Human Anatomy: Part 2 - K. A. Dyubenko, A. K. Kolomiysev, Yu. B. Chaykovsky 2008

Special Part
Sense organs, organa sensuum
Vestibulocochlear organ - Internal ear

The internal ear, auris interna, consists of the osseous and membranous labyrinths, labyrinthus osseus et membranaceus (Fig. 226 A, B).

The osseous labyrinth is embedded in the spongy bone of the petrous part of the Temporal bone. It features very dense walls and is situated between the internal acoustic meatus medially and the tympanic cavity laterally. It comprises three parts: the vestibule (in the center), the cochlea (anteriorly), and the semicircular canals (posteriorly) (Fig. 226 A).

The vestibule, vestibulum, is an roughly oval cavity measuring 6x5x3 mm. Its lateral wall is almost entirely occupied by the Base of the stapes, which is fixed in the vestibular window. Anteriorly and inferiorly on this wall lies the communication between the vestibule and the cochlea. Its superior and posterior walls feature 5 openings through which it communicates with the semicircular canals. The medial wall is divided by a small crest, crista vestibuli, into two shallow depressions: the anterior spherical recess, recessus sphericus, and the posterior elliptical recess, recessus ellipticus.

In the region of the recessus ellipticus lies a group of small openings, the superior cribriform spot, macula cribrosa superior. The vestibular aqueduct, aquaeductus vestibuli, begins in the recessus ellipticus. In its middle portion, the recessus sphericus contains a small area with openings, the middle cribriform spot, macula cribrosa media. Below the recessus sphericus is the cochlear recess, recessus cochlearis.

The osseous semicircular canals, canales semicirculares ossei (Fig. 226 A), are three arched bony channels located in three mutually perpendicular planes. The lateral semicircular canal, canalis semicircularis lateralis, forms an angle of 30° with the horizontal plane. Therefore, during a rotational test (to examine the function of this canal), the HEAD is tilted 30° forward and downward. The posterior semicircular canal, canalis semicircularis posterior, is the longest. Perpendicular to it lies the anterior semicircular canal, canalis semicircularis anterior. Of the two ends of each semicircular canal, one is expanded into a bony ampulla, ampulla ossea, while the other lacks such an expansion and is called the bony limb, crus osseum. The bony limbs of the anterior and posterior canals fuse with each other, forming a common bony limb, crus osseum commune. It opens into the vestibule posterior and superior to the limb of the lateral canal, which is termed the simple bony limb, crus osseum simplex. The ampullae open into the vestibule via separate orifices. Thus, the three canals, despite having six ends, have five openings into the vestibule. In the area of the posterior bony ampulla, There is a group of small openings, the inferior cribriform spot, macula cribrosa inferior.

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Fig. 226.

A – Right osseous labyrinth, labyrinthus osseus (external and anterior view); B – Right membranous labyrinth, labyrinthus membranaceus (cast)

The cochlea, cochlea (Fig. 226 A), consists of a spirally coiled bony canal, canalis spiralis cochleae, which makes 2.5 turns around a central pillar, the modiolus. The turns gradually taper from the base of the cochlea, basis cochleae, to its apex, the cupola, cupula cochleae. The base of the cochlea is directed toward the Cytology/practical/108.html">Fundus of the internal acoustic meatus, while the cupola points toward the auditory tube. The modiolus contains numerous perforations for the passage of nerve fibers (peripheral processes of the spiral ganglion) and Blood Vessels.

The modiolus is spirally wound by a thin osseous spiral lamina, lamina spiralis ossea (Fig. 227 A). Its free edge projects into the cochlear canal but does not reach the opposite wall, thus forming an incomplete partition of the canal. This partition becomes complete when it is supplemented by the membranous part of the cochlea. The lumen of the cochlea above the partition is called the scala vestibuli, and below it, the scala tympani. The latter opens into the tympanic cavity via the cochlear window. Anterior to this window in the scala tympani begins the cochlear canaliculus, canaliculus cochleae, which terminates in a small opening near the posterior margin of the petrous part.

In the final half-turn, the spiral lamina detaches from the modiolus and ends in a hook-shaped plate, the hamulus. An opening, the helicotrema, is formed between the concave edge of the hamulus and the modiolus, connecting the tympanic and vestibular scalae.

The membranous labyrinth, labyrinthus membranaceus, lies inside the osseous labyrinth and largely replicates its contours (Fig. 226 B). The walls of the membranous labyrinth are formed by a thin, translucent Connective Tissue membrane. The volume of the membranous labyrinth is slightly smaller than that of the osseous labyrinth. Consequently, a slit-like perilymphatic space is situated between their walls, filled with a clear fluid, the perilymph, perilympha. In the region of the semicircular canals, the perilymphatic space is divided by fibrous septa into separate compartments, which frequently restrict the spread of purulent labyrinthine processes to a single semicircular canal. Perilymph drains into the subarachnoid space, spatium subarachnoideum, via the perilymphatic duct, ductus perilymphaticus, which passes through the cochlear canaliculus. Inside the membranous labyrinth is a clear fluid, the endolymph, endolympha. The latter drains into the endolymphatic sac, saccus endolymphaticus, via the endolymphatic duct, ductus endolymphaticus, which passes through the vestibular aqueduct. The endolymphatic sac lies on the posterior surface of the petrous part within the thickness of the dura mater.

Fig. 227. A, B. Osseous cochlea (right).

A – modiolus and osseous spiral lamina; B – osseous cochlea partially sectioned

In the membranous labyrinth, as in the osseous one, a central part, three semicircular ducts, and the cochlear duct are distinguished (Fig. 226 B). The central part corresponds to the vestibule. The utricle, utriculus, is located in the recessus ellipticus, and the saccule, sacculus, in the recessus sphericus. The utricle and saccule are connected by a short connecting duct, ductus utriculosaccularis, from which the endolymphatic duct, ductus endolymphaticus, originates.

Three semicircular ducts, ductus semicirculares, open into the utricle via five orifices, perfectly mirroring the course and position of the osseous semicircular canals. The semicircular ducts also feature membranous ampullae (anterior, posterior, lateral), ampulla membranacea (anterior, posterior, lateralis), as well as simple and common membranous limbs.

The sacculus and utriculus contain receptor Cells that form specific structures, the maculae, whereas the ampullae contain the ampullary crests, cristae ampullares. The peripheral processes of the vestibular ganglion neurocytes terminate on these cells.

The cochlear part of the membranous labyrinth is formed by the cochlear duct, ductus cochlearis, which is structured as follows: two connective tissue membranes extend from the free edge of the lamina spiralis ossea to the outer wall of the spiral cochlear canal. The lower one, which essentially prolongs the osseous spiral lamina, is called the basilar membrane, lamina basilaris, while the upper one, diverging at an angle of about 45°, is the vestibular membrane, membrana vestibularis. These membranes form the upper and lower walls of the cochlear duct, separating it from the vestibular and tympanic scalae. The third (lateral) wall of the cochlear duct is formed by the stria vascularis, which lies on the spiral ligament, ligamentum spirale. The latter represents a region of thickened periosteum of the spiral cochlear canal.

The cochlear duct begins in the vestibular region as a blind pouch, receives the reuniting duct, ductus reuniens, connecting it to the saccule, then makes 2.5 turns (following the course of the spiral canal), and ends blindly at the apex of the cochlea.

Along the entire length of the cochlear duct, upon the basilar membrane, lies the spiral organ (of Corti), organum spirale (Cortii) (Fig. 228), a multicellular plate about 3.5 cm long, up to 0.5 mm wide near the base of the cochlea, and 0.05 mm near its apex. The spiral organ comprises Two Types of cells: sensory (Hair) cells and supporting cells. The peripheral processes of the cochlear ganglion neurocytes terminate on the sensory cells.

The primary Blood supply to the internal ear is provided by the labyrinthine artery, arteria labyrinthi (a branch of the Basilar artery). It enters the internal acoustic meatus and divides at its fundus into vestibular and cochlear branches.

The labyrinthine Veins, venae labyrinthi, drain into the sigmoid or inferior petrosal sinus. The vein of the vestibular aqueduct, vena aquaeductus vestibuli, carries blood to the transverse or superior petrosal sinus, and the vein of the cochlear canaliculus, vena canalis cochleae, drains directly into the Internal jugular vein, vena jugularis interna.

The Auditory Analyzer

The perception of auditory stimuli is ensured by the normal functioning of the entire Organ of Hearing. The latter is conventionally divided into sound-conducting and sound-receiving parts. The former includes the external and Middle ear, as well as the fluid media of the internal ear (perilymph and endolymph). The second part is represented by the spiral organ, auditory pathways, and centers.

As already noted, the auricle and external acoustic meatus ensure the conduction of sound vibrations to the tympanic membrane. The vibration of the latter causes oscillations of the auditory ossicle chain. The movement of the stapes within the vestibular window prompts the displacement of perilymph in the scala vestibuli and scala tympani. The middle ear Muscles facilitate the transmission of vibrations into the labyrinth. Upon contraction of the tensor tympani Muscle, the eardrum is drawn inward into the tympanic cavity and becomes tensed, the auditory ossicles shift medially toward the vestibular window, and high-frequency sound perception is enhanced. Contraction of the stapedius muscle moves the auditory ossicles in the opposite direction and relaxes the tympanic membrane, thereby improving the perception of low-frequency sounds. In the Inner ear, perilymph vibrations are transmitted to the endolymph. This induces specific physicochemical Changes in the receptor Cells of the spiral organ, exciting them. The transmission of excitation from the receptor cells to the auditory nerve fibers occurs chemically. The resulting neural impulse is then relayed to the auditory centers via the pathway described below.

Fig. 228. Cross-section through the cochlear duct