THE FACIAL NERVE CN VII

The Origin(s)

 Facial nucleus - pons

Superior salivary nucleus- pons

Gustatory nucleus - medulla oblongata

• Spinal nucleus of the trigeminal nerve - medulla oblongata

 


The Course and distribution

 

  • Exit the brain stem through cerebello pontine angle

  • Passes through the internal auditory meatus together with vestibulocochlear nerve and labyrinthine artery.

  • Has the geniculate ganglion containing cell bodies of the primary GSA, GVA, and SVA neurons

  • Runs in the facial canal between tympanic cavity and mastoid antrum

  • Exits through stylomastoid foramen

  • Enters parotid gland running between deep and superficial laminae

 

 

 


Part/ location
Branch
Distribution
In the facial canal
Greater petrosal nerve
  • Pterygopalatine ganglion

Nerve to stapedius

  • Stapedius muscle

chorda tympani
  • Taste to anterior two thirds of the tongue
  • Sub mandibular ganglion

Before parotid
Auricular nerve
  • Auricular muscle
  • Occipital belly of occipito-frontalis muscle

Nerve to digastric muscle
  • Posterior belly of digastric muscle
  • Stylohyoid muscle

In the parotid gland
Temporal nerve
  • muscles in the upper face
  • the orbicularis oris
  • frontalis muscles

Zygomatic nerve
  • middle portion of the face

Buccal nerve
  • cheek muscles
  • buccinator muscle

Marginal Mandibular nerve
  • Muscles of the lower face

Cervical nerve
  • Muscles below the chin
  • platysma muscle

   
Functional Components

•  Branchiomotor (SVE) to muscles of facial expression; and also to digastric and stylohyoid. These fibres are in the facial nerve proper.

•  General visceral efferent, GVE ( parasympathetic) to the submandibular and sublingual and lingual gland; to the lacrimal, palatine, pharyngeal, nasal and oral glands.

•  General visceral afferent (GVA) from the nasal cavity; the sinus cavities, and part of soft palate.

•  Special visceral afferent, SVA (taste) from the anterior 2/3 of the tongue

•  General somatic afferent (GSA) from the pinna of the ear and the external auditory meatus.

 



   

Probable cause and sites of injury

•  Brain stem vascular lesions

•  CP angle lesions e.g. acoustic neuroma

•  Facial canal oedema

•  Geniculate ganglion herpes zoster

•  Otitis media/mastoiditis

Birth trauma e.g. during forceps delivery

Clinical disorders of the facial nerve

  • Motor innervation of the muscles of facial expression is the principal function of the facial nerve.Accordingly, the most common symptom from damage to the nerve, nucleus or corticobulbar projections is a paralysis or paresis of these muscles. Involvement of the components of the intermediate nerve serves to localize the point of damage to the facial nerve.

  • Facial palsy is an impairment (paresis) or total paralysis of some or all of the muscles of facial expression. With damage to the facial nerve or facial nucleus, the paralysis is ipsilateral to the insult, and flaccid i.e. lower motor neuron damage. The following features are usually noticed:

     (+) Normal facial folds, wrinkles and creases due to the insertion of the muscles into the skin disappear, giving
          the affected side a smooth and expressionless appearance.

     (+) The corner of the mouth tends to droop, and there is a tendency for the patient to drool out of that of the
          corner of the mouth

     (+) The patient is unable to close the affected eye. If there is a lack of tearing the cornea tends to dry out.

      (+) With time, the muscles on the affected side tend to atrophy

      (+) Food accumulates in the vestibule of the affected side

 

  • Damage to only certain of the terminal branches of the facial nerve leads to a loss of only those muscles innervated.

  • Similarly nuclear lesions, if partial, may involve selected muscle groups because specific subnuclei of the facial nucleus innervate specific target muscles or muscle groups.

  • The term Bell's palsy describes a facial palsy resulting from damage to the facial nerve, at some point at or beyond the nerve's exit from the skull through the stylomastoid foramen. Typically, Bell's palsy does not involve the components of the intermediate nerve.

 

Supranuclear facial palsy, has a very characteristic feature, sparing of the upper muscles of facial expression (above the level of the palpebral fissure). Corticobulbar projections of facial neurons innervating the upper muscles of facial expression are both crossed and uncrossed, whereas those to neurons innervating the lower muscles of facial expression are all crossed. Hence, lesions affecting the corticobulbar projections produce contralateral paralysis of the lower muscles of facial expression, and spare the upper muscles. Patients are able to close both eyes and wrinkle both brows. In addition, because it is an upper motor neuron lesion, there is no accompanying atrophy of the muscles and there is a retention of facial reflexes i.e. the patient's ability to smile reflexly or show other emotional expressions.

 

Loss of taste in the anterior 2/3 of the tongue in combination with a facial palsy indicates damage to the facial nerve central to the exit of chorda tympani nerve. Also, the loss of the pre-ganglionic parasympathetic fibres to the submandibular ganglion causes diminished salivation (However, this sign may be difficult to detect, because the parotid gland is functional). The loss of taste over anterior 2/3 of the tongue and diminished salivation, without motor palsy suggest involvement only of the chorda tympani nerve. When anaesthesia of the anterior 2/3 of the tongue accompanies the loss of taste and diminished salivation, an involvement of the lingual nerve is likely.

 

An accompanied loss of tearing (lacrimal gland innervation) occurs only if the facial nerve lesion is central to the geniculate ganglion. Lacrimal gland innervation is via the greater petrosal nerve.

 

Hyperacusis, an increase in auditory sensitivity on the affected side, often occurs if there is paralysis of the stapedial muscle. This indicates injury to the facial nerve central to nerve to stapedius.

 

Herpes zoster infection of the geniculate ganglion primarily affects the regions with sensory supply from the facial nerve. The motor components are affected by the oedema. The collection of the effects comprises Ramsay-Hunt syndrome.