SOI II EYE AN PDF

Extrinsic muscles of the eye. Blood supply THE ORBIT Cross section: we see it holds the eyeball, which is surrounded by a special fascia. It also contains a series of striated muscles, extrinsic muscles of the eye that move the eyeball. The Lacrimal gland, located in the lateral angle of the orbit....

Extrinsic muscles of the eye. Blood supply THE ORBIT Cross section: we see it holds the eyeball, which is surrounded by a special fascia. It also contains a series of striated muscles, extrinsic muscles of the eye that move the eyeball. The Lacrimal gland, located in the lateral angle of the orbit. And lastly, a very thin layer of fat which protects + contains the blood vessels and nerves. NEURAL -- / FIBROUS ○ ○ - ○ Innermost layer = the retina, continuous with the optic nerve. Contains the axons of the optic nerve Vascular layer, of which the posterior ⅘ are the Choroid and the anterior ⅕ is the Iris (pigmented epithelium). Continuous with the arachnoid meninges which surrounds the optic nerve External-most layer = fibrous layer, which forms the Cornea anteriorly and the Sclera posteriorly. It’s also continuous with the meninges, with the dura mater, which surrounds the optic nerve Periorbita The Periorbita is the periosteum lining the orbit. This periosteum has a thickening anteriorly that forms a thin CT septum: orbital septum. It’s connected to another connective tissue structure that forms a sort of skeleton for the eyelids: the Inferior and superior tarsus. Provide consistency to the lids. Bulbar fascia (Tenon’s capsule) Thin connective tissue layer that surrounds the eyeball (around the sclera). Extends all the way to the angle, but the Cornea does not have this fascia. This fascia is pierced by muscles which cross it to reach and insert into the sclera of the eyeball. Bulbar fascia slightly thickens on the anterior and inferior aspect of the eyeball, forming the suspensory ligament. Check ligaments Cross section: we can see the bulbar fascia surrounding the eyeball, muscles surrounded by their own fascia,... The external aspect of the muscular fascia extends towards the walls of the orbit to form the Check ligaments, lateral and medial, which take insertion in the bone. They limit the movements of the eyeball. DEVELOPMENT OF THE EYEBALL - Optic nerve The eyeball originates from the optic vesicle. Origin of the Retina The retina derives from an evagination of the diencephalon: the optic vesicle, which is why we say its part of the CNS. From the forebrain, a couple of evaginations (optic grooves) form and extend laterally. Slightly after, they expand and form the optic vesicles, which head towards an area of ectoderm which is thickening due to the influence of the CNS. This ectodermal thickening is the developing lens placode. Origin of the Lens The lens derives from the lens placode. It has an ectodermal origin. The ectodermal cells go from being polygonal to being cuboid, and form the lens placode. This specialization of ectoderm is under CNS control. The lens placode becomes concave and starts invaginating. Optic cup As the optic vesicle curves and becomes concave (enclosing the lens placode), it will form the optic cup. The optic cup has 2 layers: - Internal layer - External layer The 2 are separated by an intraretinal space. The internal layer gives rise to layers 2-10 of the retina. Layers of the retina: Layer 1 = derived from the external layer. Gives rise to a pigmented epithelium, NO visual function. Layer 2-10 = derived from the internal layer. Gives rise to receptors and all the cells in the retina. When development proceeds, internal and external layers become opposed to each other. In terms of clinical consequences, this implies that they might separate. “Desprendimiento de retina”, or retinal detachment. Due to trauma to the retina. Intraretinal space appears again. Optic stalk and Optic nerve The optic cup has an optic stalk, which will give rise to the optic nerve. It has an inferior fissure, known as the Choroid fissure, through which the Hyaloid artery (branch of the ophthalmic) courses to reach the retina. During development, it provides blood supply to the lens, but in the adult its distal part disappears. Only the central part will remain as the Central artery of the retina. Transformation of the optic stalk into the optic nerve i. Image A = the first axons of the ganglionar cells begin to arise in the inner layer of the stalk. They connect with the diencephalon through their central process, which is where they originated from. Course back to the diencephalon, where they synapse with 2ary neurons. ii. Image C = Axons grow and become myelinated. End up forming the optic nerve. The choroid fissure closes and the artery becomes embedded within the optic nerve, at this point known as the Central artery of the retina. iii. The lumen of the stalk disappears. Outer and inner layers of the stalk will come in contact. - Inner layer will form the optic nerve fibers, which in the eye become the retinal layer - Outer layer will form the Vascular layer, continuous with the arachnoid. Therefore, below it will the subarachnoid space (surrounding the optic nerve) Finally, the Dura mater surrounds the whole optic nerve, overlying the arachnoid. Changes in intracranial pressure are transmitted to the optic nerve. Fluid compresses the optic nerve and has an effect on the blood vessels + visual inputs. Just by looking at the fundus of the eye we can know if there is intracranial pressure. What would cause an increase in intracranial pressure? - Edema of brain parenchyma - Hemorrhage - Inflammation of the meninges (meningitis) - Tumor Any extra tissue or fluid in the brain causes an increase in intracranial pressure, as the brain is enclosed in the meninges and calvaria (skull). EXTRINSIC MUSCLES OF THE EYE Embryonic origin of extrinsic eye muscles Striated muscle derives from paraxial mesoderm. Beyond the occipital (or postotic) somites, we have the so-called somitomeres (not fully somites). Some of these somitomeres are located rostral to the otic placode (which is why they’re called preotic somitomeres), and are the ones which migrate towards the eye. However, we must remember where they originated from. They form at the level where III, IV and VI CN arise. Therefore, as the somitomeres migrate, they bring the nerves with them. We’ll see them around the lens placode during the 6th week. B ARE ALL SOMITOMERES ↳ M anial ans somitomenes ↳ stay all other somitomers Indense into somites The myotomes from these somitomeres will give rise to 7 STRIATE MUSCLES that move the eye. Their origin is the orbit; All insert into the sclera of the eyeball except for 1 (levator palpebrae), which inserts in the superior lid to elevate it. They pierce the bulbar fascia and take insertion in the sclera, around the ora serrata. Proximal ⅓ of the eye = where the ora serrata is, which separates the colloid from ciliary bodies. INSERT more Laterally This is the point when the majority of the muscles will take insertion. We find 7: 1. Superior rectus muscle 2. Inferior rectus muscle 3. Medial rectus muscle 4. Lateral rectus muscle 5. Superior oblique muscle. It extends along the medial aspect of the orbit and gives a tendon which passes via a ring structure, the trochlea. The tendon passes through here and then it turns, extends and takes insertion in the superoposterior quadrant of the eyeball 6. Inferior oblique muscle, which originates from the medial aspect of the orbit (on maxilla) and takes insertion in the posteroinferior aspect of the eyeball. 7. Levator palpebrae superioris, which is an EXCEPTION as it takes insertion in the eyelid to elevate it, not in the Sclera. s Origin of the 4 rectus muscles - Common tendinous ring The 4 rectus muscles originate from a common tendinous ring. A thickening of periorbita around the optic canal and superior orbital fissure. Insertions of the extrinsic eye muscles The rectus and oblique muscles insert into the sclera of the eye. - The rectus muscles pull the eyeball backwards, whereas - The obliques can pull the eyeball slightly forwards The Levator Palpebrae has a tendon that pierces the orbital septum and the muscle that lies deep to the skin, the orbicularis oculis muscle, inserting itself in the skin of the superior eyelid. Some fibers also take insertion in the superior tarsus. - Function = all together, the muscle fibres will work to elevate the lid There are additional smooth muscle fibers that attach to the superior tarsus. Globally named as superior tarsal muscles. They’re activated and under the control of the autonomous nervous system. The division that activates these muscles is the sympathetic, in order to elevate the lid and be awake and ready. Sleepy, tired = PS tone, the lid tends to close. Innervation of extrinsic eye muscles - Cranial nerves Cranial nerves innervate these 7 striated muscles: III CN (oculomotor), IV CN (trochlear) and VI CN (abducens). VI CN (abducens) will travel within the cavernous sinus. Clinical consequences: when there’s an Aneurysm, it's the first nerve that becomes affected IV and III CN (trochlear and oculomotor) will travel along the walls of the cavernous sinus Trochlear nerve (IV): the IV cranial nerve, which courses most superficial, only innervates 1 muscle: superior oblique muscle, on its superior surface. How to remember? Trick: this is the muscle that passes through the trochlea. Oculomotor nerve (III): it has 2 branches: - Superior branch, for the superior rectus and levator palpebrae muscles - Inferior branch, for medial rectus, inferior rectus and inferior oblique muscles Abducens nerve (VI): also innervates only 1 muscle: lateral rectus muscle III CN→ Superior branch: superior rectus + levator palpebrae; Inferior branch: medial rectus, inferior rectus, inferior oblique IV CN→ superior oblique VI = lateral rectus Actions of individual extraocular muscles LITERALLY ○ Lateral & medial rectus pull eyeball lateral & medially, respectively APART FROM ○ Superior & inferior rectus pull the eyeball medially as well as inferio additionally pull MEDIALLY elevate and depress it ○ Because of their oblique course, the superior & inferior oblique pull the eyeball laterally as well as down and up In THE In THE FUNCTION IS NAME superior and that IN LIKEWISE FOR Rectus , opposite for oblique) rectus Pure elevation→ produced by the superior rectus & inferior oblique acting together Pure depression→ produced by inferior rectus & superior oblique The levator palpebrae superioris takes insertion in the skin. Therefore, everytime sup rectus is activated, the lid is elevated too. Simply take into account that movements of the eye are complicated. There are organized and coordinated responses and activation of different muscle groups in order to perform movement. INTRINSIC MUSCLES OF THE EYE - Visceral innervation (III CN) Simply remember that we also have intrinsic muscles (already seen): Ciliary muscle, Constrictor muscle of the pupil and Dilator muscle of the pupil. These muscles are involuntary, and therefore, receive autonomic innervation: Actions of the PS: - Contraction of the ciliary muscle = to accommodate the eye. Changes the curvature of the lens - Contraction of the constrictor muscle = decrease in pupil diametre Actions of the Sympathetic: - Contraction of dilator muscle of the pupil = increase in pupil diametre - No Symp innervation to the ciliary muscle; not necessary. Enough light coming in for sharp images to be recovered This visceral innervation is conveyed by the oculomotor nerve (III CN), which carries visceral efferents; and by sympathetic fibres from the carotid plexus which incorporate into long and short ciliary nerves to reach the eyeball. BLOOD SUPPLY TO THE EYEBALL ORIGIN IS ↑ INTERNAL CAROTID - The eye is supplied by branches of the ophthalmic artery that accompany the sensory nerves. ○ Special mention to the Central artery of the retina, which provides supply for the layers 2-10 of the retina. IMPORTANT: the central artery of the retina is a terminal branch. If there is an aneurysm, the retina will become prived of blood flow ○ The pigmented epithelium and sclera are supplied by other branches, globally known as choroidal blood vessels: - Anterior ciliary arteries Posterior ciliary arteries: - Short posterior ciliary arteries - Long posterior ciliary artery The posterior don’t divide and course through the vascular layer to reach and anastomose with the anterior ciliary artery (coming from muscular arteries), which enter the sclera. 2 circular arterial rings are formed that give a radial organization. However, these arteries stop at the cornea as THERE ARE NO BLOOD VESSELS IN THE CORNEA. It's supplied by aqueous humor, by diffusion. Although it has no bv, it does have many sensory nerves, but so tiny that they don’t interfere with vision. any lesion to the cornea = very very painful. Branches of the central artery and vein of retina The branches of the central artery of the retina have NO anastomoses. Blood vessels become very thin as they approach the fovea and are not present in the center at all, so as not to interfere with the light and vision. By looking into the fundus of the eye, we can assess intracranial pressure: - Contour of the eye is neat, clear = no intracranial pressure - If there is intracranial pressure, the veins become edematous and tortuous = blurry contour. Papilledema or edematous optic papilla VENOUS DRAINAGE OF THE EYEBALL We find 4 veins, each from a quadrant of the eyeball, which drain into either superior or inferior ophthalmic veins. Superior ophthalmic vein forms from the angular vein of the face + supraorbital vein, and it drains into the cavernous sinus (transmission & of infections, triángulo de la muerte!) Inferior ophthalmic vein can either drain into the superior ophthalmic - and cavernous sinus, or it may pass via the inferior orbital fissure to the of face pterygoid plexus in the infratemporal fossa. AMONG ITS TRIBUTARIES : - - inframbital vein Angular vein EYELIDS The eyelids are mobile skin folds that protect the eye from any agressions. Formed by: Skin + lax connective tissue where blood can accumulate, orbital septum, and superior and inferior tarsus + tarsal glands (the ones that form the fluid which seal the lids while we sleep). The superior tarsus, aside from the Levator palpebrae muscle, has smooth muscle fibres attached to it, which are innervated by the Sympathetic nervous system. HORNER’S SYNDROME - Palpebral ptosis = lids fall. No basal sympathetic tone to keep them open - Miosis = pupil is contracted (PS effect as there’s no symp tone) - Lack of sweating before a source of light (only on one side if the lesion affects one side) LACRIMAL SYSTEM Under parasympathetic control. As a trick, remember that the sympathetic generally has a vasomotor effect, while the parasympathetic has a secretomotor effect (on GI glands, tears, saliva,..) The gland is pierced by the tendon of the Levator palpebrae superioris. Tears will be released to clean the conjunctiva. They’ll spread over its surface and then head towards the medial angle of the eye. Here we find the Lacrimal caruncle, where the tears accumulate by going from lateral to medial. Additionally, on the free margin of the eyelids, there are 2 puncta which can drain the tears from the eye to the lacrimal sac: - Superior lacrimal puncta - Inferior lacrimal puncta These puncta each connect with lacrimal canaliculi that lead to the lacrimal sac. The sac is continuous with the nasolacrimal duct, which is contained within a bony duct that opens up into the inferior nasal meatus. From there they enter the nasal cavity→ nasopharynx→ esophagus. We literally swallow out tears! Everytime we blink, the sac is compressed and tears flow down.