Inner Ear Anatomy

In mammals, the anatomy of the inner ear consists of the bony labyrinth, a system of passages making up the following 2 main functional parts: (1) the cochlea, which is dedicated to hearing, and (2) the vestibular system, which is dedicated to balance. ^{[1, 2]} The inner ear is found in all vertebrates, with substantial variations in form and function. The inner ear is innervated by the eighth cranial nerve in all vertebrates. (See the image below.) ^[3]

The superficial contours of the inner ear are established by a layer of dense bone known as the bony labyrinth, which refers to the network of canals. The walls of the bony labyrinth are continuous with the surrounding temporal bone. The inner contours of the bony labyrinth closely follow the contours of the membranous labyrinth, a delicate, interconnected network of fluid-filled tubes in which the receptors are found.

The walls of the bony labyrinth consist of dense bone everywhere except at 2 small areas near the base of the cochlear spiral. The round window consists of a thin, membranous partition that separates the perilymph of the cochlear chambers from the air-filled middle ear. Collagen fibers connect the boney margins of the opening known as the oval window at the base of the stapes. ^[4]

A liquid called perilymph, the properties of which closely resemble those of cerebrospinal fluid, flows between the bony and membranous labyrinths. Another fluid, called endolymph, is contained in the membranous labyrinth. The endolymph has concentrations of electrolytes that differ from those of typical body fluids.

The bony labyrinth can be subdivided into the vestibule, 3 semicircular canals, and the cochlea. The vestibule contains a pair of membranous sacs: the saccule (sacculus) and the utricle (utriculus). Receptors in the vestibule provide for sensations of gravity and linear acceleration.

The semicircular canals enclose the slender semicircular ducts. Receptors located here are stimulated by rotation of the head. Together with the vestibule, this is called the vestibular complex. The fluid filled chambers within the vestibule are generally continuous with those of the semicircular canals.

The cochlea is a bony, spiral-shaped chamber that contains the cochlear duct of the membranous labyrinth. The sense of hearing is provided by receptors within the cochlear duct. A pair of perilymph-filled chambers is found on each side of the duct. The entire apparatus makes turns around a central bony hub, much like a snail shell. ^[4]

During week 4 of embryonic development, the human inner ear develops from the auditory placode, a thickening of the ectoderm that gives rise to the bipolar neurons of the cochlear and vestibular ganglions. As the auditory placode invaginates towards the embryonic mesoderm, it forms the auditory vesicle or otocysts.

The auditory vesicle gives rise to the utricular and saccular components of the membranous labyrinth. They contain the sensory hair cells and otoliths of the macula of utricle and of the saccule, respectively, which respond to linear acceleration and the force of gravity. The utricular division of the auditory vesicle also responds to angular acceleration, as well as the endolymphatic sac and duct that connect the saccule and utricle.

Beginning in the fifth week of development, the auditory vesicle also gives rise to the cochlear duct, which contains the spiral organ of Corti and the endolymph that accumulates in the membranous labyrinth. The vestibular wall separates the cochlear duct from the perilymphatic scala vestibuli, a cavity inside the cochlea. The basilar membrane separates the cochlear duct from the scala tympani, a cavity within the cochlear labyrinth. The lateral wall of the cochlear duct is formed by the spiral ligament and the stria vascularis, which produces the endolymph. The hair cells develop from the lateral and medial ridges of the cochlear duct, which together with the tectorial membrane make up the spiral organ of Corti. ^{[3, 5, 6, 7, 8]}

Interference with or infection of the labyrinth can result in a syndrome of ailments called labyrinthitis. The symptoms of labyrinthitis include temporary nausea, disorientation, vertigo, and dizziness. Labyrinthitis can be caused by viral infections, bacterial infections, or physical blockage of the inner ear.

Labyrinthitis is an inflammation of the inner ear and a form of unilateral vestibular dysfunction. It derives its name from the labyrinths that house the vestibular system, (which sense changes in head position). Labyrinthitis can cause balance disorders. In addition to balance control problems, a patient with labyrinthitis may encounter hearing loss and tinnitus. Labyrinthitis is usually caused by a virus, but it can also arise from bacterial infection, head injury, extreme stress, or an allergy or as a reaction to a particular medication. Bacterial and viral labyrinthitis can cause permanent hearing loss, although this is rare. Labyrinthitis often follows an upper respiratory tract infection (URI). ^[4]

Symptoms

A prominent and debilitating symptom of labyrinthitis is severe vertigo. The vestibular system is a set of sensory inputs consisting of 3 semicircular canals, which sense changes in rotational motion, and the otoliths, which sense changes in linear motion. The brain combines visual cues with sensory input from the vestibular system to determine adjustments required to retain balance.

When working properly, the vestibular system also relays information on head movement to the eye muscle, forming the vestibulo-ocular reflex, in order to retain continuous visual focus during motion. When the vestibular system is affected by labyrinthitis, rapid, undesired eye motion (nystagmus) often results from the improper indication of rotational motion. Nausea, anxiety, and a general ill feeling are common due to the distorted balance signals that the brain receives from the inner ear. This can also be brought on by pressure changes such as those experienced while flying or scuba diving.