Vestibular System Anatomy

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If you’ve read my Three Bodily Balance Systems article, you now know that the body’s balance system is controlled by the visual system, the sensory system, and the vestibular system. As a physical therapist, I don’t normally work on correcting vision or sensation to a high degree as this is not my specialty. But in the physical therapy world, we often do work with the vestibular system in order to help create compensation or correction for any deficit that may be present. Today, I want to give an overview of vestibular system anatomy. It is important to note that all the structures discussed below are in both ears, but mirrored to each other. Additionally, explanation on physiological processes will be limited in this article but will be discussed in the future.

Outer Ear

Let’s start from the outside and work our way into the ear. The part of your body that you know as the “ear” is called the auricle or pinna. This fleshy apparatus is made up of cartilage and skin. The auricle is uniquely designed to capture sound waves from your environment and funnel them into the ear canal. The ear canal is the hole you see in your ear and is approximately 1 inch in length. (1) At the end of the ear canal sits the tympanic membrane, otherwise known as the eardrum. The eardrum is used in transportation of sound waves to the inner ear and the cochlea which we will discuss momentarily. All of this together is termed the “outer ear”.

Middle Ear

https://www.aboutkidshealth.ca/Article?contentid=833&language=English

The space just medial (inside) to the tympanic membrane is termed the “middle ear”. This area contains the smallest bones in the body called ossicles with individual names consisting of the malleus (hammer), incus (anvil), and stapes (stirrups). These three bones in conjunction with the tympanic membrane convert sound waves into vibrations and transport them deeper into the ear. The stapes then attaches to a hole in the cochlear apparatus termed the oval window.

Inner Ear

The vestibular and cochlear apparatuses sit in the inner ear. These apparatuses are encased within the bony labyrinth. The bony labyrinth itself consists of the cochlea, an intermediary chamber, and three semicircular canals (involved in vestibular activity). Although technically attached, the cochlear and vestibular apparatuses have very different functions. The cochlear apparatus, which looks like a snail, is involved in hearing. The vestibular apparatus, which is made up of the the semicircular canals and two swellings, is involved in communicating spatial awareness, motion, and head position to our brain. For the purposes of this article, we will not be going into deeper detail of the cochlear apparatus.

Membranous Labyrinth

The vestibular portion of the bony labyrinth encases a membranous labyrinth. The membranous labyrinth is suspended by perilymphatic fluid and connective tissue within the bony labyrinth. Five main portions of this membranous labyrinth exist including the three membranous tubes of the semicircular canals and two otolith organs (utricle and saccule) at the base of these canals. Each semicircular canal has a widened bulb at one end for a structure called an ampulla which houses special cell receptors called hair cells. (2)

The membranous labyrinth is filled with endolymphatic fluid. Perilymphatic fluid (between the bony and membranous labyrinths) and endolymphatic fluid have differing ratios of potassium and sodium making these fluids very different and thus no mixing occurs under normal circumstances. (3)

Semicircular Canals

https://www.semanticscholar.org/paper/In-vitro-model-of-a-semicircular-canal%3A-design-and-Obrist-Hegemann/043f0746fe65b47e1094e74c547f8b496f2c48d8/figure/0

The semicircular canals function to convert rotational forces of the head into brain signals via the hair cells in the ampulla. The three semicircular canals are named the anterior canal, the posterior canal, and the lateral canal. Each canal is oriented in such a way to pick up specific rotational motions. The anterior canal picks up sagittal plane head movement such as nodding your head “yes”. The posterior canal picks up coronal plane head movement such as tilting your ear to your shoulder. The lateral canal picks up transverse plane movement such as shaking your head “no”. It is important to note that these canals all work simultaneously. This allows the brain to feel full 360 degree rotational motions as they happen. (2)

As mentioned previously, the semicircular canals all have a swelling at one end call the ampulla. This ampulla contains a gelatinous like structure called the cupula which house hair cells. Hair cells are embedded within a sensory epithelium called crista. The cupula interacts with the endolymphatic fluid during movement to initiate or inhibit hair cell nerve signaling to the brain for informational purposes. Although rotational movement initiates cupula motion, gravity does not because the cupula has the same density as the surrounding endolymphatic fluid. (2)

Otolith Organs

https://www.britannica.com/science/otolith

Just below the bases of the semicircular canals sit two swellings called the otolith organs. The otolith organs are involved in sensing gravity and linear acceleration. The first swelling is called the utricle which primarily senses forward and backward accelerations. The second is called the saccule which is better positioned to sense vertical accelerations. These primary senses can change depending on the orientation of the head. (2)

These otoliths contain structures similar to cupula called otolithic membranes. Although similar to cupula, otolithic membranes have stones embedded in them made out of calcium carbonate called otoconia. These otoconia add weight to the otolithic organs allowing them to interact with gravity. Hair cells embed themselves into the otolithic membrane, while originating from a base area called the macula. (4)

Vestibular Nerve

https://mobilephysiotherapyclinic.in/vestibulocochlear-nerve/

Hair cells of both the semicircular canals and the otolith organs send signals to the brain via a direct connection to the vestibular nerve. After the vestibular nerve leaves the vestibular apparatus it meets up with the cochlear nerve to form cranial nerve VIII, the vestibulocochlear nerve. The vestibulocochlear nerve then travels back to the brainstem to nuclei of the pons and medulla oblongata wherein the nerve signals are processed and sent elsewhere for integration in the brain as needed. (2)

References

1. School M. Ear Anatomy - Outer Ear - Otorhinolaryngology - Head & Neck Surgery. Otorhinolaryngology - Head & Neck Surgery. https://med.uth.edu/orl/online-ear-disease-photo-book/chapter-3-ear-anatomy/ear-anatomy-outer-ear/. Published 2021. Accessed June 23, 2021.

2. Herdman S, Clendaniel R. Vestibular Rehabilitation. 4th ed. Philadelphia: F.A. Davis; 2014.

3. Hain T. Fluid pathways in the inner ear. Dizziness-and-balance.com. https://dizziness-and-balance.com/anatomy/ear/endolymphatic-sac.htm. Published 2021. Accessed June 23, 2021.

4. Timothy C. Hain M. Anatomy of the Otoliths. Dizziness-and-balance.com. https://dizziness-and-balance.com/anatomy/ear/otoliths.html. Published 2021. Accessed June 23, 2021.

Thanks for reading about vestibular anatomy! I hope you learned a thing or two after diving into this article. Leave a comment below about what you found most interesting or what you would like me to go more in-depth on in future articles.

If you want to learn more about the vestibular system check out Vestibular Rehabilitation by Susan Herdman. It is THE definitive text on vestibular rehab.

Happy falling!

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