middle ear
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The middle ear is the portion of the
ear The ear is the organ of hearing and, in mammals, balance. In mammals, the ear is usually described as having three parts—the outer ear The outer ear, external ear, or auris externa is the external part of the ear, which consists ...

ear
internal to the
eardrum In the anatomy of humans and various other tetrapods, the eardrum, also called the tympanic membrane or myringa, is a thin, cone-shaped biological membrane, membrane that separates the external ear from the middle ear. Its function is to transmit ...
, and external to the
oval window The oval window (or fenestra vestibuli) is a membrane-covered opening from the middle ear to the cochlea of the inner ear. Vibrations that contact the tympanic membrane travel through the three ossicles and into the inner ear. The oval window is t ...
of the
inner ear Inner Ear (established in 2007 in Bodø, Norway) is a Norwegian record label initiated and led by the brothers and jazz musicians Tore Johansen and Roger Johansen (musician), Roger Johansen. They opened Inner Ear to release their music when their ...

inner ear
. The mammalian middle ear contains three
ossicles 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 (inner ear), labyrinth (cochlea). The a ...
, which transfer the vibrations of the eardrum into waves in the fluid and membranes of the
inner ear Inner Ear (established in 2007 in Bodø, Norway) is a Norwegian record label initiated and led by the brothers and jazz musicians Tore Johansen and Roger Johansen (musician), Roger Johansen. They opened Inner Ear to release their music when their ...

inner ear
. The hollow space of the middle ear is also known as the
tympanic cavity The tympanic cavity is a small cavity surrounding the bones of the middle ear The middle ear is the portion of the ear internal to the eardrum, and external to the oval window of the inner ear. The mammalian middle ear contains three ossicles, ...
and is surrounded by the
tympanic part of the temporal bone The tympanic part of the temporal bone The temporal bones are situated at the sides and base of the skull, and lateral to the temporal lobes of the cerebral cortex. The temporal bones are overlaid by the sides of the head known as the temple (anat ...
. The
auditory tube In anatomy, the Eustachian tube, also known as the auditory tube or pharyngotympanic tube, is a tube that links the nasopharynx to the middle ear, of which it is also a part. In adult humans, the Eustachian tube is approximately long and in di ...
(also known as the Eustachian tube or the pharyngotympanic tube) joins the tympanic cavity with the nasal cavity (
nasopharynx The pharynx (plural: pharynges) is the part of the throat behind the mouth and nasal cavity, and above the esophagus The esophagus (American English) or oesophagus (British English; American and British English spelling differences#ae and ...
), allowing pressure to equalize between the middle ear and throat. The primary function of the middle ear is to efficiently transfer acoustic energy from
compression wave Longitudinal waves are waves in which the displacement of the medium is in the same (or opposite) direction of the wave propagation. Mechanical longitudinal waves are also called ''compressional'' or compression waves, because they produce compressi ...
s in air to fluid–membrane waves within the
cochlea The cochlea is the part of the inner ear involved in hearing. It is a spiral-shaped cavity in the bony labyrinth, in humans making 2.75 turns around its axis, the modiolus (cochlea), modiolus. A core component of the cochlea is the Organ of Corti, ...

cochlea
.


Structure


Ossicles

The middle ear contains three tiny bones known as the
ossicles 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 (inner ear), labyrinth (cochlea). The a ...
: ''
malleus The malleus or hammer is a hammer-shaped small bone or ossicle of the middle ear The middle ear is the portion of the ear internal to the eardrum, and external to the oval window of the inner ear. The mammalian middle ear contains three ossicl ...

malleus
'', ''
incus The incus or anvil is a bone A bone is a Stiffness, rigid tissue (anatomy), tissue that constitutes part of the vertebrate skeleton in animals. Bones protect the various organs of the body, produce red blood cell, red and white blood cells, ...
'', and ''
stapes The stapes or stirrup is a bone A bone is a Stiffness, rigid tissue (anatomy), tissue that constitutes part of the vertebrate skeleton in animals. Bones protect the various organs of the body, produce red blood cell, red and white blood cells ...
''. The ossicles were given their Latin names for their distinctive shapes; they are also referred to as the ''hammer'', ''anvil'', and ''stirrup'', respectively. The ossicles directly couple sound energy from the eardrum to the
oval window The oval window (or fenestra vestibuli) is a membrane-covered opening from the middle ear to the cochlea of the inner ear. Vibrations that contact the tympanic membrane travel through the three ossicles and into the inner ear. The oval window is t ...
of the cochlea. While the stapes is present in all
tetrapods Tetrapods (; from Greek 'four' and 'foot') are four-limbed animals constituting the superclass Tetrapoda . It includes extant and extinct Extinction is the termination of a kind of organism In biology, an organism (from Ancient ...

tetrapods
, the malleus and incus
evolved from lower and upper jaw bones present in reptiles
evolved from lower and upper jaw bones present in reptiles
. The ossicles are classically supposed to mechanically convert the vibrations of the
eardrum In the anatomy of humans and various other tetrapods, the eardrum, also called the tympanic membrane or myringa, is a thin, cone-shaped biological membrane, membrane that separates the external ear from the middle ear. Its function is to transmit ...
into amplified pressure waves in the fluid of the
cochlea The cochlea is the part of the inner ear involved in hearing. It is a spiral-shaped cavity in the bony labyrinth, in humans making 2.75 turns around its axis, the modiolus (cochlea), modiolus. A core component of the cochlea is the Organ of Corti, ...

cochlea
(or
inner ear Inner Ear (established in 2007 in Bodø, Norway) is a Norwegian record label initiated and led by the brothers and jazz musicians Tore Johansen and Roger Johansen (musician), Roger Johansen. They opened Inner Ear to release their music when their ...

inner ear
), with a lever arm factor of 1.3. Since the effective vibratory area of the eardrum is about 14 fold larger than that of the oval window, the sound pressure is concentrated, leading to a pressure gain of at least 18.1. The eardrum is merged to the malleus, which connects to the incus, which in turn connects to the stapes. Vibrations of the stapes footplate introduce pressure waves in the
inner ear Inner Ear (established in 2007 in Bodø, Norway) is a Norwegian record label initiated and led by the brothers and jazz musicians Tore Johansen and Roger Johansen (musician), Roger Johansen. They opened Inner Ear to release their music when their ...

inner ear
. There is a steadily increasing body of evidence that shows that the lever arm ratio is actually variable, depending on frequency. Between 0.1 and 1 kHz it is approximately 2, it then rises to around 5 at 2 kHz and then falls off steadily above this frequency. The measurement of this lever arm ratio is also somewhat complicated by the fact that the ratio is generally given in relation to the tip of the malleus (also known as the umbo) and the level of the middle of the stapes. The eardrum is actually attached to the malleus handle over about a 0.5 cm distance. In addition, the eardrum itself moves in a very chaotic fashion at frequencies >3 kHz. The linear attachment of the eardrum to the malleus actually smooths out this chaotic motion and allows the ear to respond linearly over a wider frequency range than a point attachment. The auditory ossicles can also reduce sound pressure (the inner ear is very sensitive to overstimulation), by uncoupling each other through particular muscles. The middle ear efficiency peaks at a frequency of around 1 kHz. The combined
transfer function In engineering Engineering is the use of scientific method, scientific principles to design and build machines, structures, and other items, including bridges, tunnels, roads, vehicles, and buildings. The discipline of engineering encompa ...

transfer function
of the outer ear and middle ear gives humans a peak sensitivity to frequencies between 1 kHz and 3 kHz.


Muscles

The movement of the ossicles may be stiffened by two muscles. The
stapedius muscle The stapedius is the smallest skeletal muscle Skeletal muscle (also called striated muscle - although cardiac muscle is also striated) is one of three major muscle types, the others being cardiac muscle and smooth muscle. It is a form of striated m ...
, the smallest skeletal muscle in the body, connects to the stapes and is controlled by the
facial nerve The facial nerve (the labyrinthine segment) is the seventh Cranial nerves, cranial nerve, or simply CN VII. It emerges from the pons of the brainstem, controls the muscles of facial expression, and functions in the conveyance of taste sensations ...

facial nerve
; the
tensor tympani muscle The tensor tympani is a muscle within the middle ear, located in the bony canal above the bony part of the auditory tube, and connects to the malleus bone. Its role is to dampen loud sounds, such as those produced from mastication, chewing, shoutin ...
is attached to the upper end of the medial surface of the handle of malleus and is under the control of the
medial pterygoid nerve Medial may refer to: Mathematics * Medial magma, a mathematical identity in algebra Geometry * Medial axis, in geometry the set of all points having more than one closest point on an object's boundary * Medial graph, another graph that repres ...
which is a branch of the
mandibular nerve The mandibular nerve (V3) is the largest of the three divisions of the trigeminal nerve, the fifth cranial nerve (CN V). Structure The large sensory root emerges from the lateral part of the trigeminal ganglion and exits the cranial cavity through ...

mandibular nerve
of the
trigeminal nerve The trigeminal nerve (the fifth cranial nerve, or simply CN V) is a nerve responsible for sensation in the face and motor functions such as biting and chewing; it is the most complex of the cranial nerves Cranial nerves are the nerve A ner ...

trigeminal nerve
. These muscles contract in response to loud sounds, thereby reducing the transmission of sound to the inner ear. This is called the
acoustic reflex The acoustic reflex (also known as the stapedius reflex, stapedial reflex, auditory reflex, middle-ear-muscle reflex (MEM reflex, MEMR), attenuation reflex, cochleostapedial reflex or intra-aural reflex) is an involuntary muscle Muscle is a ...
.


Nerves

Of surgical importance are two branches of the
facial nerve The facial nerve (the labyrinthine segment) is the seventh Cranial nerves, cranial nerve, or simply CN VII. It emerges from the pons of the brainstem, controls the muscles of facial expression, and functions in the conveyance of taste sensations ...

facial nerve
that also pass through the middle ear space. These are the horizontal portion of the facial nerve and the
chorda tympani The chorda tympani is a branch of the facial nerve The facial nerve (the labyrinthine segment) is the seventh Cranial nerves, cranial nerve, or simply CN VII. It emerges from the pons of the brainstem, controls the muscles of facial expression, ...
. Damage to the horizontal branch during ear surgery can lead to paralysis of the face (same side of the face as the ear). The chorda tympani is the branch of the facial nerve that carries taste from the
ipsilateral Standard anatomical terms of location deal unambiguously with the anatomy Anatomy (Greek ''anatomē'', 'dissection') is the branch of biology concerned with the study of the structure of organism In biology, an organism (from Ancient ...
half (same side) of the tongue.


Function


Sound transfer

Ordinarily, when sound waves in air strike liquid, most of the energy is reflected off the surface of the liquid. The middle ear allows the
impedance matching In electronics Electronics comprises the physics, engineering, technology and applications that deal with the emission, flow and control of electrons in vacuum and matter. It uses active devices to control electron flow by amplifier, amplific ...
of sound traveling in air to acoustic waves traveling in a system of fluids and membranes in the inner ear. This system should not be confused, however, with the propagation of sound as compression waves in liquid. The middle ear couples sound from air to the fluid via the
oval window The oval window (or fenestra vestibuli) is a membrane-covered opening from the middle ear to the cochlea of the inner ear. Vibrations that contact the tympanic membrane travel through the three ossicles and into the inner ear. The oval window is t ...
, using the principle of "mechanical advantage" in the form of the "hydraulic principle" and the "lever principle". The vibratory portion of the tympanic membrane (eardrum) is many times the surface area of the footplate of the
stapes The stapes or stirrup is a bone A bone is a Stiffness, rigid tissue (anatomy), tissue that constitutes part of the vertebrate skeleton in animals. Bones protect the various organs of the body, produce red blood cell, red and white blood cells ...
(the third ossicular bone which attaches to the oval window); furthermore, the shape of the articulated ossicular chain is like a
lever A lever ( or ) is a simple machine consisting of a beam or rigid rod pivoted at a fixed hinge, or '':wikt:fulcrum, fulcrum''. A lever is a rigid body capable of rotating on a point on itself. On the basis of the locations of fulcrum, load and effo ...

lever
, the long arm being the long process of the
malleus The malleus or hammer is a hammer-shaped small bone or ossicle of the middle ear The middle ear is the portion of the ear internal to the eardrum, and external to the oval window of the inner ear. The mammalian middle ear contains three ossicl ...

malleus
, the fulcrum being the body of the
incus The incus or anvil is a bone A bone is a Stiffness, rigid tissue (anatomy), tissue that constitutes part of the vertebrate skeleton in animals. Bones protect the various organs of the body, produce red blood cell, red and white blood cells, ...
, and the short arm being the lenticular process of the
incus The incus or anvil is a bone A bone is a Stiffness, rigid tissue (anatomy), tissue that constitutes part of the vertebrate skeleton in animals. Bones protect the various organs of the body, produce red blood cell, red and white blood cells, ...
. The collected pressure of sound vibration that strikes the tympanic membrane is therefore concentrated down to this much smaller area of the footplate, increasing the force but reducing the velocity and displacement, and thereby coupling the acoustic energy. The middle ear is able to dampen sound conduction substantially when faced with very loud sound, by noise-induced reflex contraction of the middle-ear muscles.


Clinical significance

The middle ear is hollow. In a high-altitude environment or on diving into water, there will be a pressure difference between the middle ear and the outside environment. This pressure will pose a risk of bursting or otherwise damaging the tympanum if it is not relieved. If middle ear pressure remains low, the eardrum may become retracted into the middle ear. One of the functions of the
Eustachian tube In anatomy Anatomy (Greek ''anatomē'', 'dissection') is the branch of biology concerned with the study of the structure of organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''organismos'') is any in ...
s that connect the middle ear to the
nasopharynx The pharynx (plural: pharynges) is the part of the throat behind the mouth and nasal cavity, and above the esophagus The esophagus (American English) or oesophagus (British English; American and British English spelling differences#ae and ...
is to help keep middle ear pressure the same as air pressure. The Eustachian tubes are normally pinched off at the nose end, to prevent being clogged with
mucus Mucus ( ) is a slippery aqueous secretion produced by, and covering, mucous membrane A mucous membrane or mucosa is a membrane that lines various cavities in the body and covers the surface of internal organs. It consists of one or more layers ...
, but they may be opened by lowering and protruding the jaw; this is why
yawn A yawn is a reflex consisting of the simultaneous inhalation of air and the stretching of the eardrums, followed by an exhalation of breath. Yawning (oscitation) most often occurs in adults immediately before and after sleep, during tedious act ...

yawn
ing or chewing helps relieve the pressure felt in the ears when on board an aircraft.
Otitis media Otitis media is a group of Inflammation, inflammatory diseases of the middle ear. One of the two main types is acute otitis media (AOM), an infection of rapid onset that usually presents with ear pain. In young children this may result in pulling ...
is an inflammation of the middle ear.


Infections

Recent findings indicate that the middle ear mucosa could be subjected to
human papillomavirus Human papillomavirus infection (HPV infection) is an infection An infection is the invasion of an organism's body Tissue (biology), tissues by Pathogen, disease-causing agents, their multiplication, and the reaction of host (biology), host t ...
infection. Indeed, DNAs belonging to oncogenic HPVs, i.e., HPV16 and HPV18, have been detected in normal middle ear specimens, thereby underling that the normal middle ear mucosa could potentially be a target tissue for HPV infection.


Other animals

The middle ear of
tetrapod Tetrapods (; ) are four-limbed animals constituting the superclass Tetrapoda (). It includes Neontology#Extant taxa vs. extinct taxa, extant and extinct amphibians, reptiles (including dinosaurs and therefore birds), and synapsids (including mam ...
s is
analogous Analogy (from Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population is approximately 10.7 million ...
with the spiracle of fishes, an opening from the
pharynx The pharynx (plural: pharynges) is the part of the throat behind the human mouth, mouth and nasal cavity, and above the esophagus and trachea – the tubes going down to the stomach and the lungs. It is found in vertebrates and invertebrates, thou ...

pharynx
to the side of the head in front of the main gill slits. In fish embryos, the spiracle forms as a pouch in the pharynx, which grows outward and breaches the skin to form an opening; in most tetrapods, this breach is never quite completed, and the final vestige of tissue separating it from the outside world becomes the eardrum. The inner part of the spiracle, still connected to the pharynx, forms the eustachian tube. In
reptile Reptiles, as most commonly defined, are the animals in the Class (biology), class Reptilia , a paraphyletic grouping comprising all amniotes except synapsids (mammals and their extinct relatives) and Aves (birds). Living reptiles comprise turtle ...

reptile
s,
bird Birds are a group of warm-blooded vertebrates constituting the class Class or The Class may refer to: Common uses not otherwise categorized * Class (biology), a taxonomic rank * Class (knowledge representation), a collection of indiv ...

bird
s, and early fossil tetrapods, there is a single auditory ossicle, the columella (that is homologous with the stapes, or "stirrup" of mammals). This is connected indirectly with the eardrum via a mostly cartilaginous extracolumella and medially to the inner-ear spaces via a widened footplate in the fenestra ovalis. The columella is an evolutionary derivative of the bone known as the hyomandibula in fish ancestors, a bone that supported the skull and braincase. The structure of the middle ear in living
amphibian Amphibians are ectothermic, tetrapod vertebrates of the Class (biology), class Amphibia. All living amphibians belong to the group Lissamphibia. They inhabit a wide variety of habitats, with most species living within terrestrial animal, terr ...
s varies considerably and is often degenerate. In most frogs and toads, it is similar to that of reptiles, but in other amphibians, the middle ear cavity is often absent. In these cases, the stapes either is also missing or, in the absence of an eardrum, connects to the
quadrate bone 300px, A schematic of an anapsid skull showing the location of major dermal bones of the upper skull, including the quadrate bone (q).The quadrate bone is a skull bone in most tetrapods, including amphibians, sauropsids (reptiles, birds), and ear ...
in the skull, although, it is presumed, it still has some ability to transmit vibrations to the inner ear. In many amphibians, there is also a second auditory ossicle, the ''operculum'' (not to be confused with the structure of the same name in fishes). This is a flat, plate-like bone, overlying the fenestra ovalis, and connecting it either to the stapes or, via a special muscle, to the
scapula In anatomy, the scapula (plural scapulae or scapulas), also known as the shoulder bone, shoulder blade, wing bone, speal bone or blade bone, is the bone that connects the humerus (upper arm bone) with the clavicle (collar bone). Like their connec ...

scapula
. It is not found in any other vertebrates. Mammals are unique in having independently of the various single-ossicle middle ears of other land vertebrates, all during the Triassic period of geological history. Functionally, the mammalian middle ear is very similar to the single-ossicle ear of non-mammals, except that it responds to sounds of higher frequency, because these are better taken up by the inner ear (which also responds to higher frequencies than those of non-mammals). The malleus, or "hammer", evolved from the
articular The articular bone is part of the lower jaw of most vertebrates Vertebrates () comprise all species of animal Animals (also called Metazoa) are multicellular eukaryotic organisms that form the Kingdom (biology), biological kingdom Animal ...
bone of the lower jaw, and the incus, or "anvil", from the quadrate. In other vertebrates, these bones form the primary jaw joint, but the expansion of the
dentary In anatomy Anatomy (Greek ''anatomē'', 'dissection') is the branch of biology concerned with the study of the structure of organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''organismos'') is any ind ...

dentary
bone in mammals led to the evolution of an entirely new jaw joint, freeing up the old joint to become part of the ear. For a period of time, both jaw joints existed together, one medially and one laterally. The evolutionary process leading to a three-ossicle middle ear was thus an "accidental" byproduct of the simultaneous evolution of the new, secondary jaw joint. In many mammals, the middle ear also becomes protected within a cavity, the
auditory bulla The tympanic part of the temporal bone The temporal bones are situated at the sides and base of the skull, and lateral to the temporal lobes of the cerebral cortex. The temporal bones are overlaid by the sides of the head known as the temple (anat ...
, not found in other vertebrates. A bulla evolved late in time and independently numerous times in different mammalian clades, and it can be surrounded by membranes, cartilage or bone. The bulla in humans is part of the
temporal bone The temporal bones are situated at the sides and base of the skull The skull is a bone A bone is a Stiffness, rigid tissue (anatomy), tissue that constitutes part of the vertebrate skeleton in animals. Bones protect the various organs of ...

temporal bone
.


Additional images

File:Gray919.png


See also

* *


References


External links


Promenade Around the Cochlea - Middle ear at iurc.montp.inserm.fr
{{Authority control Auditory system Ear