Generality
The ear is the organ that allows the perception of sounds (the so-called sense of hearing) and which guarantees the static and dynamic balance of the body.
Dividable into three compartments - whose names are the outer ear, middle ear and inner ear - the ear is made up of cartilage-like portions, bones, muscles, nerves, blood vessels, sebaceous glands and ceruminous glands.
In the external ear, the main elements are: the auricle, the external auditory canal and the lateral surface of the eardrum; in the middle ear, the most important elements are: the tympanum, the three ossicles, the Eustachian trumpet, the oval window and the round window; finally, in the inner ear, the most relevant elements are: the cochlea and the vestibular apparatus.
What is the ear?
The ear is the organ of hearing and balance .
In humans and mammals in general, the ear has three components, which anatomists call: the external ear, the middle ear and the inner ear .
Anatomy
The ear is an equal organ, which resides at the level of the head.
It includes portions of cartilaginous nature, bones, muscles, nerves, arterial vessels, venous vessels, sebaceous glands and ceruminous glands.
External Ear
The outer ear is, substantially, the component of the ear visible to the naked eye on the sides of the head. The main parts that constitute it are: the auricle, the external auditory canal (or external acoustic meatus ) and the external face of the eardrum (or tympanic membrane).
- Auricle. Covered with skin, it is a predominantly cartilaginous structure, on which the anatomists identify various characteristic areas, including: two curved rhymes, one more external than the other, called elice and antielice ; two protrusions, called a tragus and antitragus, which tend to cover the external acoustic meatus; the basin, which is the concave region in which the opening of the external auditory canal takes place; finally, the lobe, made up of adipose tissue and located on the lower margin.
- External auditory canal. Between 2.5 and 4 centimeters long and covered with skin, it is the channel that, with a characteristic S-curve, goes from the auricle (precisely from the basin) to the tympanum.
The initial tract of the external auditory canal is cartilaginous in nature, while its final tract is of bony nature. The bone portion that constitutes the final tract belongs to the temporal bone of the skull and is called the auditory bubble ( or tympanic bubble ).
The skin that covers the external auditory canal is rich in sebaceous glands and ceruminous glands . The task of this gland is to secrete substances such as cerumen, which serve to protect the ear in general from potential threats.
- External face of the eardrum. It is the face that looks towards the opening of the external auditory canal.
Several muscles and ligaments are placed on the outer ear.
Distinct in extrinsic and intrinsic, the muscles of the human external ear are structures that are almost completely irrelevant from the functional point of view.
On the contrary, the ligaments play a role of some significance: those defined as extrinsic connect the cartilage to the temporal bone, while those defined as intrinsic maintain the cartilage in place and give shape to the auricle.
Middle Ear
The middle ear is the component of the ear between the outer ear and the inner ear. Its main constituent parts are: the tympanic membrane (or tympanum ), the tympanic cavity, in which the so-called three ossicles take place, the auditory tube, the oval window and the rounded window .
- Timpano. Located at the end of the external auditory canal and immediately before the tympanic cavity, it is a thin membrane of oval and transparent shape, which has the task of transmitting sound vibrations, penetrated through the external ear, to the chain of the three ossicles.
The tympanic membrane can be divided into two regions: the so-called pars flaccida and the so-called pars tensa .
Very often anatomists describe it as the boundary point between the outer ear and the inner ear.
- Tympanic cavity. Also known as the tympanic cavity or tympanic cavity, it is a hollow area that originates at the level of the so-called petrous rock of the temporal skull bone . In other words, the tympanic cavity is a bone hollow belonging to the temporal skull bone.
In the tympanic cavity take place the three small bones of the middle ear, namely: the hammer, the anvil and the stirrup .
Located so as to be able to communicate with each other, hammer, anvil and stirrup have the important function of receiving sound vibrations from the eardrum, amplifying them and transmitting them to the inner ear.
Of the three bones of the middle ear, the hammer that has direct relations with the eardrum and receives sound vibrations first is the hammer. In the hammer, the point of contact with the eardrum is in a region known as the handlebar of the hammer .
Taken together, the three little bones also take the name of " chain of ossicles ". The term "chain" refers to the activation in sequence of the bone elements in question, at the moment when the sound vibrations reach the eardrum: the first to move is the hammer, then the anvil, upon stimulation by the hammer, and finally the bracket, after interacting with the anvil.
- Hearing tube. Perhaps better known as the Eustachian trumpet, it is the conduit that connects the tympanic cavity with the pharynx and the so-called mastoid air cells (or mastoid cells).
The Eustachian tube has several tasks, including: ensuring the right pressure at the eardrum and preventing normal bodily noises (for example those arising from breathing or swallowing) from bumping directly on the eardrum.
- Oval window and round window. They are two membranes very similar to the eardrum, located at the border between the middle ear and the inner ear.
The task of the oval window and the round window is to transmit the sound vibrations from the bracket to a particular liquid - the endolymph - present inside the two main structures of the inner ear, namely: the vestibular apparatus and the cochlea.
To be more precise, the oval window interacts with the endolymph of the vestibular apparatus, while the round window interacts with the endolymph of the cochlea.
Regarding the position of the membranes in question, the oval window resides above the round window.
Figure : middle ear. It is interesting to point out to readers that the bracket interacts directly only with the oval window. Nevertheless, the round window still vibrates with the stirrup movement. All this is possible, because the oval window transmits the vibrations that invest it to the round window below. Image taken from en.wikipedia.org
Two muscles belong to the middle ear, which have the task of promoting the movement of the ossicles to which they are connected. The muscles in question are the stapedius muscle and the tensor tympanic muscle . The first is connected to the bracket, while the second is joined to the hammer.
Oval window and round window: middle ear or inner ear?
In some anatomy texts, the oval window and the round window are among the elements that make up the inner ear.
It is a different point of view, compared to that according to which the oval and round windows would be part of the middle ear, but equally correct.
Inner Ear
The inner ear is the deepest component of the ear.
Located in a cavity of the temporal bone, whose name is the bony labyrinth, the parts that make up the inner ear are basically two: the vestibular apparatus (or vestibular system ) and the cochlea .
In anatomy, the complex "vestibular apparatus - cochlea" takes the name of membranous labyrinth .
Inside, as well as outside, of the vestibular apparatus and of the cochlea, a characteristic fluid circulates: the fluid on the outside takes the name of perilinfa, while the fluid inside is the already mentioned endolymph .
Interposed between the bony labyrinth and the membranous labyrinth, the perilinfa acts as a shock-absorbing pad, which prevents collisions between one of the inner ear structures and the surrounding bony walls.
The endolymph, on the other hand, plays a fundamental role in the process of perception of sounds and in the mechanisms of balance.
- Vestibular apparatus. Ear structure specifically responsible for controlling the balance, it consists of two elements: the vestibule and the semicircular canals .
The vestibule includes two characteristic vesicles: an upper one, called utricle, and a lower one, called the saccule . The utricle has an elongated shape, is closely connected to the ampoules of the semicircular canals and communicates with the bracket, through the oval window. The saccule, on the other hand, has a spherical shape and is closely connected to the cochlea.
As for the semicircular canals, these are three curved ducts, which take place above the vestibule, thus representing the upper part of the entire vestibular apparatus. At the base of each semicircular canal there is a small expansion, which takes the name of ampulla .
The orientation of the semicircular canals is particular; in fact, each channel forms a right angle with each of the other two.
Inside the vestibule and semicircular canals, dispersed in the endolymph, there are the so-called otoliths (calcium carbonate crystals) and particular cellular elements, provided with cilia ( hair cells ).
Together with the endolymph, the otoliths and the ciliate cells of the vestibule and semicircular canals play a central role in the mechanisms of balance regulation.
- Auger. Similar to a snail - a resemblance to which it takes its second name - is the structure of the ear specifically assigned to the perception of sounds.
Inside the cochlea, three rooms are recognizable, whose name is: vestibular scale, cochlear duct and tympanic scale.
Of these three chambers - all three very important - we note in particular the cochlear duct, due to the fact that it contains a fundamental element for the process of auditory perception: the so-called organ of Corti . The organ of Corti is a set of very particular hair cells, appointed to interact with the endolymph.
Finally, it should be noted that the area of the cochlea connected to the round window resides on the border with the vestibule, in the immediate vicinity of the utricle.
INNERVATION OF THE EXTERNAL EAR
Having a sensory function, the main nerves that have relations with the external ear are:
- The great auricular nerve . It preserves the lower 2/3 of the front and rear surface of the external ear.
- The auricular branch of the vagus nerve (or auricular nerve or Arnold's nerve ). Innervates the floor of the external auditory canal and the basin.
- Auricolotemporal nerve . It preserves 1/3 of the upper front part of the outer ear.
- The small occipital nerve . It preserves 1/3 of the upper back part of the outer ear.
INNERVATION OF THE MIDDLE EAR
The nerves that have relations with or through the middle ear are:
- The so-called gable string . It is a branch of the seventh cranial nerve (or facial nerve). It has a sensitive function and, among the various functions it performs, also has the task of innervating the mucosa of the tympanic cavity.
- The auricolotemporal nerve, the auricular branch of the vagus nerve and the tympanic nerve (or Jacobson's nerve or tympanic branch of the glossopharyngeal nerve). They are the sensory nerves of the tympanic membrane.
- Upper and lower caroticotympanic nerves . Passing through the tympanic cavity, they contribute to the so-called tympanic plexus, a reticular complex of several sensory nerves that have the task of innervating the middle ear.
- The small petrous nerve . It is a continuation of the tympanic nerve and has sensitive functions. It is part of the tympanic plexus.
- The large petrous nerve . It is a branch of the seventh cranial nerve and has sensitive functions. Contributes to the tympanic plexus.
- The motor branch of the facial nerve responsible for the control of the stapedius muscle.
- Internal pterygoid nerve . It is a motor branch of the mandibular nerve, which in turn is part of the so-called trigeminal nerve . The task of the internal pterygoid nerve is to innervate the tensor tympanic muscle.
Innervation of the inner ear
The innervation of the inner ear rests with the vestibulocochlear nerve (or eighth cranial nerve). The vestibulocochlear nerve is an important nerve structure with a sensory function, which originates at the level of the Varolio bridge (brainstem) and is divided into: upper vestibular nerve, lower vestibular nerve and cochlear branch (or cochlear nerve ).
The upper vestibular and lower vestibular nerves have the task of transmitting nerve signals from the vestibular apparatus - with which they communicate and to which they owe the name - to the encephalon.
The cochlear nerve, on the other hand, has the function of transmitting nerve signals from the cochlea - to which it is connected and to which it owes its name - to the encephalon.
vascularization
Each external ear, middle ear and inner ear have their own network of arterial vessels, which provides them with the oxygenated blood necessary for the survival of the various constituent anatomical elements.
Specifically, the flow of oxygen-rich blood to the external ear is mainly due to the posterior auricular artery and, secondarily, to the anterior auricular artery and the occipital artery.
The blood circulation of the middle ear depends, in the first instance, on the stilo-mastoid branch of the posterior auricular artery and on the deep auricular artery and, secondly, on the middle meningeal artery, the ascending pharyngeal artery, the internal carotid artery and the pterygoid canal artery.
Finally, the influx of oxygenated blood to the inner ear belongs to: the anterior tympanic branch of the maxillary artery, the stylo-mastoid branch of the auricular artery, the petrous branch of the middle meningeal artery and the labyrinthine artery.
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Inner ear |
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Function
Ear function has already been widely discussed.
Here, therefore, attention will be paid to how the process of perception of sounds and the mechanism of control and regulation of balance take place.
HEARING PERCEPTION
The perception of sounds in the environment involves all three components of the ear.
Sound waves, in fact, penetrate the outer ear, pass through the middle ear and finally end their path at the inner ear.
Thanks to their particular anatomy, the structures forming the external ear have the task of conveying the sound waves towards the middle ear: the auricle receives the sound waves and causes them to take the external auditory canal, up to the eardrum.
With the arrival of sounds on the eardrum, it begins to vibrate.
The vibration of the eardrum marks the beginning of the participation of the middle ear in the process of perception of sounds. Vibrating, in fact, the eardrum triggers the chain of the three ossicles: the first bone to activate is the hammer, the second is the anvil and the last is the stirrup.
From the stirrup, the vibrations pass to the oval window and the round window, which work similarly to the tympanic membrane.
From this moment on, the middle ear has completed its tasks and the inner ear enters the scene.
The vibrations of the oval window and the round window, in fact, set in motion the endolymph present in the cochlea. The movements of the cochlear endolymph represent the signal that triggers the cells of the organ of Corti. Once activated, the cells of the organ of Corti deal with the important process of conversion of sound waves into nerve impulses.
After conversion, the cochlear nerve comes into play, which collects neogenerated nerve impulses and sends them to the temporal lobe of the brain .
In the temporal lobe of the brain, reworking of nerve impulses and the generation of an adequate response takes place.
Curiosity
The ear of the human being can hear sounds that have a frequency between 20 Hz and 20 kHz. Below 20 Hz, we talk about infrasound; above 20 kHz, on the other hand, we talk about ultrasound.
EQUILIBRIUM
The sense of balance is under the control of a precise portion of the ear: the vestibular apparatus of the inner ear.
In this case, utricle and saccule control the so-called static equilibrium - that is, the equilibrium for the moments in which the body is immobile or moves in a straight line - while the three semicircular canals regulate the so-called dynamic equilibrium - that is the equilibrium for the moments in which the body makes rotational movements.
As anticipated, otoliths and hair cells, present, together with the endolymph, inside the vestibular apparatus play a fundamental role in the mechanism of balance regulation. In fact, the movement of otoliths and hair cells, following the movement of the body, produces a nervous signal, which informs the brain of the aforementioned movements.
Once the encephalon knows the movements of the body, it produces a tailored response, which guarantees stability and a sense of position in space, to the subject in motion.
The means that allow the vestibular apparatus to communicate with the brain are the vestibular nerves.
Illnesses
The ear can be subject to numerous morbid conditions.
Among the diseases that affect the ear, Ménière's syndrome, otitis media, benign paroxysmal positional vertigo, labyrinthitis, vestibular neuronitis, otosclerosis, acoustic neuroma, cholesteatoma and perforation are certainly worth mentioning. of the eardrum.
MOST COMMON SYMPTOMS OF EARS DISEASES
The most common symptoms of ear diseases include: dizziness, hearing loss, deafness, tinnitus (or tinnitus), the sense of plugged ear and loss of balance.
More information on ear diseases is available on the Ear Health page.
