Extra-Occular Muscles There are two groups of muscles within the orbit: extrinsic muscles of eyeball (extra-ocular muscles) involved in movements of the eyeball or raising upper eyelids; intrinsic muscles within the eyeball, which control the shape of the lens and size of the pupil. The extrinsic muscles include the levator palpebrae superioris, superior rectus, inferior rectus, medial rectus, lateral rectus, superior oblique, and inferior oblique. The intrinsic muscles include the ciliary muscle, the sphincter pupillae, and the dilator pupillae. Extrinsic muscles Of the seven muscles in the extrinsic group of muscles, one raises the eyelids, while the other six move the eyeball itself. Levator palpebrae superioris It is the most superior muscle in the orbit, originating from the roof, just anterior to the optic canal on the inferior surface of the lesser wing of the sphenoid. A unique feature of levator palpebrae superioris is that a collection of smooth muscle fibers passes from its inferior surface to the upper edge of the superior tarsus. This group of smooth muscle fibers (the superior tarsal muscle) help maintain eyelid elevation and are innervated by postganglionic sympathetic fibers from the superior cervical ganglion. Origin The levator palpebrae superioris arises from the posterior part of the orbit. The area of origin is on the lesser wing of the sphenoid bone a little above the optic canal. The muscle passes forwards above the superior rectus and enters the upper eyelid. Insertion It ends in an aponeurosis which divides into three layers. The superficial alyer is inserted into the anterior surface of the tarsal plate of the upper eyelid; the middle layer into the upper margin of the tarsal plate, and the deep layer into the superior conjunctival fornix. Innervation Innervation is by the superior branch of the oculomotor nerve [III]. Function Contraction of the levator palpebrae superioris raises the upper eyelid. Clinical significance Loss of oculomotor nerve [III] function results in complete ptosis or drooping of the superior eyelid, whereas loss of sympathetic innervation to the superior tarsal muscle results in partial ptosis. Rectus Muscles Four rectus muscles occupy medial, lateral, inferior, and superior positions as they pass from their origins posteriorly to their points of attachment on the anterior half of the eyeball. They originate as a group from a common tendinous ring at the apex of the orbit and form a cone of muscles as they pass forward to their attachment on the eyeball. superior rectus Origin The superior rectus originates from the superior part of the common tendinous ring above the optic canal. Insertion Anterior half of eyeball superiorly. As this muscle pass forward in the orbit to attach to the anterior half of the eyeball they are also directed laterally Innervation Innervation is by the superior branch of the oculomotor nerve [III]. Function Contraction of the superior rectus elevates, adducts, and internally rotates the eyeball; Inferior Rectus Origin The superior rectus originates from the inferior part of the common tendinous ring below the optic canal. Insertion Anterior half of eyeball inferiorly. As this muscle pass forward in the orbit to attach to the anterior half of the eyeball they are also directed laterally Innervation Innervation is by the inferior branch of the oculomotor nerve [III]. Function Contraction of the inferior rectus depresses, adducts, and externally rotates the eyeball. Clinical significance To isolate the function of and to test the superior and inferior rectus muscles, a patient is asked to track a physician's finger laterally and then either upwards or downwards. The first movement brings the axis of the eyeball into alignment with the long axis of the superior and inferior rectus muscles. Moving the finger upwards tests the superior rectus muscle and moving it downwards tests the inferior rectus muscle. Medial Rectus Muscle Origin The medial rectus originates from the medial part of the common tendinous ring medial to and below the optic canal. Insertion The medial rectus muscle pass forward and attach to the anterior half of the eyeball medially. Innervation The inferior branch of the oculomotor nerve [III] innervates the medial rectus. Function Contraction of each medial rectus adducts the eyeball. Lateral Rectus Muscle Origin The lateral rectus muscle originates from the lateral part of the common tendinous ring as the common tendinous ring bridges the superior orbital fissure. Insertion The medial rectus muscle pass forward and attach to the anterior half of the eyeball laterally. Innervation The abducent nerve [VI] innervates the lateral rectus Function Contraction of each lateral rectus abducts the eyeball. Clinical significance To isolate the function of and test the medial and lateral rectus muscles, a patient is asked to track a physician's finger medially and laterally, respectively, in the horizontal plane. Oblique muscles The oblique muscles are in the superior and inferior parts of the orbit, do not originate from the common tendinous ring, are angular in their approaches to the eyeball, and, unlike the rectus muscles, attach to the posterior half of the eyeball. Superior oblique Origin The superior oblique arises from the body of the sphenoid, superior and medial to the optic canal and medial to the origin of the levator palpebrae superioris. It passes forward, along the medial border of the roof of the orbit, until it reaches a fibrocartilaginous pulley (the trochlea), which is attached to the trochlear fovea of the frontal bone. Insertion The tendon of superior oblique passes through the trochlea and turns laterally to cross the eyeball in a posterolateral direction. It continues deep to the superior rectus muscle and inserts into the outer posterior quadrant of the eyeball. Innervation The trochlear nerve [IV] innervates the superior oblique along its superior surface. Function Contraction of the superior oblique therefore directs the pupil down and out. Clinical significance To isolate the function of and to test the superior oblique muscle, a patient is asked to track a physician's finger medially to bring the axis of the tendon of the muscle into alignment with the axis of the eyeball, and then to look down, which tests the muscle Inferior oblique Origin The inferior oblique is the only extrinsic muscle that does not take origin from the posterior part of the orbit. It arises from the medial side of the floor of the orbit, just posterior to the orbital rim, and is attached to the orbital surface of the maxilla just lateral to the nasolacrimal groove. Insertion The inferior oblique crosses the floor of the orbit in a posterolateral direction between the inferior rectus and the floor of the orbit, before inserting into the outer posterior quadrant just under the lateral rectus. Innervation The inferior branch of the oculomotor nerve innervates the inferior oblique. Function Contraction of the inferior oblique directs the pupil up and out. Clinical significance To isolate the function of and to test the inferior oblique muscle, a patient is asked to track a physician's finger medially to bring the axis of the eyeball into alignment with the axis of the muscle and then to look up, which tests the muscle. Loss of innervation of the muscles around the eye Loss of innervation of the orbicularis occuli by the facial nerve [VII] causes an inability to close the eyelids tightly, allowing the lower eyelid to droop away causing spillage of tears. This loss of tears allows drying of the conjunctiva, which may ulcerate, so allowing secondary infection. Loss of innervation of the levator palpebrae superioris by oculomotor nerve [III] damage causes an inability of the superior eyelid to elevate, producing a ptosis. Usually, oculomotor nerve [III] damage is caused by severe head injury. Loss of innervation of the superior tarsal muscle by sympathetic fibers causes a constant partial ptosis. Any lesion along the sympathetic trunk can induce this. An apical pulmonary malignancy should always be suspected because the ptosis may be part of Horner's syndrome. Diplopia Objects lying in different parts of the visual field produce images over different spots on the retina. The brain judges the position of an object by the position at which its image is formed on the retina. Normally the movements of the right and left eyes are in perfect alignment, and an object casts an image on corresponding spots on the two retinae so that only one image is perceived by the brain. When a muscle of the eyeball is weak, and a movement involving that muscle is performed, the movement of the defective eye is slightly less than that of the normal eye- As a result images of the object on the two retinae are not formed at corresponding points but over two points near each other. The brain therefore 'sees' two images, one from each retina. Squint (or strabismus): This is a condition in which the two eyes do not look in the same direction. The squint becomes obvious when the eye movement involves a muscle that is paralysed or weak, because the weak muscle cannot Keep up with the muscle of the normal side. Squint will be accompanied by diplopia. However, the patient compensates for lack of movement of the eyeball by turning the head in the direction of the object and on doing so the diplopia disappears. If the normal eye is closed, the patient is unable to judge the position of objects in the field of vision correctly (because the image of the object does not fall on the part of the retina that corresponds to the true position of the object). All the features described above are those of paralytic squint. There is another type of squint called concomitant squint. This condition is congenital, and manifests itself in early childhood. Squint is present in all positions of the eyeball. There is no muscular weakness and movements are normal in all directions. There is no diplopia. Intrinsic muscles of the Eye The intrinsic muscles of the eyeball, control the shape of the lens and size of the pupil. Ciliary Muscle Origin The ciliary muscle consists of smooth muscle fibers arranged longitudinally, circularly, and radially. They arise from Muscle fibers in the ciliary body. Innervation Innervation is by parasympathetics traveling to the orbit in the oculomotor nerve [III] Function Contraction of the ciliary muscle decreases the size of the ring formed by the ciliary body. Sphincter pupillae Origin Fibers arranged in a circular pattern in the iris make up the sphincter pupillae muscle Innervation Innervation is by parasympathetics traveling to the orbit in the oculomotor nerve [III] Function Contraction of its fibers decreases or constricts the pupillary opening; Dilator pupillae Origin Fibers arranged in a radial pattern in the iris make up the dilator pupillae muscle Innervation Innervation is by Sympathetics from the superior cervical ganglion (T1) Function Contraction of its fibers increases or dilates the pupillary opening. Involuntary muscles in the orbit 1. The levator palpebrae superioris contains some smooth muscle fibres in its deeper part. These constitute the superior tarsal muscle. 2. The inferior tarsal muscle connects the inferior tarsus to fascia around the inferior rectus and inferior oblique muscles. These smooth muscle fibres present in relation to the upper and lower eyelids can be responsible for involuntary, wide opening of the eyes under sympathetic stimulation associated with fear or anger. 3. The orbitalis bridges the inferior orbital fissure. The action of tos muscle is uncertain. It could, theoretically, compress the inferior ophthalmic veins leading to their engorgement, thus pushing the eyeball forwards and making it more prominent. These three involuntary muscles are supplied by sympathetic fibres. Destruction of sympathetic nerves supplying tne head and ,neck results in Homer's syndrome two features of wnich are drooping of the upper eyelid (ptosis), and reduced prominence of the eyeball (enophthalmos). In addition there is constriction of the pupil because of paralysis of the dilator pupillae.
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