Facial nerve

Number: VII

Name: Facial

Sensory, motor, or both: Both sensory and motor

Origin/Target: Pons(cerebellopontine angle) above olive

Nuclei: Facial motor nucleus, Solitary nucleus, Superior salivary nucleus

Function: Provides motor innervation to the muscles of facial expression, posterior belly of the digastric muscle, stylohyoid muscle, andstapedius muscle. Also receives the special sense of taste from the anterior 2/3 of the tongue and provides secretomotorinnervation to the salivary glands (except parotid) and the lacrimal gland.

Description:

The facial nerve, also known as Cranial Nerve VII, is a mixed nerve with both sensory, motor, and parasympathetic components. It plays a crucial role in facial expression, taste sensation, and several glandular functions.

Nuclei:

The facial nerve arises from three nuclei:

• Facial Motor Nucleus - Responsible for motor innervation to the muscles of facial expression.

• Superior Salivatory Nucleus – Gives off parasympathetic fibers that control salivary and lacrimal glands.

• Nucleus of Tractus Solitarius - Processes sensory information, particularly taste from the anterior two-thirds of the tongue.

Emergence from the brainstem:

The facial nerve emerges from the brainstem at the pontomedullary junction, near the cerebellopontine angle, alongside the vestibulocochlear nerve. It initially travels as two roots—the larger motor root and the smaller sensory/intermediate root—before they combine.

Course of the facial nerve:

From the brainstem, the facial nerve enters the internal acoustic meatus, a passageway in the temporal bone, along with the vestibulocochlear nerve. It travels through the facial canal in the temporal bone, where it has a complex course involving several bends.

The nerve gives off three significant branches within the facial canal:

• The greater petrosal nerve (parasympathetic to the lacrimal gland),

• The nerve to stapedius (motor to the stapedius muscle in the middle ear),

• The chorda tympani (sensory for taste from the anterior two-thirds of the tongue and parasympathetic to submandibular and sublingual glands).

Exit from the Skull and Peripheral Course:

• The facial nerve exits the skull via the stylomastoid foramen.

• It gives off branches to muscles such as the posterior belly of the digastric, stylohyoid, and occipitalis muscles before entering the parotid gland.

• Within the parotid gland, the facial nerve divides into five terminal branches: temporal, zygomatic, buccal, marginal mandibular, and cervical, which innervate muscles of facial expression.

Actions of the Facial Nerve:

• Motor: Innervates the muscles responsible for facial expressions.

• Sensory: Conveys taste sensations from the anterior two-thirds of the tongue.

• Parasympathetic: Controls secretion from the lacrimal, submandibular, and sublingual glands.

Clinical Dysfunction:

Damage to the facial nerve can lead to facial paralysis, commonly known as Bell's palsy. This can result in:

• Inability to control facial muscles on the affected side, leading to drooping of the mouth, eyelid, and an inability to wrinkle the forehead.

• Loss of taste sensation on the anterior two-thirds of the tongue.

• Hyperacusis due to stapedius muscle dysfunction.

• Dry eyes or mouth due to impaired glandular function.

Upper motor neuron lesions, such as a stroke, affect the lower face on one side due to bilateral cortical innervation of the upper face. In contrast, a lower motor neuron lesion, affecting the nerve itself, paralyzes facial muscles on the entire ipsilateral side.

Understanding the facial nerve's complex path and functions is essential for diagnosing and managing related cranial nerve disorders effectively.