The Visual System

• The Lacrimal Apparatus protects the cornea from drying out, from bacterial infections, and from eyelids sticking together
• Vision is the process through which light reflected from objects in our environment is translated into a mental image
• The eyes are protected by a bony cavity called the orbit, which is formed by the facial bones of the skull
• The pupil is an opening through which light can pass into the interior of the eye. Pupil size varies with the contraction and relaxation of a ring of smooth pupillary muscle
• The lens, suspended by ligaments called zonules, is a transparent disk that focuses light. The anterior chamber in front of the lens is filled with aqueous humor, a low-protein, plasma-like fluid secreted by the ciliary epithelium supporting the lens
• Behind the lens is a much larger chamber, the vitreous chamber, filled mostly with the vitreous body, a clear gelatinous matrix that helps maintain the shape of an eyeball. The outer wall of the eyeball, the sclera, is composed of connective tissue
• Normally, aqueous humor secreted by the ciliary epithelium flows through the pupil and drains out through the canal of Schlemm in the anterior chamber of the eye. If the outflow is blocked, the aqueous humor accumulates, causing intraocular pressure to build up and possibly cause glaucoma
• The cornea is the clear covering of the anterior surface of the eye that is the continuation of the sclera.
• In bright sunlight, the pupils narrow when the parasympathetic pathway constricts the circular pupillary sphincter muscles. In the dark, dilation occurs when radial dilator muscles lying perpendicular to the circular sphincter muscles contract under the influence of sympathetic neurons.
• The cornea and lens together bend incoming light rays so they focus on the retina, the light-sensitive lining of the eye that contains the photoreceptors
• The optic disk is the location where neurons of the visual pathway form the optic nerve and exit the eye. Lateral to the optic disk is a small dark spot, the fovea. The fovea and a narrow ring of tissue surrounding it, the macula, are the regions of the retina with the most acute vision.
• When parallel light rays pass through a convex lens, the single point where the rays converge is called the focal point. The distance from the center of the lens to the focal length of the lens.
• The ciliary muscle is a ring of smooth muscle that surrounds the lens and is attached to it by the inelastic ligaments called zonules. When the ciliary muscle is relaxed, the ring is more open and the lens is pulled into a flatter shape.
• Myopia, or near-sightedness, occurs when the focal point falls behind the retina whereas hyperopia, or far-sightedness, occurs when the focal point falls behind the retina.
• Astigmatism is blurred vision caused by an irregularly shaped cornea that is not a perfect dome
• Phototransduction is the process by which animals convert light energy into electrical signals. This takes place when the light hits the retina, the sensory organ of the eye.
• Backing the photosensitive portion of the human retina is the dark pigment epithelium layer which absorbs any light rays that escape the photoreceptors, preventing distracting light from reflecting inside the eye and distorting the visual image
• The fovea is free of neurons and blood vessels that block light reception, so photoreceptors receive light directly. The fovea has the highest concentration of cones and the lowest concentration of rods.
• Sensory information about light passes from the photoreceptors to bipolar neurons, then to a layer of ganglion cells. The axons of the ganglion cells form the optic nerve which leaves the eye at the optic disk
• Rods function well in low light and are used in night vision. Cones, on the other hand, are responsible for high-acuity vision and color vision during the daytime, when light levels are higher.
• Light-sensitive visual pigments are bound to the disk membranes in the outer segments of photoreceptors. These visual pigments are transducers that convert light energy into a change in membrane potential. Rods have one type of visual pigment called rhodopsin
• Rhodopsin is composed of two molecules: opsin, a protein embedded in the membrane of the rod disks, and retinal, a vitamin A derivative that is the light-absorbing portion of the pigment. In the absence of light, the retinal binds to the binding site on opsin. When activated by light, the retinal changes shape to a new configuration. The activated retinal no longer binds to opsin and is released from the pigment in the process known as bleaching.
• When ciliary muscles are relaxed, the ligaments pull on and flatten the lens. When the ciliary muscle contracts, it releases tension on the ligaments and the lens becomes more rounded.
• The macula lutea is a yellow spot on the back of the retina in which lies the fovea. It allows light to focus when looking at an object
• The hyaloid canal is a small canal running from the optic disk to the lens. During development, it supplies blood to the developing lens.
• The uvea is the middle layer of the eye and it is the source of blood flow to the ocular tissue
• Amacrine cells play a role in speeding up the slow potential rod messages for presentation to ganglion cells
• Horizontal cells are responsible for increasing contrast via lateral inhibition and adapting both to bright and dim light conditions
• Transducin is a type of heterotrimeric G-protein with different alpha subunits in rod and cone photoreceptors. Light leads to conformational changes in rhodopsin, which leads to the activation of transducin. Transducin activates phosphodiesterase, which results in the breakdown of cGMP
• presbyopia is the loss of accommodation abilities
• Visual Pathways includes: Cones, bipolar neurons, ganglion cells's axons that form the optic nerve, the optic nerve to the optic chiasm, optic tract, lateral geniculate nucleus of the thalamus, optic radiations, and primary visual cortex areas of the occipital lobe