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Anatomy of the inner ear

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Anatomy of the inner ear

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Figure 1: Vestibulocochlear organ.

Figure 2: Anatomy of the cochlea.

Questions

USMLE® Step 1 style questions USMLE

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A 75-year-old man is diagnosed with age-related progressive bilateral sensorineural hearing loss of higher frequencies. Which of the following components of the auditory system detects high frequency vibrations?  

Transcript

How is it you can listen to your favorite song, close your eyes, dance, and not fall on your face? Well, there is a little thing called the inner ear that contains the vestibulocochlear organ which gives you the ability to perceive sounds and maintain your balance.

The inner ear is found in the petrous part of the temporal bone between the middle ear laterally, and the internal acoustic meatus medially. It is a small and important area which houses the irregularly shaped vestibulocochlear organ, which kind of looks like a snail shell attached to a few bony rings.

Now, the inner ear contains the bony labyrinth and the membranous labyrinth. The bony labyrinth is connected to the middle ear by two windows. The oval window is found on the lateral wall of the vestibule and is covered by the base of the stapes, while the round window is found at the base of the cochlea and is covered by the secondary tympanic membrane.

The bony labyrinth, within the otic capsule, is filled with perilymph and is made of a series of cavities which are the vestibule, the semicircular canals, and the cochlea. Suspended within the bony labyrinth, there’s the membranous labyrinth, which is basically a series of sacs and ducts filled with endolymph.

The membranous labyrinth is organized into the utricle and saccule within the vestibule, the three semicircular ducts and their membranous ampullae, and the cochlear duct within the cochlea.

Let’s take a look at the structures responsible for balance, which are the semicircular canals, on the one hand, and the utricle and saccule, on the other hand. There are three semicircular canals, an anterior, posterior, and lateral canal, oriented in the three different planes of space.

Each canal contains a dilated end called the ampulla that contains an area of sensory epithelium called the ampullary crest. This is lined with tiny hair cells that pick up information about rotational movements of the head in the plane of the duct within which it is contained.

The hair cells of the ampullary crest are innervated by the vestibular branch of the vestibulocochlear nerve, and the cell bodies of these neurons are found in the vestibular ganglion.

Now, the utricle and the saccule both contain areas of sensory epithelium called maculae, which contain hair cells that detect information about linear acceleration. The hair cells in the utricle detect movements in the horizontal plane; while those in the saccule detect movement in the vertical plane. The macular hair cells are also innervated by the vestibular branch of the vestibulocochlear nerve.

Now, that was a lot of info! For a quick break, feel free to pause the video and remember the parts of the inner ear that play a role in balance.

Now, let’s switch gears and look at the structures responsible for hearing, starting with the cochlea. The cochlea is a bony tube that spirals on itself, resembling the shape of a snail shell, and contains three fluid-filled cavities called the scala vestibuli, the cochlear duct and the scala tympani.

Now, the cochlear duct is between the scala vestibuli and the scala tympani and its roof is formed by the vestibular membrane, while the floor is formed by the basilar membrane.

The organ of hearing, called the spiral organ, or the organ of Corti, is fixed to the basilar membrane and contains hair cells. The tips of these hair cells insert into the tectorial membrane, which is a gelatinous membrane that overlies the spiral organ.