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Let’s continue our USMLE Review with Anatomy from the Gold Standard USMLE Step 1 Audio Review program.
Play USMLE Audio MP3 02 20 Anatomy Phases of Swallowing Below
Begin 02 20 Anatomy Phases of Swallowing Transcription
What are the three rapidly successive phases of deglutition?
- First, moving the food from the oral cavity into the oropharynx. Then, movement of the food from the oropharynx past the larynx to the esophagus. And movement of food into the esophagus. In the first phase, moving the food from the oral cavity into the oropharynx, what two gross muscle movements are happening? First, the tongue is raising up against the palate and second, the palatoglossal arches are coming together. Both of these actions squeeze the food back into the oropharynx.
Which extrinsic muscle of the tongue, then, is most involved in deglutition, pulling the tongue upward and backward?
What cranial nerve innervates styloglossus?
- Cranial nerve XII, hypoglossus.
Of course, the intrinsic muscles are at work also, causing the tongue to be arched up against the palate.
What cranial nerve innervates the palatoglossal muscles, which cause the palatoglossal arches to close together in this first phase of deglutition?
- Cranial nerve X, the vagus.
By the way, for this reason, some authors consider the palatoglossus as the fourth intrinsic muscle of the tongue and will say that cranial nerve XII, the hypoglossal, innervates three of the extrinsic muscles of the tongue and the vagus nerve, cranial nerve X, innervates the fourth extrinsic muscle of the tongue, the palatoglossus.
In the second phase, moving the food past the larynx and not allowing it to get into the larynx, what other area is the food being prevented from entering?
- The nasopharynx.
What two gross muscle movements prevent the food from entering the nasopharynx?
- Elevation of the soft palate, contraction of superior constrictor muscles.
What two gross muscle movements prevent food from entering the larynx?
- First, elevation of the hyoid bone and thyroid cartilage. Second, closure of the lateral folds of the upper larynx.
When the laryngeal entrance is elevated by elevating the hyoid bone and the thyroid cartilage, what structure is the laryngeal entrance coming up against that closes it off very effectively?
- The epiglottis.
In the third phase, the food is forced down, behind, and alongside the larynx.
What muscle is contracting first to force the food down behind the larynx into the esophagus?
- The middle constrictor muscle.
What muscle then contracts to push the food further down into the esophagus proper?
- The inferior constrictor muscle.
Ok, we’re going to review the innervations of deglutition for just a second and then we’re out of here. In the first phase, where the tongue arches against the palate and the palatoglossal arches constrict together, and the food is forced back into the back of the oropharynx.
What are the two cranial nerves that are involved?
- CN XII, the hypoglossus, and CN X, the vagus.
And which muscle and what nerve is involved in elevating the tongue upward and backward?
- Styloglossus muscle and CN XII, the hypologlossal nerve.
And what muscle and nerve is involved in constricting the palatoglossal arches?
- Palatoglossus muscle and CN X, the vagus.
In the second phase of deglutition where the food is being prevented from entering the nasopharynx and the larynx, what CN is primarily involved?
- CN X, the vagus.
And finally, food moving into the esophagus involving the middle and inferior constrictor muscles, what CN is involved?
- CN X, the vagus.
Ok, a few clinical correlations. If you have a patient who cannot swallow food and you have observed that they cannot get the food back into the oropharynx with the tongue.
What cranial nerve do you most suspect?
- CN XII, the hypoglossal.
If the tongue is paralyzed and cannot protrude at all – what kind of lesion do you suspect?
- A lesion that affects both sides of the hypoglossal nerve.
If they can protrude their tongue, but it deviates to one side, which hypoglossal nerve is the one affected?
- The CN XII that is on the same side as the deviation.
If they can get the food bolus into the back of the oropharynx, but have difficulty getting it down into the esophagus.
What CN do you primarily suspect?
- The vagus, CN X.
And in like manner, if in swallowing, the food is going up into the nose – what CN do you suspect?
- The vagus, CN X.
If CN X, the vagus, is paralyzed on one side only, what sign will you see when you examine the patient’s mouth?
- You will see the uvula elevating deviated to one side when you ask them to say “ahh”.
Will the uvula deviate toward the affected nerve or away from the affected nerve?
- It will deviate away from the paralyzed vagus.
What property of the orientation of the muscles of the palate cause the uvula to deviate away from the paralyzed vagus?
- The muscles of the palate hold the uvula at midline by opposition. When the muscles of the palate are paralyzed on one side, the muscles on the other side of the palate can pull the uvula over to that side, away from the paralyzed vagus.
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