Seventh Cranial Nerve

The seventh cranial nerve (CNVII) is also named the facial nerve. It mainly provides motor innervation to the muscles of the face, and also sensory information for the tongue and parasympathetic stimuli to glands in the mouth and face.

The four major functions of the facial nerve are:

  • Motor movement to facial muscles for facial expressions
  • Parasympathetic secretomotor innervation to submandibular and sublingual salivary glands and the lacrimal gland of the eyes
  • Taste sensation from anterior two-thirds of the tongue
  • Sensations from the ear and the external auditory meatus

Location and Course

All cranial nerves (all nerves for that matter) have a nucleus (cell body) and axon (that carries nerve impulses away from the nucleus to other structures). There are three nuclei with the CNVII - the facial motor nucleus, the parasympathetic nucleus, and the sensory nucleus. The facial nerve is divided into two main roots—the motor root (motor neurons) and the intermediate nerve (sensory and parasympathetic fibers). These two roots leave the lower pons and enter the petrous portion of the temporal bone. The facial nerve splits to follow an intracranial course (inside the skull cavity) and the extracranial course (outside the skull and along the face/neck).

Just after entering the temporal bone, these two roots merge together and are then referred to as the facial nerve. After merging, the cell bodies of facial nerve neurons join to form the geniculate ganglion. This intracranial portion of the facial nerve finally branches to form 3 internal branches composed of sensory and motor functions:

  • Greater petrosal nerve (parasympathetic fibers that stimulate the mucous and lacrimal glands)
  • Nerve to the stapedius (controls the stapedius muscle in the inner ear)
  • Chorda tympani (provides taste for part of the tongue and stimulus to salivary glands).

The extracranial portion of the facial nerve caries only motor neurons. The extracranial portion exits the skull through the stylomastoid foramen at the base of the skull then splits into 5 external branches:

  • Temporal branch (frontalis muscle of the face and portions of the orbicularis oculi (forehead and around eyes))
  • Zygomatic branch (orbicularis oculi (around eyes))
  • Buccal branch (orbicularis pros, buccinator, and zygomatic muscles (around lips, cheeks, and corners of the mouth))
  • Marginal mandibular branch (mentalis muscle (lower lip and chin))
  • Cervical branch (platysma (angle of the jaw down to the collarbone))

As you can see, the distribution of the facial nerve is somewhat complex!

A quick review of the location and destination of CNVII:

  • Starts in the PONS
  • Carries motor, sensory, and parasympathetic fibers
  • Follows an intracranial course (motor, sensory, parasympathetic fibers) and extracranial course (motor fibers)

Facial Nerve Problems

Damage to the facial nerve can occur anywhere along its long, complex pathway.

Facial Nerve Palsy and Bell's Palsy

Palsy or paresis of the facial nerve causes weakness to the muscles of facial movement. An upper motor neuron lesion early in the facial nerve pathway may cause a central facial palsy, which affects motor function of muscles on the opposite side of the face, but is limited to the lower muscles of facial expression.

A lower motor neuron lesion is more common and results in Bell's palsy, which affects the entire side of the face on the same side as the lesion. This disorder may be idiopathic (unknown cause); however, some diseases, such as Lyme disease are known to cause of Bell's palsy.

Resources: https://www.ncbi.nlm.nih.gov/books/NBK526119/

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