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Ossicles

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This article is about the bones in the human ear. For elements embedded in the body wall of echinoderms, see Ossicle (echinoderm) .
This article may be confusing or unclear to readers. Please help us clarify the article . There might be a discussion about this on the talk page . (November 2017) ( Learn how and when to remove this template message )
Ossicles
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Auditory ossicles: Malleus , incus and stapes
Auditory ossicles-en.svg
Details
Part of Middle ear
Identifiers
Latin Ossicula auditus,
ossicula auditoria
MeSH D004429
TA A02.1.17.001
FMA 52750
Anatomical terms of bone

[ edit on Wikidata ]
1871 Descent F937.1 fig03.jpg
This article is one of a series documenting the anatomy of the
Human ear
Outer ear
  • Pinna
  • Tragus
Middle ear
  • Tympanic membrane
  • Ossicles
    • Malleus
    • Incus
    • Stapes
Inner ear
  • Vestibules
    • Utricle
    • Saccule
  • Cochlea
  • Semicircular canals
  • v
  • t
  • e

The ossicles (also called auditory ossicles) are three bones in either middle ear that are among the smallest bones in the human body. They serve to transmit sounds from the air to the fluid-filled labyrinth ( cochlea ). The absence of the auditory ossicles would constitute a moderate-to-severe hearing loss . The term “ossicle” literally means “tiny bone”. Though the term may refer to any small bone throughout the body, it typically refers to the malleus , incus , and stapes (hammer, anvil, and stirrup) of the middle ear .

Contents

  • 1 Structure
    • 1.1 Development
    • 1.2 Evolution
  • 2 Function
  • 3 Clinical relevance
  • 4 History
  • 5 See also
  • 6 References
  • 7 External links

Structure[ edit ]

See also: Malleus , Incus , and Stapes

Anatomy of the three ossicles

The ossicles are, in order from the eardrum to the inner ear (from superficial to deep): the malleus , incus , and stapes , terms that in Latin are translated as “the hammer , anvil , and stirrup “.

  • The malleus ( Latin : “hammer”) articulates with the incus through the incudomalleolar joint and is attached to the tympanic membrane ( eardrum ), from which vibrational sound pressure motion is passed.
  • The incus ( Latin : “anvil”) is connected to both the other bones.
  • The stapes ( Latin : “stirrup”) articulates with the incus through the incudostapedial joint and is attached to the membrane of the fenestra ovalis , the elliptical or oval window or opening between the middle ear and the vestibule of the inner ear . It is the smallest bone in the body. [1]

Development[ edit ]

Studies have shown that ear bones in mammal embryos are attached to the dentary , which is part of the jaw . These are ossified portions of cartilage —called Meckel’s cartilage —that are attached to the jaw. As the embryo develops, the cartilage hardens to form bone. Later in development, the bone structure breaks loose from the jaw and migrates to the inner ear area. The structure is known as the middle ear, and is made up of the stapes , incus , malleus , and tympanic membrane . These correspond to the columella , quadrate , articular , and angular structures in the amphibian, bird or reptile jaw. For this reason, researchers believe that mammals and reptiles share a common ancestry. [2]

Evolution[ edit ]

Main article: Evolution of mammalian auditory ossicles

Function[ edit ]

As sound waves vibrate the tympanic membrane (eardrum), it in turn moves the nearest ossicle, the malleus, to which it is attached. The malleus then transmits the vibrations, via the incus, to the stapes, and so ultimately to the membrane of the fenestra ovalis (oval window), the opening to the vestibule of the inner ear.

Sound traveling through the air is mostly reflected when it comes into contact with a liquid medium; only about 1/30 of the sound energy moving through the air would be transferred into the liquid. [3] This is observed from the abrupt cessation of sound that occurs when the head is submerged underwater. This is because the relative incompressibility of a liquid presents resistance to the force of the sound waves traveling through the air. The ossicles give the eardrum a mechanical advantage via lever action and a reduction in the area of force distribution; the resulting vibrations would be much weaker if the sound waves were transmitted directly from the outer ear to the oval window. This reduction in the area of force application allows a large enough increase in pressure to transfer most of the sound energy into the liquid. The increased pressure will compress the fluid found in the cochlea and transmit the stimulus. Thus, the presence of the ossicles to concentrate the force of the vibrations improves the sensitivity to sound and is a form of impedance matching .

However, the extent of the movements of the ossicles is controlled (and constricted) by two muscles attached to them (the tensor tympani and the stapedius ). It is believed that these muscles can contract to dampen the vibration of the ossicles, in order to protect the inner ear from excessively loud noise (theory 1) and that they give better frequency resolution at higher frequencies by reducing the transmission of low frequencies (theory 2) (see acoustic reflex ). These muscles are more highly developed in bats and serve to block outgoing cries of the bats during echolocation (SONAR).

Clinical relevance[ edit ]

Occasionally the joints between the ossicles become rigid. One condition, otosclerosis , results in the fusing of the stapes to the oval window. This reduces hearing and may be treated surgically.[ further explanation needed ]

History[ edit ]

There is some doubt as to the discoverers of the auditory ossicles and several anatomists from the early 16th century have the discovery attributed to them with the two earliest being Alessandro Achillini and Jacopo Berengario da Carpi . [4] Several sources, including Eustachi and Casseri , [5] attribute the discovery of the malleus and incus to the anatomist and philosopher Achillini . [6] The first written description of the malleus and incus was by Berengario da Carpi in his Commentaria super anatomia Mundini (1521), [7] although he only briefly described two bones and noted their theoretical association with the transmission of sound. [8] Niccolo Massa ‘s Liber introductorius anatomiae [9] described the same bones in slightly more detail and likened them both to little hammers. [8] A much more detailed description of the first two ossicles followed in Andreas Vesalius ‘ De humani corporis fabrica [10] in which he devoted a chapter to them. Vesalius was the first to compare the second element of the ossicles to an anvil although he offered the molar as an alternative comparison for its shape. [11] The first published description of the stapes came in Pedro Jimeno’s Dialogus de re medica (1549) [12] although it had been previously described in public lectures by Giovanni Filippo Ingrassia at the University of Naples as early as 1546. [13]

The term ossicle derives from ossiculum, a diminutive of “bone” ( Latin : os; genitive ossis). [14] The malleus gets its name from Latin malleus, meaning “hammer”, [15] the incus gets its name from Latin incus meaning “anvil” from incudere meaning “to forge with a hammer”, [16] and the stapes gets its name from Modern Latin “stirrup,” probably an alteration of Late Latin stapia related to stare “to stand” and pedem, an accusative of pes “foot”, so called because the bone is shaped like a stirrup – this was an invented Modern Latin word for “stirrup,” for which there was no classical Latin word, as the ancients did not use stirrups. [17]

See also[ edit ]

This article uses anatomical terminology; for an overview, see anatomical terminology .
  • Incudomalleolar joint
  • Incudostapedial joint
  • Otolith

References[ edit ]

  1. ^ “Your Bones” . kidshealth.org.

  2. ^ Meng, Jin. “The Journey From Jaw to Ear.” Biologist. vol. 50. (2003) p. 154-158.
  3. ^ Hill, R.W., Wyse, G.A. & Anderson, M. (2008). Animal Physiology, 2nd ed..
  4. ^ O’Malley, C. D.; Clarke, E (1961). “The discovery of the auditory ossicles”. Bulletin of the History of Medicine. 35: 419–41. PMID   14480894 .
  5. ^ Alidosi, GNP. I dottori Bolognesi di teologia, filosofia, medicina e d’arti liberali dall’anno 1000 per tutto marzo del 1623, Tebaldini, N., Bologna, 1623. http://gallica.bnf.fr/ark:/12148/bpt6k51029z/f35.image#
  6. ^ Lind, L. R. Studies in pre-Vesalian anatomy. Biography, translations, documents, American Philosophical Society, Philadelphia, 1975. p.40
  7. ^ Jacopo Berengario da Carpi,Commentaria super anatomia Mundini, Bologna, 1521. https://archive.org/details/ita-bnc-mag-00001056-001
  8. ^ a b O’Malley, C.D. Andreas Vesalius of Brussels, 1514–1564. Berkeley: University of California Press, 1964. p. 120
  9. ^ Niccolo Massa, Liber introductorius anatomiae, Venice, 1536. p.166. http://reader.digitale-sammlungen.de/en/fs1/object/display/bsb10151904_00001.html
  10. ^ Andreas Vesalius, De humani corporis fabrica. Johannes Oporinus, Basle, 1543.
  11. ^ O’Malley, C.D. Andreas Vesalius of Brussels, 1514–1564. Berkeley: University of California Press, 1964. p. 121
  12. ^ Pedro Jimeno, Dialogus de re medica, Johannes Mey, Valencia, 1549. https://archive.org/details/dialogusderemed00jimegoog
  13. ^ A Mudry, Disputes surrounding the discovery of the stapes in the mid 16th century., Otology and Neurotology, 2013 Apr;34(3):588-92.
  14. ^ “Online Etymology Dictionary” . etymonline.com.
  15. ^ “Online Etymology Dictionary” . etymonline.com.
  16. ^ “Online Etymology Dictionary” . etymonline.com.
  17. ^ “Online Etymology Dictionary” . etymonline.com.

External links[ edit ]

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      Mind

      How do the hammer, anvil and stirrup bones amplify sound into the inner ear?

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      Douglas E. Vetter, Assistant Professor of Neuroscience at the Tufts University Sackler School of Biomedical Sciences, sounds out an answer to this query.

      The hammer, anvil and stirrup—also known as the malleus, incus, and stapes, respectively, and collectively, as "middle ear ossicles"—are the smallest bones in the human body. Found in the middle ear, they are a part of the auditory system between the eardrum and the cochlea (the spiral-shaped conduit housing hair cells that are involved in transmitting sound to the brain). To understand the role of these bones in hearing requires an understanding of levers. This is because the middle ear ossicles are arranged and interact with each other as a lever system.

      All levers generate a mechanical advantage. They are used to exert a large force over a small distance at one end of the lever by applying a smaller force over a longer distance at the opposite end. The leveraging capabilities of the middle ear ossicles are needed to generate the large forces that allow us to hear.

      As terrestrial animals, we live in a gaseous environment. But, our inner ear is filled with fluid, and this represents a problem. As an example, most people have first hand knowledge of hearing underwater. If someone screams at you from above the water’s surface, the sounds are tremendously muted, making it difficult to understand or even hear at all. That is simply because most of the sound is reflected off the water’s surface.

      So how do we take in airborne sounds, which are simply vibrations of the air molecules, and get them past the air-fluid interface between our ear canal and the inner ear? We need a system to use those air vibrations to push against the surface of the inner ear fluid.

      When the eardrum vibrates as sound hits its surface, it sets the ossicles into motion. The ossicles are arranged in a special order to perform their job. Directly behind and connected to the eardrum—which is essentially, a large collector of sound—is the hammer. The hammer is arranged so that one end is attached to the eardrum, while the other end forms a lever-like hinge with the anvil. The opposite end of the anvil is fused with the stirrup (so anvil and stirrup act as one bone). The stirrup then connects with a special opening in the cochlea called the "oval window." The footplate of the stirrup—the oval, flat part of the bone that resembles the part where one would rest ones foot in an actual stirrup—is loosely attached to the oval window of the cochlea, allowing it to move in and out like a piston. The piston-like action generates vibrations in the fluid-filled inner ear that are used to signal the brain of a sound event. Without the middle ear ossicles, only about 0.1 percent of sound energy would make it into the inner ear.

      Overcoming the problem of getting airborne sound into the fluid-filled inner ear is solved by two main mechanisms: the concentration of energy from the large eardrum onto the small stirrup footplate situated in the oval window; and the lever-like action between the hammer and the anvil-stirrup complex. In cats, for example, the simple concentration of forces from the eardrum to the stirrup increases pressure at the oval window to about 35 times what is measured at the eardrum. The lever action of the middle ear bones imparts a further mechanical advantage to the system—occurring because the anvil is shorter than the hammer—and further increases pressure by roughly 35 percent. In this way we overcome the problem of getting airborne vibrations into the pressurized, fluid-filled inner ear.

      Not all animals have this same middle ear bone configuration. In fact, reptiles, amphibians and birds, have a middle ear that contains just one bone, called the columella, which connects the eardrum directly to the oval window of the cochlea. When we examine the most sensitive frequency for hearing in these animals, they do very well for sounds around 1,000 hertz (1 kHz) but quickly lose their ability to hear well at higher frequencies. On the other hand, animals with three middle ear bones tend to hear at much higher frequencies. For humans, our hearing can extend to 20 kHz, although most of our lives are spent attending to sounds between 4 and 8 kHz.

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