16.2: Vision Anatomy

Vision Anatomy

Vision (sight) is perception of light emitted or reflected from objects in the environment. Visible light is electromagnetic radiation with wavelengths from 400 to 750nm. Light must cause a photochemical reaction in order to produce a nerve signal that is sent to the occipital lobes of the brain for processing. Radiation below 400 nm has so much energy it kills cells (ultraviolet or UV light). Radiation above 750 nm has too little energy to cause photochemical reaction (it only warms the tissue) (infrared).

Macroscopic Anatomy of the Eye

The human eye is an organ that reacts to light and allows vision. The eye is the specialized organ of sight which has three principal layers, the fibrous tunic (tunica fibrosa), the vascular tunic (tunica vasculosa) and the neural tunic (tunica interna). Furthermore, there are two main chambers, the anterior chamber, containing aqueous humor and the posterior chamber, that contains vitreous humor. In the neural tunic of the retina, light propagates from the ganglionic cells through the bipolar cells to the rods cells and cone cells, which, somewhat paradoxically hyperpolarize opposite the direction of light.

Tunic of the Eyeball	Layer Type	Structures	Function
tunica fibrosa (fibrous tunic)	fibrous layer	sclera, cornea	provides protection
tunica vasculosa (vascular tunic)	vascular layer (contains blood vessels)	choroid, ciliary body, iris	provides nourishment
tunica interna (neural tunic)	internal layer	retina, optic nerve	converts light to nerve signal and sends to the occipital lobes of the brain

Above: Transverse section of the eye with a superior view (left) image of eye section and (right) illustration of eye structures.

Cavity of the Eye	Location	Fluid (Humor)	Function
anterior cavity	anterior to the lens of the eye (cornea to lens)	aqueous humor (watery fluid)	aqueous humor helps to maintain the intraocular pressure and supply of nutrients to the lens and the cornea
posterior cavity	posterior to the lens of the eye (between the lens to retina)	vitreous humor (jelly-like substance)	vitreous humor holds the retina against the choroid layer and prevents the eyeball from collapsing

Similar to the eyes of other mammals, the human eye's non-image-forming photosensitive ganglion cells in the retina receive light signals which affect adjustment of the size of the pupil, regulation and suppression of the hormone melatonin and entrainment of the body clock.

The eye is not shaped like a perfect sphere, rather it is a fused two-piece unit, composed of the anterior segment and the posterior segment. The anterior segment is made up of the cornea, iris and lens. The cornea is transparent, is more curved, and is linked to the larger posterior segment, composed of the vitreous humor, retina, choroid and the outer white shell called the sclera. The cornea is typically about 11.5 mm (0.3 in) in diameter, and 0.5 mm (500 μm) in thickness near its center. The posterior chamber constitutes the remaining five-sixths; its diameter is typically about 24 mm. The cornea and sclera are connected by an area termed the limbus. The iris is the pigmented circular structure concentrically surrounding the center of the eye, the pupil, which appears to be black. The size of the pupil, which controls the amount of light entering the eye, is adjusted by the iris' dilator and sphincter muscles.

Light enters the eye through the cornea, through the pupil and then through the lens. The lens shape is changed for near focus (accommodation) and is controlled by the ciliary muscle. Photons of light falling on the light-sensitive cells of the retina (photoreceptor cone cells and rod cells) are converted into electrical signals that are transmitted to the brain by the C.N. II optic nerve and interpreted as sight and vision.

Structure	Location	Function
choroid	posterior portion of tunica vasculosa	contains melanin (pigment) that absorbs light to prevent reflection back into the eyeball, which could cause blurred vision; provides nourishment to the retina
ciliary body	composed of ciliary process and ciliary muscle, connected by suspensory ligaments	helps in adjusting the shape of the lens for near and far vision; produces aqueous humor
conjunctiva (bulbar/ocular)	covers the sclera (white part of the eye)	lubrication and protection by producing fluid and mucus; monitors microorganisms present to prepare immune response
conjunctiva (palpebral)	thin mucus membrane that lines the inside of the eyelids	secretes mucus to reduce friction and moisten the eyeball surface
cornea	the transparent, curved coat of the eyeball that covers the iris and pupil	admits light to the interior; responsible for bending light to focus toward the back of the eye; dense fibrous nature provides protection
fovea centralis	a small depression in the macula lutea	area of the sharpest vision because of the abundance of cone cells
iris	pigments portion around the central aperture (the pupil)	constrictor and dialator muscle of the iris change the diameter of the pupil to regulate the amount of light striking the retina
lens	posterior to the iris and pupil	fine-tunes bending of light to focus image on the retina; divides the eyeball into two cavities/chambers: anterior cavity/chamber and posterior cavity/chamber
macula lutea	oval-shaped central region of the retina containing higher concentration of photoreceptors; fovea centralis is located in its center	region of highest-quality vision due to the presence of lots of photoreceptors
optic disc	location where the optic nerve exits the eyeball; lacks photoreceptors	the blind spot of the eye where no photoreceptors are located due to the presence of the optic nerve
optic nerve (C.N. II)	exits the eyeball at the optic disc, through the optic canal of the skull, into the cranial cavity (optic chiasma, to optic tract, to occipital lobe which contains the visual cortex)	transmits light information from the eye to the visual cortex in the occipital lobe of the brain
ora serrata	where the retina meets the ciliary body; has a serrated (jagged) appearance	location where photosensitive tissue transitions into non-photosensitive tissue
plica semilunaris	a semilunar (crescent-shaped) fold in the bulbar/ocular conjunctiva in the medial corner of the eye	aids in movement of lacrimal fluid across the eye (for lubrication of the eye)
pupil	contractile hole located in the center of the iris of the eye	allows light to enter the retina
retina	innermost layer of the posterior eye; contains two layers (outer pigmented layer and inner neuronal layer)	outer pigmented layer prevents light scattering and absorbs light; inner neuronal layer contains two types of photoreceptors, rod cells and cone cells, that collect light information and transmit to neurons
sclera	covers the eyeball except at the cornea; covered by the bulbar/ocular conjunctiva; white part of the eye	resists punctures and protects the eye; helps maintain pressure in the eye to keep its shape and keep the retina adhered to the back of the eye; attachment point of extrinsic eye muscles

Above: Structures of the eye.

Clinical Application: "Blind Spot"

Above: Photograph of left human retina, anterior view.

There are no rods or cones (visual receptors) at the optic disc (the structure formed from where CN II (Optic n.) connects to the retina) because that space is being occupied up by the optic nerve fibers and retinal vasculature. Therefore, as light passes through the eye and onto the retina for visual processing, the light that shines on the optic disc is not processed into eyesight, causing a “blind spot” in each eye’s normal field of vision. You can “see” your blind spot for yourself using the image below. Close your right eye and look directly at the center of the plus sign with your left eye (don’t look away or at the circle). Hold the image about one foot away from your face (how close you need to view the image will depend on the size of the image (whether in print or digital format which is dependent on the zoom in your view)). While looking directly at the plus sign with your left eye, the white circle should disappear from your peripheral vision because that area in your field of vision is shining on your optic disc. You typically don’t notice your blind spots because your brain takes vision from your other eye and “fills in” your blind spots for a full field of view. Unlike the optic disc, the fovea centralis, a structure on the retina near the optic disc, has the highest concentration of cones and processes the most acute vision necessary for reading, driving, etc.

Above: Diagram for "seeing" your blind spot.

Clinical Application: Conjunctivitis

Conjunctivitis (or pink eye) is a common inflammation of the conjunctiva of the eye. Symptoms typically include redness, itching, increased tear production, and crusting around the eyes. It is highly contagious, and can be spread by direct contact (i.e. handshake or hug) or indirect contact (i.e. touching a contaminated pencil or doorknob). Although the inflammation is usually not severe and will resolve on its own, the process can be sped up with antibiotic eye drops.

Microscopic Anatomy of the Eye

The human eye can differentiate between about 10 million colors and is possibly capable of detecting a single photon. Rod and cone cells in the retina allow conscious light perception and vision including color differentiation and the perception of depth.

Above: This image captures the many layers of nerve cells in the retina. The top layer (green) is made up of cells called photoreceptors that convert light into electrical signals to relay to the brain. The two best-known types of photoreceptor cells are rod- and cone-shaped. Rods help us see under low-light conditions but can't help us distinguish colors. Cones don't function well in the dark but allow us to see vibrant colors in daylight. The top of the image is the posterior of the retina and the bottom of the image is positioned toward the posterior cavity filled with vitreous humor.

Above: Diagram of the cells of the retina. Light would enter the eye from the right of the figure and strike the retina and be detected by the rods and cones which send light information to bipolar cells and ganglion cells.