Cranial Nerves: Sensory and Motor Innervation for Swallowing

Sensory Innervation and Swallowing

Taste as Sensory Information

• Role in Swallowing: Taste provides motivation to eat. Patients who lose the ability to taste may experience reduced motivation and enjoyment of eating after an injury.

• Neurological Impact: Conditions like stroke, certain viruses (affecting cranial nerves), Parkinson's disease, ALS, and Myasthenia Gravis can impact sensory nerves, including those involved in taste and swallowing pathways.

  • Some taste loss (e.g., from certain illnesses) can be temporary.

  • The exact mechanism for temporary taste and smell loss in cases like COVID-19 is a neurological problem that requires further investigation.

Trigeminal Nerve (Cranial Nerve V)

• Branches: Has three branches; only two are relevant for swallowing: maxillary and mandibular.

  • The first branch, related to the eyes, is not discussed in the context of swallowing.

• Maxillary Branch: Innervates the upper teeth and mouth, providing sensation.

• Mandibular Branch: Innervates the lower jaw, lower teeth, and sensation inside the mouth.

• Overlap: Both branches contribute to sensation in the mouth, crucial for mastication. There is some overlap in their functions.

• Other Innervation: The trigeminal nerve also innervates the nasopharynx and the soft palate, distinguishing the branches beyond just teeth/mouth sensation.

Somatosensation

• Definition: "Soma" refers to the body. Somatosensation means sensation of the body or body sensation.

• Interchangeability: The terms "somatosensation" and "sensory innervation" are often used interchangeably or describe overlapping sensory functions.

Glossopharyngeal Nerve (Cranial Nerve IX)

• Etymology: "Glossus" means tongue, "pharyngeal" refers to the pharynx. It innervates the area where the tongue and pharynx meet.

• Location: Primarily innervates the back of the mouth, including the back of the tongue and the faucial pillars (m's).

• Role in Swallowing: This nerve provides sensation critical for:

  • Bolus Manipulation and Preparation: Sensing the bolus in the mouth is essential for chewing and preparing it for swallowing.

  • Bolus Containment: Sensation in this area allows for holding the bolus (especially liquids) to prevent premature entry into the pharynx.

  • Initiation of Pharyngeal Swallow: The anterior faucial pillars are an important landmark. When the bolus reaches this point, the glossopharyngeal nerve senses it, providing the information needed to initiate the pharyngeal swallow.

Vagus Nerve (Cranial Nerve X)

• Overall Importance: The vagus nerve is critically important for both motor and sensation in swallowing, especially regarding airway protection.

• Aspiration and Penetration: These terms describe when bolus material enters the airway:

  • Penetration: Bolus enters the airway but stays above the vocal folds.

  • Aspiration: Bolus enters the airway and goes below the vocal folds (into the trachea).

• Cough Reflex: An intact vagus nerve (providing sensory innervation to the airway) allows the body to sense material in the airway and trigger a cough reflex to remove it. Patients with impaired vagus nerve function may not sense penetration or aspiration, leading to silent aspiration.

Branches of the Vagus Nerve and Their Sensory Roles

• Pharyngeal Branch:

  • Innervation Area: Primarily the pharynx, specifically the upper part, including the superior pharyngeal constrictor muscles.

  • Muscles: Innervates the area around the soft palate, including the levator veli palatini muscle (which elevates the soft palate).

  • Clinical Relevance: Damage to this branch can impair sensation in the pharynx, impacting a patient's ability to sense a bolus in that area.

• Internal Branch of the Superior Laryngeal Nerve (SLN):

  • Innervation Area: This branch of cranial nerve X innervates structures above the vocal folds, including the base of the tongue, the vallecula, the epiglottis, the laryngopharynx (hypopharynx), and the piriform sinuses.

  • Role in Swallowing: Provides sensation for detecting penetration (material above the vocal folds), allowing a patient to react and clear the airway.

• Recurrent Laryngeal Nerve (RLN):

  • Innervation Area: This branch of cranial nerve X innervates structures below the vocal folds (the subglottic region), including the inferior pharyngeal constrictor muscles, the esophagus (upper part), and the upper trachea.

  • Role in Swallowing: Provides sensation for detecting aspiration (material below the vocal folds), also triggering a cough reflex.

• Vocal Folds as a Sensory Division: The vocal folds serve as a critical anatomical division for sensory innervation: the SLN innervates above the vocal folds, and the RLN innervates below the vocal folds.

• Esophageal Branch: Innervates the sensation in the esophagus.

• Epiglottis Taste: The vagus nerve also innervates taste sensation in the epiglottis, though this is less critical for overall taste perception compared to oral sensation.

• Clinical Connection: Due to its extensive and widespread branches, the vagus nerve can be affected during procedures like heart surgery. Irritation or damage to the vagus nerve during such surgeries can temporarily lead to swallowing problems, even though the procedure itself is not directly related to swallowing anatomy.

Relative Importance of Oral vs. Epiglottic Sensation

• Oral cavity sensation is considered more important for swallowing than sensation in the epiglottis area.

• Loss of oral sensation would have a greater negative impact than loss of epiglottic sensation.

Motor Innervation for Swallowing

Introduction to Motor Control

• Definition: Motor innervation is related to movement.

• Cranial Nerves Involved: Several cranial nerves have motor components crucial for swallowing, including: Trigeminal (V), Facial (VII), Glossopharyngeal (IX), Vagus (X), Accessory (XI), and Hypoglossal (XII).

  • Cranial nerve XI (Accessory) is solely motor and had not been discussed in sensory innervation.

Trigeminal Nerve (Cranial Nerve V) Motor Function

• Key Role: Primarily innervates the muscles of mastication (chewing).

• Muscles of Mastication (Closers):

  • Temporalis: Elevates (closes) and retracts (pulls backward) the jaw.

  • Masseter: Elevates (closes) the jaw. It is considered one of the main muscles of mastication.

  • Medial Pterygoid: Elevates (closes) the jaw. (Located deep to the masseter).

• Muscle of Mastication (Opener):

  • Lateral Pterygoid: Depresses (opens) the jaw, protrudes the mandible (moves it forward), and permits side-to-side (rotary) movements for efficient chewing. (Located deep to the masseter, with horizontal fibers).

• Tensor Veli Palatini:

  • Function: Tenses and pulls the soft palate laterally (towards the same side when unilaterally contracted, flattens bilaterally).

  • Swallowing Role: Has a minor role in nasopharyngeal closure (sealing the nasal cavity), but its main role is opening the Eustachian tube.

• Suprahyoid/Submental Muscles (Innervated by V):

  • These muscles are located in the area above the hyoid bone, underneath the chin.

  • Mylohyoid: The largest muscle forming the floor of the mouth.

  • Anterior Belly of the Digastric: Another muscle in this region.

  • Function: When the hyoid bone is stable, their contraction (shortening) pulls the jaw down, thus opening the mouth (jaw depression).

  • Inverse Function: Conversely, if the jaw is fixed, their contraction elevates the hyoid bone up and forward, which is crucial for swallowing.

Facial Nerve (Cranial Nerve VII) Motor Function

• Key Roles: Facial expressions and taste from the anterior two-thirds of the tongue.

• Swallowing Muscles:

  • Buccinator: Located in the cheeks; helps maintain oral tone and generate pressure in the oral cavity, preventing food from pocketing. Weakness can lead to problems with oral pressure generation.

  • Orbicularis Oris: The circular muscle around the lips; opens and closes the mouth, and protrudes the lips (e.g., for kissing).

  • Stylohyoid: A suprahyoid muscle that elevates the hyoid bone, helps open the mouth, and due to its posterior location, also retracts the tongue.

• Unique Innervation Pattern (Upper vs. Lower Face):

  • Upper Face (e.g., forehead, eyes): Receives bilateral innervation from upper motor neurons (from both the contralateral and ipsilateral cerebral hemispheres). This provides a protective mechanism: if one hemisphere is damaged (e.g., by a stroke), the upper face can still receive some motor signals from the other intact hemisphere, allowing for partial function (e.g., slight eyebrow lift).

  • Lower Face (e.g., mouth, cheeks): Receives contralateral innervation only (from the opposite cerebral hemisphere). If a hemisphere is damaged, the lower face on the affected side will experience a complete loss of motor function (e.g., inability to open the jaw or chew on that side).

Glossopharyngeal Nerve (Cranial Nerve IX) Motor Function

• Innervates: One of the "long muscles of the pharynx" (vertical muscles, as opposed to the horizontal constrictor muscles).

• Stylopharyngeus: Elevates the pharynx.

• Pharyngeal Plexus: Cranial nerves, especially those involved in swallowing (IX and X), travel very closely. In some areas, their fibers bundle together to form a "plexus," meaning they contribute in combination to innervate certain muscles.

Vagus Nerve (Cranial Nerve X) Motor Function

• Vocal Folds as Division: The vocal folds divide the sensory innervation for the Superior Laryngeal Nerve (SLN - above) and Recurrent Laryngeal Nerve (RLN - below). This also applies to their motor functions, though the present discussion is primarily on sensory. We will learn about muscles they innervate.

• Pharyngeal Branch Muscles (Long Muscles of the Pharynx):

  • Palatoglossus: Forms the anterior faucial pillar; lowers the soft palate.

  • Palatopharyngeus: Forms the posterior faucial pillar; elevates the pharynx.

  • Salpingopharyngeus: Elevates the pharynx.

  • Levator Veli Palatini: The main elevator of the soft palate. When contracted unilaterally, it elevates the soft palate to the same side; bilaterally, it elevates the entire soft palate.

  • Swallowing Role: Elevation of the soft palate (velum) is critical to seal the entrance to the nasopharynx, preventing food/liquid from entering the nasal cavity during swallowing.

  • Overall Function: Injury to the vagus nerve's pharyngeal branch can lead to problems elevating the larynx and hyoid bone, impacting swallowing. In general, the vagus nerve innervates most of the long muscles of the pharynx.

Key Terms and Concepts

• Soma: Body

• Veli (Villi): Soft palate

• Levator: Elevates

• Recurrent Laryngeal Nerve (RLN): Branch of vagus nerve, innervates below vocal folds.

• Superior Laryngeal Nerve (SLN): Branch of vagus nerve, innervates above vocal folds.

• Glottis: Space between the vocal folds.

• Subglottic Region: Area below the vocal folds.

• Rotary Chewing: Efficient chewing involves not just up-and-down movements but also side-to-side movements.