Vision

• More than half of the sensory receptors in the human body are located in the eye
• Accessory structures of the eyes

  • eyebrows and eyelashes: protection against foreign objects entering the eye
  • eyelids: voluntary muscle and skin lined with conjuctiva (mucous membrane)
• Lacrimal apparatus is a structure that secrete tears and drains them from the surface of the eyeball into the nasal passages
• Tears provide lubrication for the eye and antibacterial properties for protection
• Muscles of the eye

  • extrinsic eye muscles
  • intrinsic eye muscles
  • iris
  • ciliary muscles
• Extrensic eye muscles are skeletal muscles that attach to the outside of the eyeball and bones of the orbit
• Intrinsic eye muscles are smooth muscles (involuntary movement like dilation of pupils) located within the eye
• Iris regulates the size of the pupil
• Ciliary muscle controls the shape of the lens
• Humors

  • aqueous humor is a clear watery fluid that fills chambers of the anterior cavity, often leaks out when the eye is injured
  • vitreous humor is a semisolid material that fills the posterior cavity, helps maintain sufficient intraocular pressure to give the eyeball its shape
• Three layers of the eyeball

  • outer: sclera, cornea
  • middle: choroid (ciliary body, suspensory ligament, iris)
  • inner: retina
• Outer layer of the eye

  • sclera is a tough, white, fibrous tissue
  • cornea: transparent anterior portion that lies over the iris
• Middle layer contains many blood vessels and a large amount of pigment
• Choroid: pigmented membrane lining more than two thirds of the posterior fibrous outer coat. Separated into:

  • ciliary body: thickening of choroid
  • suspensory ligament: attached to the ciliary processes and holds the lens in place
  • iris: colored part of the eye, consists of circular and radial smooth muscle fibers
• The inner layer of the eyeball is the retina containing specialized neurons called photoreceptor cells. They are visual receptors sensitive to light rays
• Photoreceptor cells

  • rods: absent from the fovea, increased in density toward the periphery of the retina
  • cones: less numerous than rods, most densely concentrated in the fovea centralis
• Rods allow us to see shades of grey and very dim light, producing black and white vision. There are approximately 120 million rods in the retina
• Cones produce vision in bright light and give sharp images. Each type of cone (blue, red or green sensitive) is sensitive to different wavelengths of light. Our perception of color is a combination of the 3 types of cones.
• Color blindness is the abnormal function of the cones
• Information from photoreceptors pass to sensory neurons that extend out of the retina through an area called the optic disk onwards to the brain to the occipital lobe of the cerebral cortex
• The optic disk contains no rods or cones, it is a blind spot. It is not normally noticed because each eye compensates for the other
• Image formation

  1. Refraction of light rays to focus them on the retina
  2. Accommodation: change the shape of the lens to focus the light for near of far vision
  3. Constriction of the pupil to control the amount of light entering the eye
  4. Convergence of eyeballs for binocular vision
• Refraction of light is the bending of light rays as they pass through a substance such as a magnifying glass
• 75% of light entering the eye is refracted at the cornea. The lens further refracts the light so it can focus directly on the retina.
• The resulting image of refraction is inverted and reversed bu the brain interprets information and adjusts
• Accomodation: lens adjusts shape depending on distance of object to allow for the image to focus on the retina

  • distant objects: ciliary muscle relaxes = flat lens
  • close-up vision: ciliary muscle contracts = pulls choroid = lens becomes rounder
• Visual disorders of refraction abnormalities

  • myopia
  • hyperopia
  • presbyopia
  • astigmatism

  They can all be corrected with glasses or LASIK eye surgery to change the shape of the cornea
• Myopia: nearsightedness (can see near but not far), the eyeball is too long so lens cannot accommodate enough to focus images of distant objects onto retina
• Hyperopia: farsightedness (can see far but not near), eyeball is too short so lens cannot accommodate enough to focus images of nearby objects onto retina
• Presbyopia: with age the lens loses some of its elastic properties and leads to hyperopia
• Astigmatism: irregular curvature of cornea or lens
• Constriction of the pupil controls the amount of light allowed into the eye
• Convergence: eyes rotate inward as object comes closer to maintain binocular (3D) vision