglion is a collection of nerve cell bodies). Lastly, the nerve gives rise to three branches, the greater petrosal nerve, the nerve to the stapedius and the chorda tympani.
After exiting the skull, the facial nerve turns superiorly to run just anterior to the outer ear. The first extracranial branch to arise is the posterior auricular nerve. It provides motor innervation to the some of the muscles around the ear. Immediately distal to this, motor branches are sent to the posterior belly of the digastric muscle and to the stylohyoid muscle. The main trunk of the nerve, now termed the motor root of the facial nerve, continues anteriorly and inferiorly into the parotid gland (note – the facial nerve does not contribute towards the innervation of the parotid gland, which is innervated by the glossopharyngeal nerve). Within the parotid gland, the nerve terminates by splitting into five branches- temporal branch, zygomatic branch, buccal branch, marginal mandibular branch and the cervical branch.
Branches of the facial nerve are responsible for innervating many of the muscles of the head and neck, all of which are derivatives of the second pharyngeal arch. The first motor branch arises within the facial canal; the nerve to stapedius which passes through the pyramidal eminence to supply the stapedius muscle in the middle ear. Between the stylomastoid foramen and the parotid gland, three more motor branches are given off-
Within the parotid gland, the facial nerve terminates by bifurcating into five motor branches. These innervate the muscles of facial expression-
In conclusion, the facial nerve (CNVII) provides motor, sensory and special sensory functions, and its anatomical course consists of both an intracranial and an extracranial part. Within the skull, motor branches arise from the facial nerve which supply the stapedius muscle, posterior belly of the digastric muscle and stylohyoid muscle. Outside of the skull, the facial nerve terminates by bifurcating into five motor branches which are responsible for innervating the muscles of facial expression.
The facial nerve (CN VII) is the seventh paired cranial nerve. In this article, we shall look at the anatomy of the facial nerve - its anatomical course, functions and clinical correlations.
The facial nerve is associated with the derivatives of the second pharyngeal arch:
The course of the facial nerve is very complex. There are many branches, which transmit a combination of sensory, motor and parasympathetic fibres. Anatomically, the course of the facial nerve can be divided into two parts:
The facial nerve begins in the pons, an area of the brainstem. It arises as two roots; a large motor root, and a small sensory root (the part of the facial nerve that arises from the sensory root is sometimes known as the intermediate nerve).
Motor functions of the facial nerve are responsible for innervating the orbicularis oris, buccinator and zygomaticus (buccal branch), depressor labii inferioris, depressor anguli oris and mentalis (marginal mandibular branch) and the platysma (cervical branch).
The chorda tympani branch of the facial nerve is responsible for innervating the anterior 2/3 of the tongue with the special sense of taste. The nerve arises in the facial canal, and travels across the bones of the middle ear, exiting via the petrotympanic fissure, and entering the infratemporal fossa. Within the infratemporal fossa, the chorda tympani ‘hitchhikes’ upon the lingual nerve. The parasympathetic fibres of the chorda tympani stay with the lingual nerve, but the main body of the nerve leaves to innervate the anterior 2/3 of the tongue.
The parasympathetic fibres of the facial nerve are carried by the greater petrosal and chorda tympani branches.
The greater petrosal nerve arises immediately distal to the geniculate ganglion within the facial canal. It then moves in an anteromedial direction, exiting the temporal bone into the middle cranial fossa. From here, it travels across (but not through) the foramen lacerum, combining with the deep petrosal nerve to form the nerve of the pterygoid canal. The nerve of the pterygoid canal then passes through the pterygoid canal (Vidian canal) to enter the pterygopalatine fossa, and synapses with the pterygopalatine ganglion. Branches from this ganglion then go on to provide parasympathetic innervation to the mucous glands of the oral cavity, nose and pharynx, and the lacrimal gland.
The chorda tympani also carries some parasympathetic fibres. These combine with the lingual nerve (a branch of the trigeminal nerve) in the infratemporal fossa and form the submandibular ganglion. Branches from this ganglion travel to the submandibular and sublingual salivary glands.
The facial nerve has a wide range of functions. Thus, damage to the nerve can produce a varied set of symptoms, depending on the site of the lesion.
Intracranial lesions occur during the intracranial course of the facial nerve (proximal to the stylomastoid foramen). The muscles of facial expression will be paralysed or severely weakened. The other symptoms produced depend on the location of the lesion, and the branches that are affected:
The most common cause of an intracranial lesion of the facial nerve is infection related to the external or middle ear. If no definitive cause can be found, the disease is termed Bell’s palsy.
Extracranial lesions occur during the extracranial course of the facial nerve (distal to the stylomastoid foramen). The symptoms that arise will depend on which branches of the facial nerve are affected. Extracranial lesions of nerves such as the facial and seventh cranial nerves can be caused by head and neck trauma, surgery, tumours, and other medical conditions.
In summary, the facial nerve (CN VII) is responsible for various functions including those related to motor (facial expression), sensory (a small area around the concha of the external ear), special sensory (taste on the anterior 2/3 of the tongue) and parasympathetic (mucous glands of the head and neck, and the lacrimal gland) functions. Damage to the facial nerve can result in a multitude of symptoms, depending on the site of the lesion (intracranial or extracranial).
The two roots of the facial nerve travel through the internal acoustic meatus, a 1cm long opening in the petrous part of the temporal bone. Here, they are in very close proximity to the inner ear.
Still within the temporal bone, the roots leave the internal acoustic meatus, and enter into the facial canal. The facial canal is a 'Z' shaped structure. Within the facial canal, three important events occur. Firstly, the two roots fuse together to form the facial nerve. Next, the nerve forms the geniculate ganglion (a ganglion is a collection of nerve cell bodies). Lastly, the nerve gives rise to a number of useful branches, including the greater petrosal nerve (which is responsible for conveying parasympathetic fibres to mucous glands and the lacrimal gland), the nerve to stapedius (which provides motor fibres to the stapedius muscle of the middle ear) and the chorda tympani (which is responsible for special sensory fibres to the anterior 2/3 tongue, as well as parasympathetic fibres to the submandibular and sublingual glands).
The facial nerve then exits the facial canal (and the cranium) via the stylomastoid foramen. This is an exit located just posterior to the styloid process of the temporal bone.
Once the facial nerve has exited the skull, it turns in a superior direction and runs just anterior to the outer ear. The first extracranial branch to arise is the posterior auricular nerve, which provides motor innervation to the some of the muscles around the ear. Immediately distal to this, motor branches are sent to the posterior belly of the digastric muscle and to the stylohyoid muscle. The main trunk of the nerve is now known as the motor root of the facial nerve, and subsequently continues anteriorly and inferiorly into the parotid gland (note - the facial nerve does not contribute towards the innervation of the parotid gland, which is innervated by the glossopharyngeal nerve).
Within the parotid gland, the nerve terminates by splitting into five branches, including the temporal branch, zygomatic branch, buccal branch, marginal mandibular branch and cervical branch. These branches are responsible for innervating the muscles of facial expression.
Branches of the facial nerve are responsible for innervating many of the muscles of the head and neck, all of which are derivatives of the second pharyngeal arch.
The first motor branch arises within the facial canal; the nerve to stapedius. The nerve passes through the pyramidal eminence to supply the stapedius muscle in the middle ear.
Between the stylomastoid foramen, and the parotid gland, three more motor branches are given off. The posterior auricular nerve ascends in front of the mastoid process, and innervates the intrinsic and extrinsic muscles of the outer ear. It also supplies the occipital part of the occipitofrontalis muscle. The nerve to the posterior belly of the digastric muscle innervates the posterior belly of the digastric muscle (a suprahyoid muscle of the neck). It is responsible for raising the hyoid bone. The nerve to the stylohyoid muscle innervates the stylohyoid muscle (a suprahyoid muscle of the neck). It too is responsible for raising the hyoid bone.
Within the parotid gland, the facial nerve terminates by bifurcating into five motor branches. These innervate the muscles of facial expression as follows:
The chorda tympani branch of the facial nerve is responsible for innervating the anterior 2/3 of the tongue with the special sense of taste.
The nerve arises in the facial canal, and travels across the bones of the middle ear, exiting via the petrotympanic fissure, and entering the infratemporal fossa. Within the infratemporal fossa, the chorda tympani 'hitchhikes' upon the lingual nerve. The parasympathetic fibres of the chorda tympani stay with the lingual nerve, but the main body of the nerve leaves to innervate the anterior 2/3 of the tongue.
The parasympathetic fibres of the facial nerve are carried by the greater petrosal and chorda tympani branches.
The greater petrosal nerve arises immediately distal to the geniculate ganglion within the facial canal. It then moves in an anteromedial direction, exiting the temporal bone into the middle cranial fossa.
The facial nerve (CN VII) is a mixed nerve that has multiple functions, such as motor movement, special sensory reception, and parasympathetic control. Its anatomical course starts within the skull, continuing through the stylomastoid foramen, and finally branching out into numerous fibers. Its motor function primarily covers the muscles responsible for facial expressions, while any damage to this nerve can cause either a paralysis or a severe weakening of one side of the face. Intracranial lesions may arise due to an external or a middle ear infection, or an extracranial lesion may come about due to pathology of the parotid gland, nerve infection, or compression during forceps delivery. All these may cause bell's palsy, an idiopathic cause.
The facial nerve then travels across (but not through) the foramen lacerum, and combines with the deep petrosal nerve to form the nerve of the pterygoid canal. This nerve passes through the pterygoid canal (or the Vidian canal) and enters the pterygopalatine fossa, where it synapses with the pterygopalatine ganglion. The branches from this ganglion supply parasympathetic innervation to the mucous glands of the oral cavity, nose and pharynx, and even to the lacrimal gland.
The chorda tympani also contains some parasympathetic fibers, which come together with the lingual nerve (a branch of the trigeminal nerve) in the infratemporal fossa to form the submandibular ganglion. Branches from this ganglion reach out to the submandibular and sublingual salivary glands.
In summary, the facial nerve is a multi-faceted complex of functions. Its anatomy and pathways are intricate and require a deeper understanding of how the nerve works in order for it to be properly diagnosed and treated. It is always important to be mindful of the many potential causes of problems with the facial nerve, and to seek professional help when necessary, as the effects can be quite significant.
