The ear can be divided into three parts- the outer ear, middle ear and inner ear. The inner ear is the innermost part of the ear, and houses the vestibulocochlear organs. It has two main functions- to convert mechanical signals from the middle ear into electrical signals, which can then be transferred to the auditory pathway in the brain, and to maintain balance by detecting position and motion. In this article, we shall look at the anatomy of the inner ear – its position, structure, neurovasculature, innervation, and function.
The inner ear is located within the petrous part of the temporal bone. It lies between the middle ear and the internal acoustic meatus, which are located laterally and medially respectively. The inner ear has two main components – the bony labyrinth and membranous labyrinth.
The inner ear has two openings into the middle ear, both of which are covered by membranes. The oval window is located between the middle ear and the vestibule, whilst the round window separates the middle ear from the scala tympani (which is part of the cochlear duct).
The bony labyrinth is a series of bony cavities within the petrous part of the temporal bone. It is composed of three parts – the cochlea, vestibule and the three semi-circular canals.
The vestibule is the central part of the bony labyrinth. It is separated from the middle ear by the oval window, and communicates anteriorly with the cochlea and posteriorly with the semi-circular canals. Two parts of the membranous labyrinth; the saccule and utricle, are both located within the vestibule.
The cochlea houses the cochlear duct of the membranous labyrinth - the auditory part of the inner ear. It twists upon itself around a central portion of bone called the modiolus, producing a cone-shape which points in an anterolateral direction. Branches from the cochlear portion of the vestibulocochlear (VIII) nerve are found at the base of the modiolus. Extending outwards from the modiolus is a ledge of bone known as the spiral lamina, which attaches to the cochlear duct, holding it in place. The presence of the cochlear duct creates two perilymph-filled chambers above and below it - the scala vestibuli and scala tympani respectively.
The inner ear contains three semi-circular canals - the anterior, lateral and posterior. They each contain a semi-circular duct, which is responsible for balance (along with the utricle and saccule). The canals are situated superoposteriorly to the vestibule, at right angles to each other. Each canal has a swelling at one end, known as the ampulla.
The membranous labyrinth is a continuous system of ducts filled with endolymph. It lies within the bony labyrinth, surrounded by perilymph. It is composed of the cochlear duct, three semi-circular ducts, saccule and the utricle. The cochlear duct is situated within the bony scaffolding of the cochlea and is held in place by the spiral lamina. The presence of the duct creates two canals above and below it – the scala vestibuli and scala tympani respectively.
The cochlear duct can be described as having a triangular shape - the lateral wall is formed by a thickened periosteum, known as the spiral ligament.
The inner ear is located within the petrous part of the temporal bone and lies between the middle ear and the internal acoustic meatus. It is composed of two main components – the bony labyrinth and the membranous labyrinth. The bony labyrinth consists of three parts – the cochlea, vestibule and three semi-circular canals. The vestibule is the central part of the bony labyrinth and is separated from the middle ear by the oval window. It communicates anteriorly with the cochlea and posteriorly with the semi-circular canals. The cochlea is the part of the inner ear which houses the cochlea duct of the membranous labyrinth and is the auditory part of the inner ear. It is cone-shaped and twists upon itself around a central portion of bone called the modiolus, which points in an anterolateral direction. Branches from the cochlear portion of the vestibulocochlear (VIII) nerve can be found at the base of the modiolus.
The membranous labyrinth is a series of ducts suspended within the bony labyrinth. It consists of two sacs – the saccule and the utricle, and the three semi-circular ducts, along with the cochlear duct. The saccule and utricle are organs of balance which detect movement or acceleration of the head in the vertical and horizontal planes, respectively. The saccule is globular in shape and receives the cochlear duct, whereas the utricle is the larger of the two and receives the three semi-circular ducts.
The membranous labyrinth is separated from the bony labyrinth by two thin membranes – the Reissner’s membrane and the basilar membrane. The Reissner’s membrane forms the roof of the cochlear duct and separates it from the scala vestibuli. The basilar membrane forms the floor of the cochlear duct and separates it from the scala tympani. It also houses the epithelial cells of hearing, known as the Organ of Corti. Endolymph drains from the saccule and utricle into the endolymphatic duct, which travels through the vestibular aqueduct to a sac where endolymph can be secreted and absorbed.
The semi-circular ducts are located within the semi-circular canals and share their orientation. Upon movement of the head, the flow of endolymph within the ducts changes speed and/or direction and sensory receptors in the ampullae of the semi-circular canals detect this change, sending signals to the brain, allowing for the processing of balance.
The bony labyrinth and membranous labyrinth have different arterial supplies. The bony labyrinth receives its blood supply from three sources – the anterior tympanic branch of the maxillary artery, the petrosal branch of the middle meningeal artery and the stylomastoid branch of the posterior auricular artery. The membranous labyrinth is supplied by the labyrinthine artery, a branch of the inferior cerebellar artery (or, occasionally, the basilar artery). It divides into three branches – the cochlear branch, which supplies the cochlear duct, and the two vestibular branches, which supply the vestibular apparatus. Venous drainage of the inner ear is through the labyrinthine vein, which empties into the sigmoid sinus or inferior petrosal sinus.
The inner ear is innervated by the vestibulocochlear nerve (CN VIII) which enters the inner ear via the internal acoustic meatus, where it divides into the vestibular nerve and the cochlear nerve. The vestibular nerve enlarges to form the vestibular ganglion and then splits into superior and inferior parts to supply the utricle, saccule and three semi-circular ducts. The cochlear nerve enters at the base of the modiolus and its branches pass through the lamina to supply the receptors of the Organ of Corti. The facial nerve (CN VII) also passes through the inner ear but does not innervate any of the structures present.
Meniere’s disease is a disorder of the inner ear which is characterized by episodes of vertigo, low-pitched tinnitus, and hearing loss. Symptoms are thought to be caused by an excess accumulation of endolymph within the membranous labyrinth, causing progressive distension of the ducts and resulting pressure fluctuations which can damage the thin membranes of the ear that detect balance and sound.
Extending outwards from the modiolus is a ledge of bone known as spiral lamina, which attaches to the cochlear duct, holding it in position. This creates two chambers filled with perilymph – Scala vestibuli located superiorly to the cochlear duct, and Scala tympani which terminates at the round window inferiorly. Both of these canals are continuous with the vestibule.
The membranous labyrinth is a continuous system of ducts surrounded by perilymph and filled with endolymph. It is composed of the cochlear duct, three semi-circular ducts, saccule, and the utricle. The cochlear duct has a triangular shape, with a lateral wall formed by the thickened periosteum, or spiral ligament, a roof formed by the Reissner's membrane which separates the cochlear duct from the scala vestibuli, and a floor formed by the basilar membrane which separates it from the scala tympani. This basilar membrane houses the epithelial cells of hearing, known as the Organ of Corti.
The saccule and utricle are two membranous sacs located in the vestibule. They are organs of balance which detect movement or acceleration of the head in the vertical and horizontal planes, respectively. The utricle is the larger of the two, while the saccule is globular in shape and receives the cochlear duct. Endolymph drains from the saccule and utricle into the endolymphatic duct, which travels through the vestibular aqueduct to the posterior aspect of the petrous part of the temporal bone. Here, the duct expands to a sac where endolymph can be secreted and absorbed.
The semi-circular ducts are located within the semi-circular canals, and share their orientation. Upon movement of the head, the flow of endolymph within the ducts changes speed and/or direction. Sensory receptors in the ampullae of the semi-circular canals detect this change, and send signals to the brain, allowing for the processing of balance.
The bony labyrinth and membranous labyrinth have different arterial supplies. The bony labyrinth receives its blood supply from three arteries, which also supply the surrounding temporal bone – the anterior tympanic branch from the maxillary artery, the petrosal branch from the middle meningeal artery, and the stylomastoid branch from the posterior auricular artery. The membranous labyrinth is supplied by the labyrinthine artery, a branch of the inferior cerebellar artery (or, occasionally, the basilar artery). This artery divides into three branches – a cochlear branch which supplies the cochlear duct, and two vestibular branches which supply the vestibular apparatus.
Venous drainage of the inner ear is through the labyrinthine vein, which empties into the sigmoid sinus or inferior petrosal sinus.
The inner ear is innervated by the vestibulocochlear nerve (CN VIII). It enters the inner ear via the internal acoustic meatus, where it divides into the vestibular nerve (responsible for balance) and the cochlear nerve (responsible for hearing). The vestibular nerve enlarges to form the vestibular ganglion, which is then split into superior and inferior parts to supply the utricle, saccule, and three semi-circular ducts. The cochlear nerve enters at the base of the modiolus and its branches pass through the lamina to supply the receptors of the Organ of Corti. The facial nerve (CN VII) also passes through the inner ear, but does not innervate any of the structures present.
Meniere’s disease is a disorder of the inner ear, characterised by episodes of vertigo, low-pitched tinnitus, and hearing loss. The symptoms are thought to be caused by an excess accumulation of endolymph within the membranous labyrinth, causing progressive distension of the ducts. The resulting pressure fluctuations damage the thin membranes of the ear that detect balance and sound.
