Neurology of the Swallow
For the swallow response to occur, sensory and motor information are necessary. Sensory and motor control are from afferent and efferent systems. Sensory feedback comes from the trigeminal, facial and glossopharyngeal nerves to help initiate a swallow. Information about motor movement is received from the muscle spindles in the tongue via the hypoglossal nerve.
Sensory and motor information is carried to the swallowing center, which is believed to be located in the medulla, within the nuclei of the reticular formation; specifically the nucleus ambiguous. When the swallow response is initiated, this center causes messages to be sent to the glossopharyngeal, the vagus, and the hypoglossal nerves. The glossopharyngeal is considered the major nerve for the swallowing center.
Six of the cranial nerves provide the innervation for both swallowing and speech.
- CN. V The Trigeminal Nerve
- CN. VII The Facial Nerve
- CN. IX The Glossopharygeal Nerve
- CN. X The Vagus Nerve
- CN. XI The Spinal Accessory Nerve
- CN. XII The Hypoglossal Nerve
The Trigeminal Nerve (CN. V):
The efferent portion of the trigeminal nerve innervates the muscles involved in chewing. These include the temporalis, the masseter, the medial, and the lateral pterygoid.
The trigeminal nerve also innervates the tensor veli palatine muscle, which tenses the velum.
In addition, the trigeminal assists the glossopharyngeal nerve in raising the larynx and pulling it forward during the laryngeal substage of the pharyngeal swallow.
The trigeminal nerve carries feedback about all kinds of sensation, except taste, from the anterior 2/3 of the tongue. CN. V also carries sensory information from the face, mouth and mandible.
It emerges from the brainstem between the pons and the medulla.
Bell’s palsy: paralysis of facial muscles on affected side and loss of taste sensation due to facial nerve damage.
The facial nerve controls the muscles of facial expression, and innervates the lip muscles including the orbicularis oris and the zygomaticus. The muscles must contract during the oral preparatory and oral transport stages of the swallow to prevent food from spilling out of the mouth.
The facial nerve also innervates the buccinator muscles of the cheeks. These must remain tense during the oral component of the swallowing process to prevent the pocketing of food between the teeth and the cheeks.
The submandibular and sublingual glands receive their parasympathetic input from the facial nerve via the submandibular ganglion.
The facial nerve carries information about taste from the anterior 2/3 of the tongue.
The Glossopharyngeal Nerve (CN. IX)
The glossopharyngeal nerve and the vagus nerve control similar functions and are tested together.
It innervates the parotid gland, an important salivary gland. The saliva mixes with the chewed up food to form a bolus.
CN. IX has motor, sensory, and autonomic nervous system nerve fibers. It, along with the vagus (CN. X), provides some innervation to the upper pharyngeal constrictor muscles (Zemlin, 1997).
It innervates the stylopharyngeus muscle which elevates the larynx and pulls it forward during the pharyngeal stage of the swallow. This action also aids in the relaxation and opening of the cricopharyngeus muscle.
The glossopharyngeal nerve mediates all sensation, including taste, from the posterior 1/3 of the tongue.
CN. IX also carries sensation from the velum and the superior portion of the pharynx. A lesion may have impaired the gag reflex unilaterally (Zemlin, 1997).
The vagus nerve is one of the main nerves for swallowing and can cause major problems if injured.
The vagus is responsible for raising the velum as it innervates the glossopalatine and the levator veli palatine muscles.
The vagus along with CN. IX innervates the pharyngeal constrictor muscles. CN IX and X, also both innervate the intrinsic musculature of the larynx. It is responsible for vocal fold adduction (closing) during the swallow.
The vagus also innervates the cricopharyngeus muscle.
The vagus controls the muscles involved in the esophageal stage of the swallow as well as those that control respiration. (This is the only cranial nerve that influences structures inferior to the neck.)
The vagus carries sensory information from the velum and posterior and inferior portions of the pharynx.
The vagus also mediates sensation in the larynx.
CN. XI innervates the palatopharyngeus muscle which depresses the velum and constricts the pharynx.
It also innervates the muscularis uvula which tenses the velum. It, along with CN.X, innervates the levator veli palatini (CN. XI is strictly a motor nerve.)
The hypoglossal innervates all extrinsic and intrinsic tongue muscles, which move the tongue for speech and swallowing. It is strictly a motor nerve.
Check out how to perform a cranial nerve exam to check if nerves are damaged during swallowing.
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