Anatomy and physiology of the ear

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Anatomy and physiology of the ear

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Anatomy and physiology of the ear

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The pressure measured by the force per unit area is (greater/lesser) at the oval window than at the tympanic membrane in order to overcome the inertia of the cochlear fluid.

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Content Reviewers:

Rishi Desai, MD, MPH

Our ears help us hear and balance ourselves in space, and they have three parts.

The first part is the outer ear which is the part you see and hang earrings on, called the pinna, as well as the ear canal.

The second part is the middle ear, which is a tiny chamber that houses even tinier ear bones—the malleus, incus, and stapes.

The third part is the inner ear, which contains the cochlea, a special structure that converts sound waves into electrical impulses for the brain, as well as the semicircular canals which help with balance.

Let’s start with the external ear. The pinna, also called the auricle, is made up of cartilage that gives our ears their various shapes and sizes, and it also has a fleshy bit at the bottom- called the ear lobe, or lobule.

The pinna directs sound waves towards the opening of the ear canal.

The ear canal, or the external acoustic meatus, is a short, curved tube that burrows through the temporal bone for about 1 inch - or 2 and a half centimeters - and ends at the tympanic membrane.

On the inside, the ear canal is covered by skin, along with hair follicles and ceruminous glands - which secrete cerumen, or the sticky, yellow-ish, earwax.

Cerumen helps prevents foreign objects or tiny insects from getting in and damaging the tympanic membrane. That’s a creepy thought.

The tympanic membrane is also called the eardrum, and it’s a thin, translucent membrane that separates the external ear from the middle ear. It’s shaped a bit like a cone, protruding slightly into the middle ear.

When sound waves reach the eardrum, it vibrates and transmits those vibrations to the tiny bones in the middle ear.

Now, the middle ear is an air-filled cavity inside the temporal bone, shaped like tiny chamber with 4 walls, a floor and a roof.

The eardrum makes up the lateral wall of this cavity - and opposite from it there is the medial, or internal wall, that separates the middle ear from the inner ear.

The internal wall has two windows: an oval window above, and a round window below.

The two other walls of the middle ear are the posterior wall - towards the back of our head - and the anterior wall - towards the front.

The posterior wall has an opening called the mastoid antrum, and it connects the middle ear with the mastoid cavity within the temporal bone.

The anterior wall has an opening for the eustachian tube, which connects the middle ear to the nasopharynx.

The eustachian tube has three main functions—equalizing pressure across the tympanic membrane, protecting the middle ear from reflux of fluids going up from the nasopharynx, and clearing out middle ear secretions.

The roof of the middle ear is dome-shaped, and it’s called the epitympanic recess.

Finally, the floor of the middle ear is a thin layer of bone that sits right above our friendly basement neighbor - the jugular vein.

Now, inside the tiny middle ear chamber, there are three tiny bones that are arranged from the eardrum to the oval window: the malleus, incus, and stapes - named after their resemblance to a hammer, an anvil, and stirrups, respectively.

The “handle” of the malleus rests on the eardrum, and the base of the stapes is on the oval window - so when the eardrum vibrates, the vibrations are transmitted from the malleus to the incus, then to the stapes, and finally to the oval window, which transfers the vibrations over to the inner ear.

The inner ear, sometimes called the labyrinth, is a marvelous bit of anatomical engineering.

On the outside, the inner ear has a tough bony shell - the bony labyrinth; and inside the bony labyrinth, there is the membranous labyrinth.

Sources
  1. "Medical Physiology" Elsevier (2016)
  2. "Physiology" Elsevier (2017)
  3. "Human Anatomy & Physiology" Pearson (2018)
  4. "Principles of Anatomy and Physiology" Wiley (2014)
  5. "An actin molecular treadmill and myosins maintain stereocilia functional architecture and self-renewal" Journal of Cell Biology (2004)
  6. "Extracellular current flow and the site of transduction by vertebrate hair cells" The Journal of Neuroscience (1982)