Blood Vessels and Nerves of the Eye
The human eye is a highly evolved structure of our anatomy and has many coexisting and interdependent elements. It is capable of moving and follow the objects along with accommodating to near and far; the eyes also can see in varying light, and in colour. Our two eyes working together give us stereoscopic vision, and depth perception. This article covers the anatomy, function and clinical relevance of the vessels and nerves of the eye. We will conclude the article will some review questions to test understanding of the article content.
The eyeball is filled with vitreous humor, with the aqueous humor lying in the small anterior chamber of the eye. The eye itself is coated with three layers, the sclera and cornea (opaque and transparent layer respectively); the choroid (filled with blood vessels) and the retina (with the rod cells for black and white, and the cone cells for colour). The optic nerve feeds in the posterior surface of the eye and carries all visual information from the retina, ultimately to the primary orbital cortex, which lies within the depths of the calcarine sulcus on the occipital lobe of the brain.
The fovea centralis is the region of the retina with a high concentration of cone cells (colour detecting cells). The fovea and the surrounding area are not supplied by direct branches from the ophthalmic artery, and are in fact supplied directly from the choroid. As well as staying wet enough to maintain a transparent layer (tear film), the eye must also be supplied with nutrients, and be able to move for our vision to be effective. The pupil must be able to dilate and constrict to accommodate to light and autonomic stimuli, and the lens must be able to relax and contract for near and far vision.
Blood Supply and Drainage
The eye is supplied by the ophthalmic artery, which is the first branch of the internal carotid artery, when it has travelled passed the cavernous sinus. The ophthalmic artery has numerous branches that supply the muscles that move the eye and surround the eye, the eyelid and the eyeball itself. The branches of the ophthalmic artery are divided into the orbital (supply the orbit and related structures) and the optical group (supply the eye and its muscles).
Lacrimal artery- This artery arises from the ophthalmic artery near the optic canal. It runs with the lacrimal nerve to supply the lacrimal gland. The terminal branches of the artery pass through the lacrimal gland and supply the eyelids and conjunctiva as the lateral palpebral arteries, and pass medially to supply the upper and lower eyelids respectively. These arteries then anastomose with the medial palpebral artery and form a complete arterial circle.
Supraorbital artery- This artery arises from the ophthalmic artery just as it passes over the optic nerve, and runs forwards, along the medial border of levator palpebrae superioris and superior rectus muscles, and passes through the supraorbital foramen in order to supply the upper eyelid, frontal sinus, levator palpebrae superioris and part of the scalp.
Posterior ethmoidal artery- Once the ophthalmic artery reaches the medial wall of the orbit, it runs forwards and gives off the posterior ethmoidal artery. This artery enters the nasal cavity by passing through the posterior ethmoidal canal, and supplies the posterior ethmoidal sinuses as well as continuing to enter the skull and supply the meninges.
Anterior ethmoidal artery- This artery branches from the ophthalmic artery within the orbit, and accompanies the nasociliary nerve through the anterior ethmoidal foramen into the middle and anterior air cells as well as the frontal sinus. Before the artery passes into the cranium, it provides a meningeal branch to supply the dura mater. There are also nasal branches that pass into the nasal cavity via a small opening adjacent to the crista galli, and goes on to supply a section of the dorsum of the nose.
Medial palpebral artery- This artery has two branches i.e. the superior and inferior palpebral arteries. They arise opposite the superior oblique muscle. They supply the upper and lower eyelids respectively.
Long posterior ciliary arteries- These arteries branch from the ophthalmic artery near the optic nerve, and run anteriorly on both sides of the eyeball. They run between the sclera and the choroid layers, and they run to supply the ciliary muscle where they divide further. These two arteries merge and form the circulus arteriosus major around the iris, which run inwards to form a smaller circle of arteries (the circulus arteriosus minor). In all they supply the choroid, ciliary body and iris.
Short posterior ciliary arteries- There are around 6-12 of these arteries for each eye. They branch from the ophthalmic artery as it curves medially to cross over the optic nerve. They pierce the back of the eye and run between the sclera (which they supply) and choroid, and supply up to the ciliary processes. They also give off smaller branches that supply the optic disc. They do this by forming an arterial ring known as the circle of Zinn-Haller.
Anterior ciliary arteries- There are 7 of these arteries per eye, and they supply the sclera, and rectus muscles. They branch from the ophthalmic artery and pass forwards to the anterior aspect of the eyeball, where they pierce the sclera, near the cornea, and terminate in the circulus arteriosus major, that surrounds the iris. Medial, inferior and superior rectus are supplied by two branches each, with the lateral rectus receiving the remaining single branch.
Central retinal artery- This artery runs underneath the optic nerve and lies within the dural sheath of the nerve to reach the eyeball. It pierces the optic nerve itself near the back of the eye, and sends numerous branches over the internal aspect of the retina. In diabetic retinopathy, there can be haemorrhages and aneurysms that can form in this artery and its branches.
Drainage- The central retinal vein runs through the optic nerve. From here it drains into the cavernous sinus or superior ophthalmic vein. The superior ophthalmic vein is usually the largest and is the principal vein. It is formed when the supraorbital and angular veins unite just behind the trochlea (pulley like structure). The veins that drain into it are named as the arteries of the region (medial palpebral, lacrimal, anterior ethmoidal, inferior ophthalmic, central retinal and muscular). The inferior ophthalmic vein runs over the surface of the inferior rectus muscle, and drain to the cavernous sinus or the superior ophthalmic vein. It drains the inferior rectus muscle, inferior oblique muscle, lacrimal sac and lower lid. There is sometimes a middle ophthalmic vein which some consider a second inferior ophthalmic vein. The veins of the region run with the arteries and follow a similar course.
Sight- The special sense of sight is transmitted by cranial nerve II, the Optic Nerve. This nerve leaves the skull via the optic canal, and provides us with our sense of sight. The pair of optic nerves are in fact a direct structure of the brain, and not technically a distinct cranial nerve like the other 11 pairs. They unite together at the optic chiasm that lies superior to the pituitary gland (which lies in the sella turcica). The peripheral fields/nasal retinal fields cross over at the chiasm. The optic tracts then travel to synapse in the lateral geniculate nucleus of the thalamus, and from there they travel through Meyer’s loop (from inferior retina/superior visual field) and Baum’s loop (from the superior retina/inferior visual field), and from there on to the primary visual cortex.
Pupil constriction/Miosis- This occurs when the eye is exposed to light. This originates from the Edinger Westphal nucleus which carries parasympathetic fibers that run as the outer part of the oculomotor nerve, and eventually synapse with the ciliary ganglion (which is a parasympathetic ganglion that lies in the posterior orbit. This then gives off the short ciliary nerves, which innervate the constrictor pupillae.
Lens Relaxation (looking at near objects) and lens tensing (looking at far objects)/Accommodation- The short ciliary nerves described above also innervate the ciliaris muscle, which contract, and release the tension on the zonular fibers, causing the lens to become more convex (rounder) and hence focus on near objects. When we need to focus on a distant object, the impulse to the ciliary muscles is withdrawn, the suspensory ligaments become taut, and the lens tenses and become broader.
Pupil Dilation/Mydriasis- This occurs when the eye is in the dark, when we are experiencing sympathetic output i.e. adrenaline (fight, flight and fright response), and in response to some drugs. Anatomically, the long ciliary nerves (sympathetic nerves) mediate the reflex.
Movement- Cranial nerve III is the Oculomotor nerve, and as the name suggests it moves the eye. It leaves the skull through the superior orbital fissure. It innervates the inferior rectus, medial rectus and inferior oblique. When the eye is deviated away from the midline the inferior rectus is responsible for depressing the eyeball. The medial rectus acts to adduct the eyeball. When the eyeball is adducted, the inferior oblique acts to elevate the eyeball. Cranial nerve IV, the Trochlear nerve originates from the posterior aspect of the brainstem (the only cranial nerve to do so), and is a long slender nerve that winds around the brainstem to run forwards. It then leaves the skull via the superior orbital fissure and innervates the superior oblique muscle. This muscle runs through a pulley, and attaches the eyeball in an oblique fashion. This muscle is responsible for depressing the adducted eye. If the muscle acts alone when the eye is facing directly forwards, it moves the eye down and out. Hence it is known as the ‘tramp’s’ muscle. Cranial nerve VI is the Abducens, which innervates the lateral rectus muscle, and leaves the skull via the superior orbital fissure. The muscle it supplies abducts the eye, and its function links with the name of the nerve that supplies it.
Sensation- V1- Ophthalmic division of the trigeminal nerve. This nerve exits the skull via the superior orbital fissure. It provides sensation to the eyeball, the upper eyelid, as well as the bridge of the nose as far down as the nasal tip.