Muscles of the Eyeball
he eyeball is attached by membranes to the capsule of the orbital fat body, and it can move in all directions. Movements are achieved by six extra-ocular muscles, namely four rectus muscles and two oblique muscles. The tendons of origin of the rectus muscles form a funnel-shaped ring around the optic canal, the common annular tendon(AB1). The superior rectus muscle (A – C2) (oculomotor nerve) runs above the eyeball in a slightly oblique, outward direction. The inferior rectus muscle (A – C3) (oculomo-tor nerve) runs beneath the eyeball in the same direction. At the nasal aspect of the eyeball lies the medial rectus muscle (AC4) (oculomotor nerve), and at the temporal aspect lies the lateral rectus muscle(A – C5) (abducens nerve). At a distance of 0.5 – 1 cm from the margin of the cornea, the flat tendons of the rectus muscles attach to the sclera of the eyeball. The superior ob-lique muscle (AC6) (trochlear nerve) origi-nates medially at the lesser wing of the sphenoid bone and extends almost to the margin of the orbit. Here, its tendon passes through the trochlea (A7), a wide loop con-sisting of fibrous cartilage and lined with a synovial sheath. The tendon then turns at an acute angle in posterolateral direction and attaches underneath the superior rectus muscle on the temporal side of the upper eyeball. The inferior oblique muscle (BC8) (oculomotor nerve) originates medially at the infraorbital margin and runs to the tem-poral side of the eyeball. Levator muscle ofupper eyelid (B9).
! Rotation around the vertical axis toward the nose (adduction) and toward the temple (abduction)
! Rotation around the horizontal axis up-ward (elevation) and downward (depres-sion)
! Rotation around the sagittal axis with roll-ing of the upper half of the eyeball toward the nose (inward rotation, or intorsion) and toward the temple (outward rotation, or extorsion)
The medial rectus muscle (C4) causes adduc-tion; thelateral rectus muscle(C5) abduction.The superior rectus muscle (C2) raises the eyeball and causes also a slight adduction and intorsion; the inferior rectus muscle (C3) lowers the eyeball and causes also a slight adduction and extorsion.
The superior oblique muscle (C6) rotates the upper pole of the eyeball inward slightly depresses and abducts the eyeball; the infe-rior oblique muscle(C8) rotates the upperpole of the eyeball outward and slightly ele-vate and abducts the eyeball.
This functional description applies only when looking straight ahead (medial gaze) and when both eyeballs have parallel axes of vision. During ocular movements and simultaneous reactions of convergence and divergence, the func-tions of individual muscles changes, e.g., the two medial rectus muscles are synergists during con-vergence and antagonists during lateral gaze. The change in function is determined by the deviationof the visual axis from the anatomical axis of the orbit. When the two axes overlap during abduc-tion of the eyeball by 23!, the superior rectusmuscle (D10, looking straight ahead) and theinfe-rior rectus muscle lose their accessory functions;the first turns into a true levator of the eyeball (D11) and the latter into a true depressor of the eyeball. During maximal adduction of the eyeball up to 50!, the superior oblique muscle (E12, look-ing straight ahead) turns into a true depressor ofthe eyeball (E13) and theinferior oblique muscleinto a true levator of the eyeball. All extra-ocular muscles are involved with tension and relaxation during every eye movement, and the position of the eyeball determines the function of each muscle.Precision and speed of muscle function rely on structural characteristics. Apart from the intra-fusal fibers of muscle spindles, numerous extra-fusal fibers are supplied with sensory anulospiral nerve endings. The motor units are extremely small; about six ocular muscle fibers are supplied by one nerve fiber. For comparison, one nerve fiber innervates 100 – 300 muscle fibers in the muscle of the fingers, in other muscles often more than 1500 fibers.
Clinical Note: Paralysis of individual eyemuscles causes double vision, when objects in the surroundings are perceived twice. The relative position of the two images—side by side or shifted obliquely above or obliquely below one another— indicates which muscle is paralyzed.
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