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The cochlear duct (also known as the scala media) is an endolymph-filled cavity located between the scala vestibuli (upper) and the scala tympani (lower) in the cochlea which is part of the inner ear along with the vestibular apparatus 1,4. The cochlea is located in the bony labyrinth, itself found in the temporal bone 2.
The cochlear duct is a cavity filled with endolymph and is a component of the membranous labyrinth of the ear 4. It is held in position by the lamina of the modiolus 1. The cochlear duct starts at the saccule and ends blindly at the apex of the cochlea. The cochlear duct subdivides the bony labyrinth into two perilymph chambers, namely the scala vestibuli anteriorly (opens into the vestibule) and the scala tympani posteriorly (ends at the round window) 1. It is separated from the scala vestibuli by Reissner's membrane (vestibular membrane) 5. The organ of Corti, the sensory organ for hearing, lies within the cochlear duct 2.
The cochlear duct is described as being triangular in shape and has 1,4:
- outer wall: consists of thickened periosteum, known as the spiral ligament
- roof (vestibular membrane): separates the cochlear duct from the scala vestibuli
- floor: separates the cochlear duct from the scala tympani. It also consists of the lamina modiolus and basilar membrane, which supports the organ of Corti
- proper cochlear artery (main cochlear artery), a subdivision of the common cochlear artery, itself a branch of the labyrinthine artery (also known as the auditory artery or internal auditory artery) 2,4
Adequate blood supply is crucial for auditory transduction and therefore the function of the cochlea 2.
Venous drainage of the cochlear duct occurs through the cochlear veins and vestibular veins 4. These merge and form the labyrinthine vein, which drains either into the sigmoid sinus or the inferior petrosal sinus 2,4.
The cochlear nerve is one branch of the vestibulocochlear nerve (CN VIII) which innervates the cochlear duct 1,4.
The cochlear duct is a structure of the membranous labyrinth that cannot be distinguished on high-resolution CT or MRI as it is too small but sits in the central area of the cochlea 3.