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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 19 Anatomy Swallowing Below
Begin 02 19 Anatomy Swallowing Transcription
What complex part of digestion do four of the cranial nerves, VII, XI, X, and XII work together in?
- Deglutition – swallowing.
Alright, let’s turn it around. What cranial nerves are involved in deglutition (swallowing)?
- CN VII, XI, X, and XII.
We’re not going to get into all the muscles involved in deglutition because it would be much too complex and it’s fairly low-yield for the boards. I’ll just hit on a few points here.
CN VII participates in swallowing via two muscles. What are they?
- Digastric muscle, posterior belly. And stylohyhoid muscles.
Ok, in like manner, CN XI, the glossopharyngeal nerve, innervates only one muscle and it is peripherally involved in deglutition.
What is the muscle?
More importantly, CN XI, the glossopharyngeal, gathers sensation from three areas. What are these 3 areas?
- The posterior wall of the pharynx, all the way from the nasopharynx down to the beginning of the esophagus. The soft palate and tonsils. And the posterior 1/3 of the tongue.
What types of sensation are gathered by CN XI from the posterior 1/3 of the tongue?
- Taste and somatic sensation.
How does CN X, the vagus, participate?
- It supplies motor innervation to a number of muscles involved in deglutition.
We’ll just look at the groups of the muscles.
There are five groups of muscles. What are the first two groups?
- First, some of the muscles of the palate. Second, some of the muscles of the pharynx.
Next three specific muscles make up the other three groups which are very important innervated by the vagus. What are they?
- The superior pharyngeal constrictor muscle, the middle pharyngeal constrictor muscle, and the inferior pharyngeal constrictor muscle.
Finally, CN XII, the hypoglossal, innervates what 3 groups of muscles?
- The intrinsic muscles of the tongue, the extrinsic muscles of the tongue, and some of the neck strap muscles.
What is the main task of the intrinsic muscles of the tongue?
- To change the shape of the body of the tongue.
What are the 3 intrinsic muscles of the tongue innervated by CN XII, hypoglossal?
- On top, the superior longitudinal. On the bottom, inferior longitudinal. And in between, in the middle of the tongue, transverse and vertical. And this one in the middle of the tongue has fibers that go both transversely and vertical. It’s usually treated as one muscle.
When performing surgery on the tongue, cutting what part of the tongue will result in the least bleeding?
- Cuts along the midline of the tongue.
And why is this? What structure is there?
- The septum lingui, which is a wall of fascia separating the tongue into two halves.
What are the 3 extrinsic muscles of the tongue?
- Hyoglossus, styloglossus, and genioglossus.
Now both the intrinsic and extrinsic muscles work together to move the tongue. They can either work together or against each other. For simplicity, when we talk about movements of the tongue, we generally talk about the extrinsic muscles.
Which extrinsic muscle pulls the tongue upward and backward?
Which extrinsic muscle pulls the body of the tongue forward and downward?
And which extrinsic muscle flattens the tongue?
If a patient has a paralyzed CN XII, hypoglossal nerve, on one side and you ask the patient to protrude their tongue. What will happen?
- The tongue will deviate to the paralyzed side.
Why is this?
- Because the two genioglossus muscles pull against each other.
- Because genioglossus runs from the inside front of the mandible at the midline to the body of the tongue fanning out at each side, the active side then in pulling the tongue forward will also pull the tongue across the midline toward the paralyzed side.
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