General characteristics
The membranous labyrinth (labyrinthus membranaceus) is a closed system located within the bony labyrinth. The main feature of the membranous labyrinth is that it topographically mirrors the curves of the bony one, yet is considerably smaller.
The entire internal cavity of this structure is filled with a fluid called endolymph.
Anatomically, the membranous labyrinth is conventionally divided into several key regions, each serving a specific functional role:
- utricle (utriculus);
- saccule (sacculus);
- membranous semicircular ducts (ductus semicirculares);
- cochlear duct (ductus cochlearis).
Vestibule: utricle and saccule
Two important expansions of the membranous labyrinth responsible for vestibular functions are located in the region of the vestibule.
Utricle (utriculus) lies in the elliptic recess (recessus ellipticus). Saccule (sacculus) is located in the spherical recess (recessus sphericus).
Special receptor areas called maculae are located on the inner surface of these structures:
- macula utriculi (macula of the utricle);
- macula sacculi (macula of the saccule).
These areas contain crystals called statoconia (statoconia). Their main function is the perception of linear acceleration, as well as registering changes in head position in space.
Both structures (utricle and saccule) are not isolated: they communicate with each other via the ductus utriculosaccularis. This duct further continues into the endolymphatic duct (ductus endolymphaticus), which in turn ends blindly in the endolymphatic sac (saccus endolymphaticus).
Membranous semicircular ducts
The membranous semicircular ducts (ductus semicirculares) lie inside the corresponding bony canals.
Each of the three ducts has a characteristic expansion at one of its ends — the membranous ampulla (ampulla membranacea). Inside the ampullae are receptor structures named ampullary crests (crista ampullaris). Unlike the maculae of the utricle and saccule, the receptors of the ampullary crests specialize in the perception of angular acceleration (rotational movements).
Cochlear duct and spiral organ
The cochlear duct (ductus cochlearis) is located inside the spiral canal of the cochlea (canalis spiralis cochleae). In cross-section, this duct has a characteristic triangular shape.
The walls of the cochlear duct are formed by the following structures:
- Basilar membrane (membrana basilaris) — the lower wall.
- Vestibular membrane or Reissner's membrane (membrana vestibularis) — the upper wall.
- Outer wall — represented by the stria vascularis (stria vascularis). It is the stria vascularis that is responsible for the secretion (production) of the endolymph filling the membranous labyrinth.
The most important structure of the auditory analyzer — the spiral organ (organum spirale), also known as the organ of Corti — is located on the basilar membrane. It serves as the peripheral receptor apparatus for hearing.
The spiral organ includes:
- inner and outer hair cells (cellulae pilosae internae et externae), which act as sensors;
- supporting cells;
- tectorial membrane (membrana tectoria), which overhangs the hair cells.
Briefly: answers to topical questions
What space is located between the bony and membranous labyrinths and what fluid is it filled with?
Between the walls of the bony and membranous labyrinths is the perilymphatic space (spatium perilymphaticum), which is filled with perilymph.
Where is the endolymphatic sac located anatomically?
The endolymphatic sac (saccus endolymphaticus) is located within the thickness of the dura mater on the posterior surface of the petrous part of the temporal bone.
How are the macula of the utricle and the macula of the saccule oriented in space relative to each other?
The macula of the elliptic saccule (utricle) is located at the base of the utricle parallel to the base of the skull, while the macula of the spherical saccule is located vertically on the medial wall of the saccule.
Through which anatomical duct does the cochlear duct communicate with the saccule?
The cochlear duct communicates with the spherical saccule (sacculus) via the ductus reuniens (ductus reuniens).
To remember the location: the saccule (Sacculus) lies in the spherical pocket (Sphericus) — both start with the letter "S".
Class and exam questions
Where is endolymph produced?
Endolymph is produced in the stria vascularis, which forms the outer wall of the cochlear duct.
What are the functions of the utricle and saccule?
Their receptor zones contain statoconia, which ensure the perception of linear acceleration and head position relative to the gravity vector.
What is the function of the ampullary crests?
The ampullary crests, located in the membranous semicircular ducts, specialize in the perception of angular (rotational) acceleration.
What constitutes the peripheral receptor apparatus for hearing?
This is the spiral (Corti's) organ, located on the basilar membrane of the cochlear duct. It contains hair cells that transform sound vibrations into a nerve impulse.
What else to study on this topic
- Cellular structure of the spiral (Corti's) organ
- Mechanism of endolymph circulation and the role of the saccus endolymphaticus
- Histology of hair and supporting cells
- Connection of membranous structures with the bony labyrinth
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