Motor nucleus of facial nerve
Nucleus nervi facialis
- Synonym: Nucleus of facial nerve; Facial nucleus
Definition
The motor nucleus of facial nerve is special visceral efferent nucleus (or a branchiomotor nucleus) of the facial nerve, located in the lower pons.
Afferent connections:
The motor nucleus of facial nerve receives input from both cerebral cortices via upper motor neurons, also known as corticonuclear fibers.
Efferent connections:
Facial colliculus: The nucleus contains the cell bodies of efferent lower motor neurons. After emerging, these neurons proceed backward to encircle the abducent nerve nucleus at the brainstem's posterior. This looping results in the facial colliculus, a bump on the floor of the fourth ventricle. Once the loop is complete, the efferent fibers advance and exit the brainstem via the motor branch of the facial nerve. These special visceral efferent neurons continue their path through the facial nerve as it traverses the internal acoustic meatus.
Within the facial canal: As these fibers pass through the facial canal, some take a specific route to innervate the stapedius muscle within the middle ear. This muscle modulates the vibration of the stapes bone, preventing excessive movement. However, paralysis of the facial nerve impairs the stapedius function, leading to hyperacusis, a condition where sounds are perceived as excessively loud.
Continued pathway: The remaining efferent fibers exit the facial canal via the stylomastoid foramen, dispersing through various branches of the facial nerve to innervate muscles including the occipital belly of the occipitofrontalis, the stylohyoid, the posterior belly of the digastric, the posterior auricular muscle, and all muscles responsible for facial expressions.
Voluntary control of facial muscles: Understanding the innervation of the motor nucleus of facial nerve is crucial for comprehending voluntary facial muscle control and aids clinicians in localizing neurological lesions during cranial nerve examinations. The upper half of the motor nucleus innervates the upper facial muscles and receives corticonuclear fibers from both cerebral cortices. In contrast, the lower half innervates the lower facial muscles and receives fibers only from the contralateral side.
Effects of facial nerve lesions:
Lower motor neuron lesion | Upper motor neuron lesion | ||
A lower motor neuron lesion of the facial nerve, such as in Bell's palsy or due to nerve trauma during parotid surgery, results in paralysis of all muscles on the same side as the lesion, leading to hemiparalysis. For example, a lesion on the left side causes paralysis of the entire left side of the face, resulting in a flattened nasolabial fold and pulling the face to the right. | An upper motor neuron lesion, such as a cortical stroke, causes different effects. The upper half of the motor nucleus of facial nerve remains functional because it receives bilateral input, while the lower half of the nucleus, receiving input only from the contralateral side, is impaired. Consequently, only the lower quadrant of the opposite side of the face is paralyzed. Thus, a lesion in the right cerebral cortex would paralyze the lower left side of the face. | ||
Summary: Paralysis of half the face indicates a lower motor neuron lesion of the facial nerve or its motor nucleus on the same side. However, if only the lower quadrant of the face is paralyzed, it suggests an upper motor neuron lesion above the pons-level on the opposite side. | |||
References
Blumenfeld, H. (2010). ‘Chapter 13: Brainstem II: Eye movements and Pupillary Control’, in Neuroanatomy through clinical cases. (2nd ed.) Sunderland, Mass.: Sinauer Associates, pp. 479-484.
Snell, R.S. (2010). ‘Chapter 11: The cranial nerve nuclei and their central connections and distribution’, in Clinical Neuroanatomy. (7th ed.) Philadelphia: Wolters Kluwer Health/Lippincott Williams & Wilkins, pp. 346-348.
Dulak D, Naqvi IA. Neuroanatomy, Cranial Nerve 7 (Facial) [Updated 2023 Jul 24]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2024 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK526119/
