Ear Anatomy and Function | A Guide to Internal and External Ear
But nope, only hearing, or perception of sound is not the only job of the two protruding structures of the head. It maintains the body balance too. So, a person suffering from an ear problem can face dizziness and turbulence too.
Structure of Ear: Ear has many structures which can be classified according to its regions. It is classified into three different locations as
- External ear
- Middle ear
- Internal ear.
External ear Anatomy
External ear starts from the part of ear we see, that protruding part, and limited in the inner side by the tympanic membrane.
Structures: External, although may seem devoid of many structures, also has a lot.
A. Pinna: Pinna is the outer helical structure that we can see. It can’t be moved in human, but in lower animals, it can be moved, like in rabbit, dog, etc.
The structure of pinna is cartilage covered up with skin. It is a single piece of yellow cartilage, except for the lower lobule part, which is barely a piece of flesh and skin.
- Functions of the pinna: Pinna acts like a condenser, a collector of sound waves from distant sources and reflect it into the tympanic membrane. It’s an astonishing piece of art of nature that shows perfect engineering. Perfect reflection of sound is achieved via this pinna.
Pinna has further functions too. The skin of Pinna is used in grafting like nasal ala, or of hands. The cartilage is used in reconstruction surgery of the middle ear, or nasal bridge.
B. External Acoustic Canal: The concha of pinna leads to the long bent canal which ends into tympanic membrane. In human, it is 24mm long. The outer part is directed upward, backward and medially and inner part being directed downward, forward and medially, pinna has to be drawn upward, backward and laterally to give a look at the tympanic membrane.
Structures of the external acoustic canal are rather simple, having two parts.
The outer one-third of the canal is made up of cartilage. It has the skin with hair follicles, which extend up to this part only. So cerumen glands and pilosebaceous glands are seen in this part and are a seat for Staphylococcus bacterial infections too.
The inner two-thirds of the canal is made of bone. It is devoid of glands, and hair follicles. But cerumen secreted in outer part can be seen here too.
- Functions of the external acoustic canal include the carrying of sound waves to the tympanic membrane, and keeping all the infections and dust away from the inner ear.
C. Tympanic Membrane: Tympanic membrane acts as the barrier between the middle ear and external ear. It is 9-10mm tall, 8-9mm wide and 0.1mm in thickness, placed obliquely, posterosuperior part being more lateral than the anteroinferior part.
Structures of the tympanic membrane include two parts – pars tensa and Pars flaccida. Pars tensa is quite tensed, i.e. stretched with an attachment of head of the malleus and its characteristic Umbo. Whereas Pars flaccida is more flaccid, reddish and has malleal folds.
- Functions of the tympanic membrane include converting the sound energy to mechanical energy. Its vibrations are transmitted to the inner ear ossicles that in the course, help in hearing.
Middle Ear is the continuation of the tympanic membrane. A closed room, with the tympanic membrane on one side and oval and round windows on another side, the only aperture of this chamber is situated at its base, called Eustachian tube.
Structures of Middle Ear
The middle ear is a densely packed portion of the ear. It is loaded with ear ossicles, three in number.
A. Ear ossicles: Three ear ossicles are found in a human named- malleus, incus, and stapes.
Malleus is hammer-shaped bone, having an attachment with the pars tensa part of the tympanic membrane. And the other end is attached with the next ossicle called incus.
Incus is the next ossicle, attached with malleus on one side and with the next ossicle on the other side.
Stapes is the last ossicle. It is the smallest bone of the human body. It is attached with incus on one side, with the annular ligament on the attic of the middle ear, and on the other side, it is attached to the oval window.
- Functions of Ear Ossicles: Ear ossicles play a critical role in hearing. The ear ossicles transfer the vibration they receive from the tympanic membrane to the round window.
Not only this, they act as a safety valve. They can shut down hearing temporarily if some sound of large amplitude hits the tympanic membrane, by losing the annular ligament of stapes. Moreover, they amplify the sound by many times as it reaches the round window.
Two muscles are also attached with these bones, called tensor tympani and stapedius. Stapedius is, no doubt the smallest muscle of the human body.
B. Eustachian Tube: Eustachian tube is a few millimeter long tube, connecting the middle ear with the pharynx. It is the only vent to the otherwise closed middle ear.
- Functions of Eustachian tube: Eustachian tube maintains an important function indirectly helping to maintain painless hearing. The eustachian tube maintains the air pressure of the middle ear equal with atmospheric pressure. Else, the pressure will directly act on the tympanic membrane, bending it or even tearing it, causing immense pain, and resulting in the person deaf.
There are other structures in the middle ear, like the mastoid air spaces, the temporal air sinuses, etc., helping in hearing in indirect ways.
Internal Ear Anatomy
The internal ear has two primary functions – one is obviously hearing, and the other being body balance. First, let us see the body balancing function:
1. Body Balance:
Body balance is maintained primarily by the cerebellum, a part of the brain. In-ear, it is maintained by the bony labyrinth.
Bony labyrinth: Bony labyrinth is formed of bone, as the name suggests. It is divided into 3 parts:
A. Vestibule: This is the central chamber of the bony labyrinth. It has two recesses lodging into the utricle. It is the seat of the dynamic body balance.
B. Semilunar Canals: This is the Main seat of body balance in the bony labyrinth. Having calcium carbonate crystals they help in the static balance. It has three semi-circular canals which act independently. All of them open into vestibule by five openings. All the canals are at a right angle to each other.
Hearing, the primary function of the ear is performed by the Membranous labyrinth.
The membranous labyrinth is mainly formed of membranes, as the name suggests. Parts are:
A. Cochlea: Although cochlea is a bony spirally coiled tube, the cochlear duct is Membranous one. The cochlea has three parts namely
- Scala vestibule
- Scala tympani
- Scala media
Fluids are all around in the internal ear. It covers up all the chambers of the cochlea. The fluid present in scala tympani and vestibule is called perilymph, whereas, that present in the scala media is the endolymph. Both the scala tympani and the vestibule are connected with each other by an opening called Helicotrema. Scala tympani connects with subarachnoid space CSF via aqueduct of Cochlea.
B. Cochlear duct: It is actually part of cochlea itself. It consists of three membranes namely
- Basilar membrane, on which lies the organ of Corti.
- Reissner’s membrane, which separates the organ of Corti from scala vestibule.
- Functions of Internal ear: Among all the functions except visual beauty, others are maintained and completed by internal ear only.
It has a bony labyrinth, which helps in the maintaining of body balance via its parts vestibule and semicircular canals.
The Membranous labyrinth has an organ of Corti which helps in the hearing. It has hair cells which move during the vibrations which are passed by the stapes to round window, and this opens the sodium-potassium channels among the perilymph and endolymph. The endolymph resembles the intracellular fluid, and perilymph resembles the extracellular fluid. The change in concentration of these cations leads to change in membrane potential of Basilar membrane, which is taken as an Impulse by the auditory nerve, the 8th cranial nerve. And we can hear; Simple
The ear is the only organ of sono-reception. It lets us hear the sounds of the world.
Many diseases are caused to ear starting from minor diseases like infection, temporary deafness, to diseases of significant concern like Chronic suppurative otitis media CSOM, etc.