Photoreceptors

Photoreceptors Abstract We will discuss in this essay the workings of the Photoreceptors and how they determine vision, the perception of light. Also we will briefly discuss how photoreceptors work in other animals and how they work, if at all, in humans without sight. Photoreceptors 1 The peripheral nervous system is described as all parts of the nervous system except the brain and spinal cord, which is considered the part of the central nervous system. The peripheral nervous system, or the PNS, connects the central nervous system (CNS) to the remainder of the body, and is the conduit through which neural signals are transmitted to and from the CNS. (Gale Encyclopedia of Neurological Disorders, Hoyle and Arthur, 2005). These neurons are broken down into three categories, sensory, motor and associated neurons. Motor neurons control the muscles and glands, the sensory neurons are associated with the eyes, ears and skin, while the associated neurons connect the two. (The Living World, Johnson and Lobos, 2010) We will concentrate on a particular part of the PNS, the sensory receptors, and in this case, the photoreceptors. Photoreceptors respond to light, which is what vision is, the perception of light. Eyes contain sensory receptors called rods and cones that respond to photons of light. The light energy is absorbed by the pigments in the rods and cones, which respond by triggering nerve impulses in sensory neurons, sending it to the CNS, particularly the brain. After sending these signals from the photoreceptors through the retina (the eyeballs, the locations of the rods and cones) to the cerebral cortex, the brain interprets this information and provides the images of what we are seeing. The key difference between humans and certain animals is the different types of vision we have. Humans and Photoreceptors 2 primates function with binocular vision, which gives them ability to determine depth perception. But the advantage that animals have with eyes on the side of their head is of a wider field of vision.(The Living World, Johnson and Lobos, 2010) Some people are born completely blind—lacking the eye’s photoreceptors known as rods and cones. But how about partially blind or people with bad vision' (interesting subject because without glasses I cannot see three feet in front of me!) The number one reason for blindness is cataracts, according to the WHO website (www.WHO.int, 2010). Other causes are macular degeneration and diabetes. Cataracts are the clouding of the crystalline lens. Cataracts block light from entering the retina, not allowing the rods, cones and photoreceptors to send impulses to the brain. Macular degeneration, Diabetes and retinitis pigmentosa (or tunnel vision) are conditions that do involve the dying of photoreceptors, and to this point not much hope has been given to patients who slowly lose their vision.(www.why files.org, 2010) But hope is on the horizon in the form of stem cell research and in particular neuron stem cells. Dr Michael Young of the Schepens Eye Research Center at the University of Harvard have made progress in rats by injecting neuron stem cells into a blind rats retina. The cells populated in the retina and became normal looking, as well as the nerve cells. (www.schepenes.Harvard.edu , 2010) Current research has moved on to pigs, and in the future maybe humans. References Gale Encyclopedia of Neurological Disorders, Hoyle and Arthur, 2005 The Living World, Johnson and Lobos, 2010 www.schepens.harvard.edu 2010 www.whyfiles.org 2010 www.WHO.int 2010