Neuroanatomy of the Eye PDF

Pag. 1 a 10 International Medical School – NEUROAN #6 – prof. Dellavia – The eye pt.1 NEUROAN #6 The eye pt.1 Prof. Dellavia – 22/10/21 – Author: Carola Plebani – Reviser: Axel Duhamel The eyeball is a non...

Pag. 1 a 10 International Medical School – NEUROAN #6 – prof. Dellavia – The eye pt.1 NEUROAN #6 The eye pt.1 Prof. Dellavia – 22/10/21 – Author: Carola Plebani – Reviser: Axel Duhamel The eyeball is a non-perfect sphere; it is a combined system of 2 spheres: a small one, projecting anteriorly, creates a segment that merges with the segment of a larger one, projecting posteriorly; together they form the anterior surface of the eye. The anterior surface of the eye is the most prominent. This permits to properly create the images, which are collected at the level of the most prominent anterior area (the smallest sphere); this portion is lined by the cornea, the external layer which is exposed to the environment. From here, the light enters, passes through the entire globe and arrives on the posterior surface of the eye, where is located an area dedicated to the concentration of the light rays with the maximum power of resolution. The ability to focus the images perfectly depends on the size of the eye: a disproportionate eyeball can lead to problems in vision. If the eyeball is too long (disproportion in the antero-posterior dimensions of the diameter), the individual will focus anteriorly with respect to the posterior surface of the eyeball, thus presenting myopia (correction through a lens of divergence); if the eyeball is too short, the focus will be posteriorly with respect to the posterior surface, thus leading to hypermetropia (correction through a lens of convergence). The lenses used for the correction present the same system which is used inside the eye: they allow the light rays to converge on the correct point on the retina, the layer where the receptors are found. On average, the dimensions of the whole eyeball are: - Antero-posterior: 24 mm - Lateral: 25 mm - Cranio-caudal: 23.5 mm Since the eyeball is not a perfect spheres, the diameters calculated from different points are different. Even a small difference in the length can vary the vision greatly. The light rays during their course pass through some fluids and gelatinous structures, important for the consistency of the eye: they can allow the light rays to cross over them or they can deflect or absorb part of the light. There are 2 eyes and both are need for 3D vision. The 2 eyes are positioned into the orbits so that they have a orientation that can be regulated by (ocular?) muscles.  Pag. 2 a 10 International Medical School – NEUROAN #6 – prof. Dellavia – The eye pt.1 1. Site of the eye and axes The anatomical/optical axis (vertical line in black in the L picture) is defined as the conjunction between the anterior pole (the most prominent point anteriorly: the centre of the cornea, and the posterior pole (the most prominent point of the posterior surface: the centre of the sclera). The 2 anatomical axis are approximately parallel. In the conventional anatomical position, the 2 anatomical eyes are parallel, whereas while moving they can converge or diverge. The visual axis (reported with the red font in the R image) depends on the connection of the anterior pole to the portion of the eye where there is the maximum power of resolution, the point where there is an high concentration of the cells with the maximum discriminant characteristic (the cones). This point is located at the center of a small concave fossa, the fovea, and it is named fovea centralis. This axis is divergent with respect to the anatomical axis because the fovea is located more laterally with respect to the most prominent point of the posterior surface. The 2 visual axis are divergent posteriorly. The orbital axis follows the orientation of the orbital cavity. When the fibres of the optic nerve (forming the visual image onto the retina) abandon the orbit, they enter into the optic foramen, located inside the attachment of the lesser wings of the sphenoid bone; the foramen is located more medially. The fibres go along the more medial direction of this axis, because the nerve forms more medially on the posterior surface of the eye with respect to centre of the posterior surface itself. The formation of the nerve corresponds to the optic disc. The 2 orbital axis are divergent anteriorly. Summarizing, on the posterior surface of the eye there are 3 main points: the anatomical centre (where the anatomical axis ends); more laterally, the fovea centralis (where the visual axis ends) and more medially, the optic disc (where the orbital axis ends). 2. Eyeball The eyeball has a stratification of 3 layers that englobe the condenser From superficial to deep:  Pag. 3 a 10 International Medical School – NEUROAN #6 – prof. Dellavia – The eye pt.1 a. Fibrous layer It is the external layer, made of a fibrous membrane. It can be subdivided in 2 parts: - in the anterior portion, the cornea (covers the lesser segment of the sphere) and - posteriorly the sclera; These 2 layers are in continuation at what is named the cornea-sclera angle. Even though they both present fibrous tissue, they do not have the exact same composition so the passage from one layer to the other is detectable. Together they form an envelope protecting the eye but only the cornea (more prominent) is exposed to the environment. The cornea is transparent to allow the collection of the light rays which pass through it, while the sclera doesn’t have this necessity. The fibrous tissue requires a certain thickness in order to give insertion to the oculomotor muscles (skeletal muscles, insert on the sclera) that are needed to move the eye. b. Vascular layer/uvea It is the intermediate layer, covered by the fibrous layer externally. It is characterized by the presence of many vessels (from here the name vascular layer). It is formed by 3 structures that, from anterior to posterior, are in continuation: the iris, the ciliary body and the choroid. Every part has its own specificities, but they are all characterized by the presence of many vessels in order to supply the high amount of nutrients required by the eye and to control the temperature. From the blood there is also the formation of the fluids present inside the eye. c. Nervous layer It is the inner most layer and thus the most protected. It is called nervous even though not the whole layer is made of nervous apparatus. It is formed by the retina, composed of a proper nervous portion and a non- optic or non-nervous portion (where cells of the nervous system lack). In the nervous portion, the receptors can be found. The nervous system of the eye has similar organization to the cortex (mesencephalon): a very complex organization with many cell layers and interconnections with the possibility to create cellular circuits.  Pag. 4 a 10 International Medical School – NEUROAN #6 – prof. Dellavia – The eye pt.1 Inside this “envelope”, the actual structure of the eyes are contained. 3. Dioptric devices Some components of the eye can form dioptric devices in order to modify the light course. The light rays are collected from the environment at the level of the cornea (convex lens). The light rays pass into the space formed by the convex curvature of the cornea (therefore located posterior to it), the anterior chamber, filled by a fluid called the aqueous humor. This chamber is not completely closed posteriorly: the anterior wall is formed by the cornea (complete), whereas the posterior wall is made by the iris, which contains a hole in the centre, the pupil (incomplete). The fluid inside the anterior chamber can penetrate into the pupil, passing inside the posterior chamber, whose posterior wall is formed by the lens. The lens, differently from the cornea, presents a double convexity (biconvex lens, prominent both anteriorly and posteriorly). The aqueous humour is actually produced and first released in the posterior chamber, passing across the pupil into the anterior chamber just later on. At the level of the sclero-corneal angle, this fluid will be collected by one vein. Posterior to the lens, the light passes into the vitreous body, a fosse formed by the convexity of the lens itself. The vitreous body is a gelatinous sphere with a concavity on its anterior surface; since it is made by a gelatinous substance, it is more consistent than the aqueous humor and therefore it is able to maintain the spherical structure of the eye. It is completely covered by the different layers (from deep to superficial: the retina, the choroid and the sclera). Summarizing, the light course starts in the cornea and passes, in order, through the aqueous humor (located inside the anterior chamber), through the pupil into the posterior chamber, through the lens and lastly, into the vitreous body. The dioptric devices are: Cornea Aqueous humor Lens Vitreous body ANTERIOR CHAMBER POSTERIOR CHAMBER VITREOUS BODY  Pag. 5 a 10 International Medical School – NEUROAN #6 – prof. Dellavia – The eye pt.1 4. Anterior and posterior chambers The anterior chamber is quite large whereas the posterior one accounts for a very small space. However, the aqueous humor is released inside the posterior chamber after being formed by a portion of the ciliary body (part of the vascular layer): the vessels found inside can pass through the connective tissue and the epithelium, releasing the already modified liquid through a transudation effect into the posterior chamber. From here, the fluid moves into the anterior chamber and it is reabsorbed at the level of the angle between iris and cornea. The ciliary body is a continuation of the iris, and it forms a ring eccentric to the pupil’s ring. The iris is perforated by the pupil, while the ciliary body has a portion inside that performs 2 main functions: on one side, it is responsible for the formation of the aqueous humor and on the other side, for the attachment for ligaments’ fibres that tense (for the suspension of the lens) and regulates the convexity of the lens; the lens is therefore attached to the ciliary body. There is also a muscle that can regulate the convexity of the lens, the ciliary body therefore holds a role in the suspension and contraction of the lens. It therefore adapts the vision in respect to the distance to the object.  Pag. 6 a 10 International Medical School – NEUROAN #6 – prof. Dellavia – The eye pt.1 5. Aqueous humour The aqueous humour is a key component in regulating the pressure on the cornea. It is therefore responsible for the maintenance of the curvature of the cornea. It also pushes to the ciliary body and maintains in position all the structures of the eye. On the picture, the aqueous humour (red arrows) is released from the ciliary body. There are veins inside the sclero-corneal angle. These veins are located in the region of the arrival of the conjunctiva (in between the sclera, internally, and the conjunctiva, externally). The conjunctiva is the mucous membrane of the palpebra. This portion located on the periphery of the cornea is prominent as it contains the venous system inside and the conjunctiva outside (covering the external surface). This forms an important system of protection for the eye. An obstruction of the flux of production and drainage of the aqueous humour causes a glaucoma. The increased pressure, caused by the obstruction, causes a constriction of the retina, in fine leading to blindness. 6. The lens The lens presents a simple structure. The lens contains proteins that are transparent and a thin layer of epithelial cells, delimiting this structure. These cells are in a continuous proliferation, causing an increased density and a modified convexity of the lens. In normal physiological conditions, the anterior surface has a different (smaller) convexity than the posterior surface. The lens is in a biconvex shape with an anterior radius of 10mm whereas the posterior radius is of 16mm. An alteration of the structure of the lens, causing a thickening of it, is a cataract (structural alteration of the lens or its capsule, present in 10% of all 80-years-olds). 7. Vitreous body The lens creates a pressure over the vitreous body. Specific groups of fibers form ligaments that attach and maintain the contact between the lens and the vitreous body. The vitreous body is attached very closely, to avoid movement, at the level of the passage between the ciliary body and the choroid, behind the lens. The spaces between choroid and the vitreous body, intercalated between the points of attachment are left for the contraction of the muscles.  Pag. 7 a 10 International Medical School – NEUROAN #6 – prof. Dellavia – The eye pt.1 The function of the vitreous body is to stabilize the eyeball and to protect against retinal detachment. It is very gelatinous: 98% water and 2% hyaluronic acid and collagen. It is devoid of nerves and vessels. A canal is present inside, the hyaloid canal and is occupied by vessels and nerves only during intrauterine life. It is closed after birth. If failure to do so, this explains the reason of impaired vision after birth. 8. The cornea – the fibrous layer The cornea is present in the anterior portion of the fibrous layer. It is a stratified transparent membrane. It is responsible for the very large amount of refractive power. It is more dense in the periphery (1.2mm) than in the centre (0.6mm). It has a radius of curvature that modifies with age: the radius reduces with age, although the extent of such affects can vary depending on the individuals. The cornea is formed of 5 layers (from superficial to deep): (1) Stratified non keratinized squamous epithelium (dioptric device): the basement membrane is a network responsible for the high refracting power as the collagen fibers are oriented in different directions. It is found in an antero-posterior direction. It is keratinized in order to create less interference for the light rays that need to pass through it. Being an epithelium, this layer presents different vessels inside, brining nutrients from the aqueous humor. (2) Anterior lamina membrane limitans (Bowman’s): forms the barrier between the epithelium and the connective tissue. (3) Connective tissue: creates consistency and support; it presents bundles of collagen forming a network of refractive collagen layers with different orientation; it is avascular and creates also rigidity, together with the pressure of the aqueous humor. (4) Posterior lamina membrane limitans (Descemet’s): separates between the connective tissue and the epithelium. (5) Endothelium (anterior wall of the anterior chamber): one layer of specialized flat cells, considered as the wall of capillaries since it brings nutrients from the aqueous humor. 9. The sclera – fibrous layer The sclera is a dense fibrous CT that is used for the attachment of extraocular muscles. It is formed of several layer of collagen and elastic fibers. Important for the insertion of the extra-ocular muscles: superior rectus, medial rectus, lateral rectus, inferior rectus, superior oblique and inferior oblique muscles. They are mostly innervated by the oculomotor nerve, except for the superior oblique (trochlear nerve) and the lateral rectus (abducens nerve).  Pag. 8 a 10 International Medical School – NEUROAN #6 – prof. Dellavia – The eye pt.1 10. Uvea – vascular layer The uvea is responsible for delivering oxygen and nutrients. It is a vascular layer: it contains many capillaries organized in different circles. The first ring of vessels, which is an arterial lesser ring/circle, is surrounding the pupil in the iris. The second ring, an arterial grater ring/circle, is located in the ciliary body. Both rings are interconnected by small anastomotic arteries. The pupil has different layers: (1) A pigmented epithelium, responsible for the coloring of the eye. The pigmented epithelium is made of two layers located on the posterior surface. Respectively to the cornea, the iris faces the posterior chamber, containing the aqueous humour. (2) The stroma, containing vessels and melanocytes. (3) Smooth muscles: inside the iris is located a proper smooth muscle, the constrictor of the pupil (or phincter). It is represented by a circle of fibers located around the pupil. Their contraction causes the reduction in diameter of the pupil. It is innervated by the oculomotor nerve, under the control of the parasympathetic nervous system (constriction). The muscle can be over-dilated under (such as under the influence of drugs): the overdilation is caused by a myoepithelium that acts as a dilator muscle,  Pag. 9 a 10 International Medical School – NEUROAN #6 – prof. Dellavia – The eye pt.1 under the control of the orthosympathetic system. During physiological conditions, mainly the parasympathetic system is active, whereas the sympathetic system activates only in extreme situations 11. Ciliary body The iris is in continuity with the ciliary body. The ciliary body has two important roles. The organisation of the ciliary body, cutting the eye and seeing from the pupil, shows that the ciliary body, starting from the pupil and moving more peripherally, is formed of the pars plicata (closer to the lens) and the pars plana. The pars plicata is creating ondulations (=plicae) on the surface. In the plicae are located many ciliary processes (in white), composed of folds of an epithelium, that allow the aqueous humor to be secreted from capillaries located inside these folds and pass inside the connective tissue. More externally, the pars plana can be seen. Inside the pars plicata, in between the ciliary processes can be seen the zonular fibers. The zonular fibres are located very close to the iris and are responsible for the attachment of lens. They constitute the suspensory apparatus of the lens. This suspensory system is only able to maintain the position of the lens. Inside the ciliary body are located many muscle fibers. The ciliary body is therefore formed of two epithelial layers, one inner continuous with the optical and one outer continuous with the pigmented retina, of loose CT, rich in vessels penetrating the ciliary muscle, and smooth muscle. The smooth muscle fibers form the ciliary muscle and are organised in specific directions. They are both circular and radial fibers located inside the ciliary body. These fibers attach the lens in a way such that during contraction, the choroid is pulled forward and the zonular fibres are relaxed. This contraction allows the lens to assume a more convex shape to accommodate for near vision (more convergence). This is in contrast to the pupil, where one circular muscle is around the pupil and other radial muscle fibers are inside the pupil.  Pag. 10 a 10 International Medical School – NEUROAN #6 – prof. Dellavia – The eye pt.1 12. The choroid The choroid is made of 3-4 layers and it is located in the posterior portion of the uvea. It is the most highly vascularized region of the body. In the passage between the iris and the ciliary body, is located the grater arterial ring, whereas the lesser arterial ring is located inside the iris. These 2 rings are in connection with each other, forming a capillary network that has its continuation inside the choroid. This vascularization is important not only because it provides nutrients but also because it regulates the temperature of the eye and provides blood supply to the outer portions of the retina. The blood supply comes from branches of the ophthalmic artery (enters from the optic foramen with optic nerve) in a segmental distribution. A central artery of the retina, located at the center of the bundle passing through the optic foramen and other vessels detaching, entering and spreading in the choroid are present here. There are 2 types of supply: - The retinal supply: provided from the central retinal artery. It arrives at the center of the retina in the optic disk. - The uveal supply: it configures as other collaterals entering the choroid in two layers (one inner and one outer). These collaterals are the posterior ciliary artery and the anterior ciliary body. They supply the uvea and the outer and middle layers of the retina, until anastomosing with the anterior ciliary artery at the level of the ciliary body. The venous drainage is done by the vorticose veins (piercing the sclera behind the equator and opening into the superior or inferior ophthalmic vein). 13. The retina The retina has a visual optic nerve portion and a non-optic portion. The posterior border of the ciliary body has a dentate shape, called the ora serrata. The non-visual retina is continuous anteriorly to the ora serrata. The rest of the retina is the optic part of retina and has a complex organization.