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The inner ear, which is also known as the internal ear, primarily functions to help in hearing and keeping balance. The inner ear is mainly divided into two parts – the auditory system which is primarily composed of the cochlea, and the vestibular system, which is primarily composed of semicircular canals and vestibule and takes charge of balance.
AUDITORY SYSTEM
The auditory system, from the name itself, takes charge of the hearing mechanism. The main portion of the auditory system is the cochlea.
COCHLEA
The cochlea is known as the organ of hearing and it participates in the auditory transduction process. The term cochlea originates from the Greek word “kokhliās” which means “snail” or “shell of the snail,” a perfect representation of its anatomical shape or structure. The cochlea is roughly around 30 millimeters long. Since it is the organ of hearing, it works closely with the middle ear through the two holes that are closed by membranes – the oval window and the round window.
The oval window is scientifically or medically known as the fenestra ovalis, a connective tissue membrane at the boundary of the middle and inner ear. Consequently, the round window, also known as fenestra rotunda, is located inferior and posterior to the oval window. The oval and round windows aid in sound transmission. The basic concept of sound transmission here is that vibratory music enters the oval window. The music is then transmitted to the cochlea and exits through the round window and into the middle ear cavity.
The cochlea has several landmarks which include the modiolus, osseous spiral lamina, Rosenthal’s canal, scala vestibule, scala tympani, promontary, helicotrema, and the round window. The canals of the cochlea are known as the scala tympani (the lowermost portion and is closed by the round window), scala vestibular (the uppermost compartment and is closed by the oval window), and scala media (the middle portion).
The cochlea spirals about two to three turns and the bony portion where the cochlea spins is known as the MODIOLUS. Within that is the cranial nerve’s acoustic branch which passes through other structures that it innervates. The aforementioned acoustic branch then goes to different spaces where it will be called the “spiral ganglion.” A sheath of membrane called “spiral lamina” then covers the spiral ganglion. This will terminate to the Organ of Corti, the location of the hair cells, tectorial membrane, and other support structures. The Organ of Corti is referred to as the epithelial cells of hearing which is housed on the basilar membrane. Also, the Organ of Corti consists of 30,000 finger-like cilia projections. The cilia is known as the hair cell. The Organ of Corti is responsible for transducing mechanical sounds into electrical signals.
The cochlea is also related to the cochlear duct, an endolymph-filled cavity between the scala tympani (lower) and scala vestibuli (upper).
VESTIBULAR SYSTEM
As previously mentioned, the vestibular system is the balance part of the ear. It aids in maintaining the balance in collaboration with the eye movement and somatosensory input. The vestibular system is primarily composed of the vestibule and semicircular canals.
VESTIBULE
The vestibule is the central portion or the central egg-shaped cavity of the inner ear. This is marked by three prominent recesses: the spherical, elliptical, and cochlear recesses. The elliptical and spherical recesses are separated by the vestibular crest which also encloses the cochlear recess for the cochlear nervous fibers (Wageih Felfela, 2017).
The vestibule has two end organs with membranous labyrinth, which are the utricle and saccule. The utricle is located superiorly while the saccule lies on the medial wall.
The vestibule communicates in front with the Cochlea’s scala vestibuli. It is also lateral to the oval window and footplate of stapes.
The vestibule is also related to the vestibular duct or scala vestibuli, a perilymph-filled cavity inside the inner ear’s cochlea. The vestibular ducts are responsible for the sound vibrations to the cochlear duct. These vestibular ducts are narrow, bony canals from the medial wall of the vestibule. These structures are considered enlarged if they exceed 1.5 millimeters.
SEMICIRCULAR CANALS
Overall, there are three semicircular canals in the ear: the anterior vertical/superior, lateral/horizontal, and posterior vertical. These three are named after the position they occupy. Moreover, these semicircular canals are positioned at right angle to each other. These canals are placed superoposterior to the vestibule and they have a swelling at one end which is called as the “ampulla.” Each ampulla contains a crista, a saddle-shaped ridge of tissue. Physiologically speaking, the semicircular canals also have the sense organ for body movement.
MEMBRANOUS VS BONY LABYRINTH
Aside from classifying the inner ear into the vestibular and auditory systems, the inner ear can also be divided into the bony labyrinth and the membranous labyrinth. Basically, the components of the inner ear are categorized based on their structure– either bony or membranous.
The bony labyrinth is composed of the bony cavities found in the temporal bone’s petrous part. The parts that make up the bony labyrinth are the vestibule, semicircular canals, and the cochlea.
Consequently, the membranous labyrinth lies within the bony labyrinth. Moreover, the membranous labyrinth is filled with endolymph, a fluid substance. This membranous labyrinth consists of the utricle, saccule, and cochlear duct.
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