What is the internal ear?
The internal ear is the innermost compartment of the ear and is contained within the petrous temporal bone of the skull. The internal ear contains the vestibulocochlear organs that are responsible for the reception of sound and maintenance of balance.
The internal ear consists of the sacs and ducts of the endolymph fluid-filled membranous labyrinth (fig. 1).
The membranous labyrinth is suspended by delicate filaments in a perilymph-filled bony labyrinth.
These fluids are involved in stimulating the end organs of hearing and balance.
(Gray, 1995; Moore et al., 2009)
The bony labyrinth
The bony labyrinth is a series of cavities contained within the otic capsule of the petrous part of the temporal bone. It has 3 main regions:
- vestibule of the bony labyrinth
- semicircular canals
Figure 2 depicts the otic capsule. The otic capsule is made of bone denser than petrous temporal bone. It is often wrongly illustrated and identified as the bony labyrinth.
The bony labyrinth is in fact the FLUID-FILLED SPACE that is surrounded by the otic capsule.
The cochlea is the shell shaped component of the bony labyrinth and contains the organs concerned with hearing.
It contains the cochlear duct and features a round window (fig. 2) that is closed off by a secondary tympanic membrane.
The vestibule is a small oval chamber that contains the utricle and the saccule (fig. 3) and other parts of the balancing apparatus (otherwise known as the vestibular labyrinth).
It features an oval window on the lateral wall.
The vestibule is continuous with the cochlea anteriorly, semi-circular canals posteriorly and the posterior cranial fossa by the vestibular aqueduct which transmits the endolymphatic duct and expands into the endolymphatic sac (fig. 3).
There are 3 semicircular canals that lie at right angles to one another: anterior (superior), posterior and lateral (horizontal) (fig. 3). Within the semicircular canal is a semicircular duct.
The semicircular canals open into the vestibule of the bony labyrinth at the ampullae (which are just expansions of the bone or duct).
(Gray, 1995; Moore et al., 2009)
The membranous labyrinth
The membranous labyrinth consists of a series of communicating sacs and ducts suspended in the bony labyrinth.
It has 2 divisions:
- vestibular labyrinth
- cochlear labyrinth
This course will only consider the vestibular labyrinth as this is the component of the inner ear responsible for maintaining balance.
The vestibular labyrinth, otherwise known as the vestibular apparatus is composed of the:
- 3 semicircular ducts
The utricle communicates with the saccule through the utricosaccular duct, from which the endolymphatic duct arises (fig. 4).
The saccule is continuous with the cochlear duct through the ductus reuniens (fig. 4).
The semicircular ducts open into the utricle through five openings that are reflective of the way the surrounding semicircular canals open into the vestibule.
In the walls of the membranous labyrinth, five areas of sensory epithelium upon which terminal fibres of the vestibular division of the vestibulocochlear nerve (cranial nerve VIII) exist (fig. 5).
- The macula of the utricle is in the floor of the utricle.
- The macula of the saccule is vertically placed on the medial wall of the saccule.
- Cristae ( within the ampullae walls near the utricular openings of each semicircular duct)
- Each semicircular duct has an ampullary crest - the hair cells within these, like the maculae, stimulate primary sensory neurones whose cell bodies are in the vestibular ganglion.
Hover your mouse over the information icons ("i") in figure 5 for more detail!
Test your understanding! Fill in the blanks.
The , and make up the vestibular apparatus.
The is on the lateral wall of the vestibule of the bony labyrinth.
The is the fluid filled space that is immediately surrounded by the otic capsule.
There are two each located within the saccule and utricle.
There are three areas of specialised epithelium within the semicircular ducts named .
The saccule and the cochlear duct are continuous by the .
Test your reasoning! True or false?
- The membranous semicircular ducts contain perilymph.
- The cochlea contains the organs for the balance function.
- The membranous labyrinth is contained within the bony labyrinth.
- The ampullae are expansions of the semicircular canals that open to the saccule.
- The bony labyrinth consists of the cochlea, semicircular canals and otic capsule.
Test your knowledge! Label the diagram of the right vestibular system:
- Cochlear duct
- Oval window
- Round window
- Endolymphatic sac
- Endolymphatic duct
- Ampulla of semicircular duct
- Ampulla of semicircular canal