The muscles of the eye

Six striated muscles are attached to the eyeball: four straight muscles - superior, inferior, lateral and medial, and two oblique muscles - superior and inferior. All the straight muscles and the superior oblique begin in the depth of the orbit on a common tendinous ring (anulus tendineus communis), fixed to the sphenoid bone and periosteum around the optic canal and partially on the edges of the superior orbital fissure. This ring surrounds the optic nerve and ophthalmic artery. The muscle that raises the upper eyelid (m. levator palpebrae superioris) also begins from the common tendinous ring. It is located in the orbit above the superior rectus muscle of the eyeball and ends in the thickness of the upper eyelid. The rectus muscles run along the corresponding walls of the orbit, on the sides of the optic nerve, pierce the vagina of the eyeball (vagina bulbi) and with short tendons are woven into the sclera in front of the equator, 5-8 mm away from the edge of the cornea. The rectus muscles rotate the eyeball around two mutually perpendicular axes: vertical and horizontal (transverse).

The lateral and medial rectus muscles (mm. recti lateralis et medialis) rotate the eyeball outward and inward around the vertical axis, each in its own direction, and the pupil rotates accordingly. The superior and inferior rectus muscles (mm. recti superior et inferior) rotate the eyeball up and down around the transverse axis. When the superior rectus muscle contracts, the pupil is directed upward and slightly outward, and when the inferior rectus muscle works, it is directed downward and inward. The superior oblique muscle (m. obliquus superior) lies in the superomedial part of the orbit between the superior and medial rectus muscles. Near the trochlear fossa, it passes into a thin round tendon enveloped in a synovial sheath, which is thrown over the trochlea, constructed in the form of a ring of fibrous cartilage. After passing through the trochlea, the tendon lies under the superior rectus muscle and is attached to the eyeball in its superolateral part, behind the equator. The inferior oblique muscle (m. obliquus inferior), unlike the other muscles of the eyeball, originates on the orbital surface of the maxilla, near the opening of the nasolacrimal canal, on the lower wall of the orbit. The muscle is directed between the lower wall of the orbit and the inferior rectus muscle obliquely upward and backward. Its short tendon is attached to the eyeball from its lateral side, behind the equator. Both oblique muscles rotate the eyeball around the anteroposterior axis: the superior oblique muscle rotates the eyeball and pupil downward and laterally, the inferior one - upward and laterally. The movements of the right and left eyeballs are coordinated due to the combined action of the extraocular muscles.

The oculomotor apparatus is a complex sensorimotor mechanism, the physiological significance of which is determined by its two main functions: motor and sensory.

The motor function of the oculomotor apparatus ensures the guidance of both eyes, their visual axes and the central pits of the retina to the object of fixation, the sensory function ensures the fusion of two monocular (right and left) images into a single visual image.

The innervation of the oculomotor muscles by the cranial nerves determines the close connection between neurological and ocular pathology, which is why a comprehensive approach to diagnosis is necessary.

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Anatomical and physiological features of the eye muscles

The movements of the eyeball are carried out with the help of six oculomotor muscles: four straight muscles - external and internal (m. rectus externum, m. rectus internum), upper and lower (m. rectus superior, m. rectus inferior) and two oblique muscles - upper and lower (m. obliguus superior, m. obliguus inferior).

All the rectus muscles and the superior oblique muscle of the eye originate at the tendinous ring located around the optic canal at the apex of the orbit and fused with its periosteum. The rectus muscles are directed forward in the form of bands parallel to the corresponding walls of the orbit, forming the so-called muscular funnel. At the equator of the eye, they pierce Tenon's capsule (the sheath of the eyeball) and, before reaching the limbus, are woven into the superficial layers of the sclera. Tenon's capsule supplies the muscles with a fascial covering, which is absent in the proximal part at the place where the muscles begin.

The superior oblique muscle of the eye originates at the tendinous ring between the superior and medial rectus muscles and goes forward to the cartilaginous trochlea, located in the upper-inner angle of the orbit at its edge. At the trochlea, the muscle turns into a tendon and, passing through the trochlea, turns back and outward. Situated under the superior rectus muscle, it is attached to the sclera outside the vertical meridian of the eye. Two-thirds of the entire length of the superior oblique muscle are between the apex of the orbit and the trochlea, and one-third is between the trochlea and the point of attachment to the eyeball. This part of the superior oblique muscle determines the direction of movement of the eyeball during its contraction.

Unlike the five muscles mentioned, the inferior oblique muscle of the eye begins at the lower inner edge of the orbit (in the area of the entrance of the nasolacrimal canal), goes back and forth between the wall of the orbit and the inferior rectus muscle towards the external rectus muscle and is fan-shapedly attached under it to the sclera in the posterolateral part of the eyeball, at the level of the horizontal meridian of the eye.

From the fascial membrane of the extraocular muscles and Tenon's capsule, numerous strands extend to the walls of the orbit.

The fascial-muscular apparatus ensures a fixed position of the eyeball and gives smoothness to its movements.

The innervation of the eye muscles is carried out by three cranial nerves:

• oculomotor nerve - n. oculomotorius (III pair) - innervates the internal, superior and inferior rectus muscles, as well as the inferior oblique;
• trochlear nerve - n. trochlearis (IV pair) - superior oblique muscle;
• abducens nerve - n. abducens (VI pair) - external rectus muscle.

All these nerves pass into the orbit through the superior orbital fissure.

The oculomotor nerve, after entering the orbit, divides into two branches. The upper branch innervates the superior rectus muscle and the muscle that raises the upper eyelid, the lower branch innervates the internal and inferior rectus muscles, as well as the inferior oblique.

The nucleus of the oculomotor nerve and the nucleus of the trochlear nerve (provides the work of the oblique muscles) located behind and next to it are located at the bottom of the Sylvian aqueduct (the cerebral aqueduct). The nucleus of the abducens nerve (provides the work of the external rectus muscle) is located in the pons under the bottom of the rhomboid fossa.

The rectus oculomotor muscles of the eye are attached to the sclera at a distance of 5-7 mm from the limbus, the oblique muscles - at a distance of 16-19 mm.

The width of the tendons at the point of muscle attachment varies from 6-7 to 8-10 mm. Of the rectus muscles, the widest tendon is that of the internal rectus muscle, which plays a major role in the function of bringing the visual axes together (convergence).

The line of attachment of the tendons of the internal and external eye muscles, i.e. their muscle plane, coincides with the plane of the horizontal meridian of the eye and is concentric with the limbus. This determines the horizontal movements of the eyes, their adduction, rotation toward the nose - adduction with contraction of the internal rectus muscle and abduction, rotation toward the temple - abduction with contraction of the external rectus muscle. Thus, these muscles are antagonists in the nature of their action.

The superior and inferior rectus and oblique muscles of the eye perform mainly vertical movements of the eye. The line of attachment of the superior and inferior rectus muscles is located somewhat obliquely, their temporal end is located further from the limbus than the nasal. As a result, the muscular plane of these muscles does not coincide with the plane of the vertical meridian of the eye and forms an angle with it equal to an average of 20° and open to the temple.

This attachment ensures that the eyeball rotates when these muscles act not only upward (when the superior rectus muscle contracts) or downward (when the inferior rectus contracts), but also simultaneously inward, i.e. adduction.

The oblique muscles form an angle of about 60° with the plane of the vertical meridian, open to the nose. This determines the complex mechanism of their action: the superior oblique muscle lowers the eye and produces its abduction, the inferior oblique muscle is a lifter and also an abductor.

In addition to horizontal and vertical movements, the four vertical eye muscles mentioned above perform torsional movements of the eyes clockwise or counterclockwise. In this case, the upper end of the vertical meridian of the eye deviates toward the nose (intorsion) or toward the temple (extorsion).

Thus, the oculomotor muscles of the eye provide the following eye movements:

• adduction, i.e. its movement towards the nose; this function is performed by the internal rectus muscle, additionally by the superior and inferior rectus muscles; they are called adductors;
• abduction, i.e. movement of the eye towards the temple; this function is performed by the external rectus muscle, and additionally by the superior and inferior oblique muscles; they are called abductors;
• upward movement - by the action of the superior rectus and inferior oblique muscles; they are called elevators;
• downward movement - by the action of the lower rectus and upper oblique muscles; they are called depressors.

The complex interactions of the extraocular muscles of the eye are manifested in the fact that when moving in some directions they act as synergists (for example, partial adductors - the superior and inferior rectus muscles), in others - as antagonists (the superior rectus - elevator, the inferior rectus - depressor).

The oculomotor muscles provide two types of coordinated movements of both eyes:

• one-sided movements (in the same direction - right, left, up, down) - the so-called versional movements;
• opposite movements (in different directions) - vergence, for example towards the nose - convergence (convergence of the visual axes) or towards the temple - divergence (divergence of the visual axes), when one eye turns to the right, the other to the left.

Vergence and version movements can also be performed in vertical and oblique directions.

The functions of the oculomotor muscles described above characterize the motor activity of the oculomotor apparatus, while the sensory activity is manifested in the function of binocular vision.

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Pathology of the oculomotor system

Disorders of the oculomotor apparatus may manifest themselves in incorrect positioning of the eyes (strabismus), limitation or absence of their movements (paresis, paralysis of the oculomotor muscles), and impaired fixation ability of the eyes (nystagmus).

Strabismus is not only a cosmetic defect, but is also accompanied by a pronounced disorder of monocular and binocular visual functions, depth vision, diplopia; it complicates visual activity and limits a person’s professional capabilities.

Nystagmus often leads to low vision and visual disability.