Human Anatomy (with the Fundamentals of Dynamic and Sports Morphology) - Ivanitsky M. F. 2008

The Doctrine of Sense Organs
Vestibulocochlear Organ

The Vestibulocochlear Organ is divided into three parts: the outer, middle, and Inner ear (Fig. 137).

The outer ear consists of the auricle (pinna) and the external acoustic meatus.

The auricle has a cartilaginous framework composed of elastic Cartilage, except for its lower part, known as the lobule (earlobe), where the cartilage is replaced by adipose tissue. The margin of the auricle is curved, forming the helix. Anterior to it lies the antihelix. At the beginning of the external acoustic meatus, anterior to the external acoustic opening, is a projection called the tragus, opposite which, i.e., posterior to the opening, is the antitragus. In its central part, the auricle features a depression known as the concha of the auricle.

The external acoustic meatus consists of cartilaginous and osseous parts. A distinctive feature of the Skin lining the external acoustic meatus is the presence of ceruminous glands (which secrete earwax) and Sebaceous Glands.

The boundary between the outer and middle ears is the tympanic membrane (eardrum), which is situated obliquely so that its upper margin faces laterally and its lower margin faces medially (see Fig. 137). The tympanic membrane has a connective-tissue Structure; it is retracted in the center, strong, and thin. On its inner side, it is covered by a thin layer of mucous membrane, while its outer surface is continuous with the epithelial lining of the external acoustic meatus.

The Middle ear consists of the tympanic cavity, the auditory tube (Eustachian tube), and the mastoid air Cells.

The tympanic cavity has six walls: medial, anterior, posterior, superior, inferior, and lateral. The medial wall is formed by the petrous part of the Temporal bone, is in contact with the bony labyrinth, and is termed the labyrinthine wall. Approximately in the center of this wall is a prominence called the promontory, above which lies an oval opening, the fenestra vestibuli (oval window), closed by the Base of the stapes—one of the auditory ossicles. Below the promontory is a round opening, the fenestra cochleae (round window). The anterior wall of the tympanic cavity, known as the carotid wall, contains the opening of the auditory tube, which connects the tympanic cavity to the Pharynx. On the posterior wall of the tympanic cavity, referred to as the mastoid wall, is the entrance to the air-filled mastoid Cells of the temporal bone, the largest of which, located anteriorly, is called the mastoid antrum. The superior wall, or tegmental wall, separates the tympanic cavity from the cranial cavity. The inferior wall is termed the jugular wall and separates the tympanic cavity from the Internal jugular vein. The lateral wall of the cavity is formed by the tympanic membrane and the surrounding bony structures. This wall is called the membranous wall.

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Fig. 137. Diagram of The structure of the vestibulocochlear organ in a frontal section

1 — superior semicircular canal; 2 — vestibule; 3 — cochlea; 4 — vestibulocochlear n.; 5 — internal carotid a.; 6 — auditory tube; 7 — tympanic cavity; 8 — tympanic membrane; 9 — external acoustic meatus; 10 — external acoustic opening; 11 — auricle; 12 — malleus

The central part of the tympanic membrane is noticeably retracted into the tympanic cavity, where it is fixed to the tip of the handle of the malleus. This retraction occurs because the tensor tympani Muscle, located in the musculotubal canal, attaches to the malleus and pulls it inward.

There are three auditory ossicles located in the tympanic cavity: the malleus, incus, and stapes (see Fig. 137). They are connected by joints, which makes these bones movable relative to one another. Within the tympanic cavity is the stapedius muscle, which helps hold the stapes against the oval window.

The inner ear is situated within the petrous part of the temporal bone and represents the most functionally vital and complexly structured part of the vestibulocochlear organ. The inner ear is formed by the so-called labyrinth, which comprises the bony labyrinth and the membranous labyrinth contained within it.

The bony labyrinth has rigid walls composed of compact Bone tissue. It consists of the central vestibule, three semicircular canals located posterior to the vestibule, and the cochlea, which lies anterior to the vestibule.

In the region of the vestibule, the bony labyrinth forms two depressions: the elliptical recess and the spherical recess. The elliptical recess communicates via five openings with the three bony semicircular canals, while the spherical recess communicates with the cochlear canal. Additionally, the vestibule features the oval window and the round window on its lateral wall, which open toward the tympanic cavity.

There are three bony semicircular canals: anterior, posterior, and lateral. They lie in three mutually perpendicular planes. The anterior canal is oriented at a right angle to the axis of the temporal bone pyramid, the posterior canal lies parallel to its posterior surface, and the lateral canal projects its convexity laterally and slightly into the tympanic cavity. Each canal has two limbs (one of which features a bony dilation called the ampulla) that open into the vestibule. However, there are only five openings for these limbs rather than six, because the anterior and posterior canals join together and open into the vestibule via a common limb.

The cochlea is a coiled bony tube with two and a half turns. It has a base, which faces toward the internal acoustic meatus, and an apex (cupola), directed toward the tympanic cavity. The spiral canal of the cochlea surrounds its central axis, which is composed of spongy bone tissue and is called the modiolus. The osseous spiral lamina, winding around the modiolus along the cochlear canal, projects its free edge into the lumen of the canal, forming an incomplete partition.

Fig. 138. Diagram of the STRUCTURE OF THE bony and membranous labyrinths:

1 - endolymphatic sac; 2 - dura mater of the Brain; 3 - bone; 4 - apex of the cochlear duct; 5 - connection between the scala vestibuli and scala tympani; 6 - cochlear duct; 7 - scala vestibuli; 8 - scala tympani; 9 - saccule; 10 - uniting duct; 11 - perilymphatic space of the vestibule; 12 - round window; 13 - tympanic cavity; 14 - cecal pouch of the cochlear duct; 15 - oval window; 16 - posterior membranous ampulla; 17 - posterior semicircular canal (perilymphatic space); 18 - posterior semicircular branch; 19 - utricle; 20 - anterior semicircular canal (perilymphatic space); 21 - anterior semicircular duct; 22 - duct of the utricle and saccule; 23 - anterior membranous ampulla; 24 - endolymphatic duct (after Spalteholz)

The membranous labyrinth (Fig. 138) somewhat mirrors the shape of the bony labyrinth, but its cavity is significantly smaller. A space remains between the two labyrinths, filled with a clear fluid known as perilymph. The walls of the membranous labyrinth consist of fibrous tissue and are lined internally by a simple squamous epithelium. The membranous labyrinth is a complex system of communicating channels and cavities filled with a clear fluid called endolymph. Both perilymph and endolymph can drain from the inner ear toward the cranial cavity via specialized narrow channels: perilymph drains into the subarachnoid space, whereas endolymph drains into the endolymphatic sac located within the dura mater.

The central part of the membranous labyrinth, situated within the Vestibule of the bony labyrinth, features two dilations: the utricle and the saccule.

The utricle connects with three membranous semicircular ducts located within the bony semicircular canals. The anterior and posterior ducts join the utricle via a common limb. Each duct has one limb with a dilation known as the ampulla.

The saccule is connected to the cochlear duct.

The semicircular ducts, utricle, and saccule serve for the perception of static stimuli arising during both linear and rotational body movements, as well as for transmitting impulses during changes in THE POSITION OF not only the HEAD but the entire body, thereby participating—along with other analyzers—in the orienting Reactions of the Organism to its environment.

In the region of the ampullae of the semicircular ducts, there are specialized crests, while the utricle and saccule contain maculae covered with statoconia (mineral dust particles). Endolymph shifts caused by various body movements are detected by the receptor cells located within these crests and maculae. The central processes of these cells form the vestibular component of the Vestibulocochlear nerve (cranial nerve VIII), which exits the pyramid of the temporal bone via the internal acoustic meatus, enters the Medulla Oblongata, and reaches the corresponding vestibular nuclei. From the medullary nuclei, information is relayed to the Cerebellum, Spinal Cord, reticular formation, Midbrain nuclei, and thalamus, and subsequently to the cortical center of the vestibular analyzer—the cortex of the middle temporal gyrus—where it is processed. Through the connections between the Cerebral Cortex AND other brain regions, involuntary muscle contractions can occur to help restore and maintain balance.

The membranous labyrinth of the cochlea is represented by the cochlear duct (Fig. 13 9), which runs the entire length of the cochlea within the bony canal, making two and a half turns just like the canal itself. The cochlear duct originates from the spherical saccule in the vestibule via the uniting duct and terminates at the cochlear apex. In a cross-section taken through the axis of the cochlea, the cochlear duct appears triangular in shape. It is situated between two so-called scalae (walls): the upper one—the scala vestibuli—and the lower one—the scala tympani, both of which are filled with perilymph. The cochlear duct itself is filled with endolymph.

The wall of the cochlear duct bordering the scala tympani is formed by the basilar membrane, which consists of numerous fibrous strands extending between the osseous spiral lamina and the opposing wall of the bony cochlear canal.

Fig. 139. Spiral organ:

1 — cochlear duct; 2 — scala vestibuli; 3 — scala tympani; 4 — osseous spiral lamina; 5 — spiral ganglion of cochlea; 6 — spiral limbus; 7 — dendrites of Nerve Cells; 8 — vestibular wall of the cochlear duct; 9 — basilar membrane; 10 — spiral ligament; 11 — epithelium lining the scala tympani; 12 — stria vascularis; 13 — Blood vessel; 14 — tectorial membrane; 15 — outer Hair cells; 16 — inner hair cells; 17 — inner supporting cells; 18 — outer supporting cells; 19 — pillar cells (rods); 20 — tunnel (after M. S. Gracheva)

These fibers vary in length. It is generally accepted that humans possess over 24,000 of them. The fibers resemble strings that vibrate in specific Regions of the basilar membrane in response to fluctuations in the perilymph and endolymph. Situated on this membrane is the spiral organ, which contains auditory receptors in the form of hair cells whose peripheral processes end freely in the endolymph of the cochlear duct. Positioned above the receptors is another membrane that glides freely across the hair-like projections of the cells.

Sound perception occurs as follows. Sound waves are transmitted through the outer ear to the tympanic membrane, then via the auditory ossicles—the malleus, incus, and stapes—and further through the oval window to the perilymph of the scala vestibuli, which communicates with the scala tympani at the apex of the cochlea. Through the scala tympani, the vibrations travel to the round window, which is sealed by a thin membrane known as the secondary tympanic membrane. From the perilymph, the vibrations are transferred to the endolymph, causing specific segments of the spiral basilar membrane to vibrate. The central processes of the hair cells form the cochlear component of the vestibulocochlear nerve (cranial nerve VIII), emerge through the internal acoustic meatus, and project to the cochlear nuclei in the Rhomboid fossa. From the medulla oblongata, passing through a series of subcortical Relay centers—such as the medial geniculate body and the inferior colliculi of the midbrain tectum—the impulses reach the thalamus and subsequently the cortex of the middle portion of the superior temporal gyrus, where auditory sensations are ultimately generated. A significant network of connections between the auditory pathways and other areas of the Central Nervous system enables automatic motor responses to auditory stimuli.