In the external ear, the main elements are: the auricle , the external auditory canal and the lateral surface of theeardrum ; in the middle ear, the most important elements are: the tympanum, the three ossicles, the Eustachian tube, 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 being human and mammals in general, the ear has three components, which anatomists call: outer ear , middle ear and inner ear .
The ear is an even organ, which resides at the level of the head.
It includes portions of a cartilaginous nature, bones, muscles, nerves, arterial vessels, venous vessels , sebaceous glands and ceruminous glands.
The outer ear is basically the component of the ear visible to the sides of the head with the naked eye. The main parts that make it up are: the auricle , the external auditory canal (or external acoustic meatus ) and the external face of the eardrum (or tympanic membrane).
- Auricular Pavilion . Covered with skin , it is a predominantly cartilaginous structure, on which anatomists identify various characteristic areas, including: two curved rhymes, one outermost of the other, called helix and anti- helix ; two protrusions, called 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 leather , it is the canal 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 of a cartilaginous nature, while its final tract is of a bone nature. The bony portion that constitutes the final part belongs to the temporal bone of the skull and is called the auditory bubble ( or tympanic bubble ).
The skin that lines the external auditory canal is rich in sebaceous glands and ceruminous glands. The task of such glands is to secrete substances such as earwax , which serve to protect the ear in general from potential threats.
- External face of the tympanum . It is the face that looks towards the opening of the external auditory canal.
Different muscles and ligaments are placed on the outer ear .
Distinguished in extrinsic and intrinsic, the muscles of the human outer ear are structures that are almost completely irrelevant from a functional point of view.
On the contrary, the ligaments have a role of a certain relevance: those defined as extrinsic connect the cartilage to the temporal bone, while those defined as intrinsic keep the cartilage in place and shape the auricle.
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 .
- Tympanum . Located at the end of the external auditory canal and just before the tympanic cavity, it is a thin oval and transparent membrane, which has the task of transmitting the 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 fortress of the temporal bone of the skull . In other words, the tympanic cavity is a bone socket belonging to the temporal bone of the skull.
In the tympanic cavity there are the three small bones of the middle ear, that is: the hammer , the anvil and the stirrup .
Located in such a way as to be able to communicate with each other, the 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 small bones of the middle ear, the one that has direct relations with the eardrum and is the first to receive sound vibrations is the hammer. In the hammer, the point of contact with the eardrum is in a region known as the hammer handlebar .
Taken together, the three ossicles also take the name of the ” ossicular chain “. The term “chain” refers to the activation in sequence of the bone elements in question, when the sound vibrations reach the eardrum: the first to move is the hammer, then the anvil, on stimulation of the hammer, and finally the stirrup , after interacting with the anvil.
- Auditory tuba . Perhaps better known as the Eustachian tube, it is the conduit that connects the tympanic cavity with the pharynx and the so-called air cells of the mastoid (or mastoid cells).
The Eustachian tube has several tasks, including: guaranteeing the right pressure at the level of the eardrum and preventing normal body noises (such as those resulting from breathing or swallowing) from hitting directly on the eardrum.
- Oval window and round window . They are two membranes very similar to the eardrum, located on 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 stirrup 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.
As for 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 only interacts directly with the oval window. Nonetheless, the round window still vibrates with the movement of the bracket. 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 muscle of the eardrum . 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.
This is a different point of view, compared to that according to which the oval and round windows are part of the middle ear, but equally correct.
The inner ear is the deepest component of the ear.
Located in a cavity of the temporal bone, whose name is a bone labyrinth , the parts that make up the inner ear are essentially two: the vestibular system (or vestibular system ) and the cochlea .
In anatomy, the complex “vestibular apparatus – cochlea” is called a membranous labyrinth .
Inside, as well as outside, the vestibular apparatus and the cochlea circulate a characteristic fluid: the fluid outside is called perilymp , while the fluid inside is the aforementioned endolymph .
By interposing between the bone labyrinth and the membranous labyrinth, the perilymph acts as a cushioning cushion, which prevents impacts between one of the structures of the inner ear and the surrounding bone 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 system . Ear structure specifically assigned to balance control, consists of two elements: the vestibule and the semicircular canals .
The vestibule includes two characteristic vesicles: an upper one, called the 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 far as the semicircular canals are concerned, 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 dilation, which is called an ampoule .
The orientation of the semicircular canals is particular; each channel, in fact, 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, equipped with cilia ( ciliated cells ).
Together with the endolymph, otoliths and hair cells of the vestibule and semicircular canals play a central role in the mechanisms of regulation of balance.
- Auger . Similar to a snail – similarity to which it owes its second name – is the structure of the ear specifically assigned to the perception of sounds.
Inside the cochlea, three chambers are recognizable, whose name is: vestibular staircase, cochlear duct and tympanic staircase.
Of these three chambers – all three very important – the cochlear duct is particularly noteworthy , for the fact that it contains a fundamental element for the auditory perception process: the so-called Corti organ . Corti’s organ is a group of very particular hair cells, responsible for interacting 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 OUTER EAR
Having sensory function, the main nerves that have relationships with the external ear are:
- The large auricular nerve . Innervates the lower 2/3 of the anterior and posterior surface of the external ear.
- The auricular branch of the vagus nerve (or auricular nerve or Arnold’s nerve ). It innervates the floor of the external auditory canal and the basin.
- The auriculotemporal nerve . Innervates 1/3 of the anterior upper part of the external ear.
- The small occipital nerve . Innerva 1/3 of the posterior upper part of the outer ear.
INNERVATION OF THE MIDDLE EAR
The nerves that have relationships with or through the middle ear are:
- The so-called gable rope . It is a branch of the seventh cranial nerve (or facial nerve ). It has a sensitive function and, among the various functions it performs, it also has the task of innervating the mucous membrane of the tympanic cavity.
- The auriculotemporal nerve , the auricular branch of the vagus nerve and the tympanic nerve (or Jacobson nerve or tympanic branch of the glossopharyngeal nerve ). They are the sensory nerves of the tympanic membrane.
- The upper and lower carotidympanic nerves . Passing through the tympanic cavity, they contribute to the so-called tympanic plexus , a reticular complex of different sensory nerves that have the task of innervating the middle ear.
- The small petrosal nerve . It is the continuation of the tympanic nerve and has sensory functions. It is part of the tympanic plexus.
- The large petrosal nerve . It is a branch of the seventh cranial nerve and has sensory functions. Contributes to the tympanic plexus.
- The motor branch of the facial nerve responsible for controlling the stapedius muscle.
- The 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 muscle of the eardrum.
Innervation of the inner ear
Innervation of the inner ear rests with the vestibulocochlear (or eighth cranial nerve) nerve. The vestibulocochlear nerve is an important nervous structure with a sensory function, which originates at the level of the Varolio bridge ( brain stem ) 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 the nerve signals from the vestibular apparatus – with which they communicate and to which they owe their name – to the brain .
The cochlear nerve, on the other hand, has the function of transmitting the nerve signals from the cochlea – to which it is connected and to which it owes its name – to the brain.
The outer ear, middle ear and inner ear each have their own network of arterial vessels , which supplies them with the oxygenated blood necessary for the survival of the various anatomical constituent elements.
More specifically, the inflow of oxygen-rich blood to the external ear belongs mainly to the posterior auricular artery and secondarily to the anterior auricular artery and the occipital artery.
The blood supply of the middle ear depends, in the first instance, on the stylo-mastoid branch of the posterior auricular artery and on the deep auricular artery and, secondly, on the middle meningeal artery, the ascending pharyngeal artery, theinternal and artery of the pterygoid canal.
Finally, the flow of oxygenated blood to the inner ear is up to: the anterior tympanic branch of the maxillary artery, the stylo-mastoid branch of the auricular artery, the petrosal branch of the middle meningeal artery and the labyrinthine artery.
Ear functions have 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 are carried out.
The perception of the sounds present in the environment involves all three components of the ear.
The sound waves, in fact, penetrate the outer ear, pass through the entire middle ear and finally conclude their path in correspondence with the inner ear.
Thanks to their particular anatomy, the structures that form the outer ear have the task of conveying the sound waves towards the middle ear: the auricle receives the sound waves and causes them to enter the external auditory canal, up to the eardrum.
With the arrival of sounds to the eardrum, this 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 three bones: the first bone to be activated 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 function similarly to the tympanic membrane.
From this moment forward, 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 the Corti. Once activated, the cells of Corti’s organ deal with the important process of converting sound waves into nerve impulses.After conversion, the cochlear nerve comes into play, which collects the newly generated nerve impulses and sends them to the temporal lobe of the brain .
In the temporal lobe of the brain, the reworking of nerve impulses and the generation of an adequate response takes place.
The ear of the human being can hear sounds that have a frequency between 20 Hz and 20 kHz. Below 20 Hz, we speak of infrasound; above 20 kHz, however, we speak of ultrasound.
The sense of balance is under the control of a specific 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 moving in a straight line – while the three semicircular channels regulate the so-called dynamic equilibrium – that is, the equilibrium for the moments in which the body performs rotation movements.
As anticipated, otoliths and hair cells, present, together with the endolymph, inside the vestibular apparatus, play a fundamental role in the mechanism of regulation of balance. In fact, the movement of the otoliths and hair cells, following the movements of the body, produces a nervous signal, which informs the brain of the aforementioned movements.
Once the brain knows the movements of the body, it produces a tailored response, which guarantees stability and sense of position in space, to the moving subject.The means that allow the vestibular apparatus to communicate with the brain are the vestibular nerves.