Anatomy & Physiology

Anatomy & Physiology Study Guide limit of clear vision, called the near point of vision, is calculated by the degree of elasticity in the lens. Children can usually focus on something 7–9 cm from the eye, but the lens tends gradually becomes stiffer and less responsive. A young adult can focus on objects 15–20 cm away. As aging occurs, this distance increases; the near point at age 60 is typically about 83 cm. If light traveling through the cornea and lens is not refracted properly, the visual image will be distorted. In the condition called astigmatism , the degree of curvature in the cornea or lens varies from one axis to another. Image Reversal An object in view is a complex light source that must be regarded as a large number of solitary points. Light from each point is centered on the retina. The result is the creation of a miniature image of the original, but the image arrives upside down and reversed. The brain compensates for this image reversal, and we are not aware of any difference between the orientation of the image on the retina and that of the object. Visual Acuity Clarity of vision, or visual acuity, is evaluated against a “normal” standard. The standard vision rating of 20/20 is defined as the level of detail seen at a distance of 20 feet by an individual with normal vision. Vision rated as 20/15 is better than average because at 20 feet the person can see details that would be clear to a normal eye only at a distance of 15 feet. When visual acuity falls below 20/200, even with the help of glasses or contact lenses, the individual is considered to be legally blind. The term blindness indicates a total absence of vision due to damage to the eyes or the optic pathways. Abnormal blind spots, or scotomas , may appear in the field of vision at positions other than at the optic disc. Visual Physiology The rods and cones of the retina are called photoreceptors because they detect photons, basic units of visible light. Light energy is a form of radiant energy that travels in waves with a characteristic wavelength (distance between wave peaks). Our eyes are sensitive to wavelengths of 700–400 nm, the spectrum of visible light. This spectrum, seen in a rainbow, can be remembered by the acronym ROY G BIV (Red, Orange, Yellow, Green, Blue, Indigo, Violet). Photons of red light carry the least energy and have the longest wavelength, and those from the violet portion of the spectrum contain the most energy and have the shortest wavelength. Rods supply the central nervous system with information about the presence or absence of photons, with little regard to their wavelength. Cones provide information about the wavelength of arriving photons, giving us a perception of color. Rods and Cones The elongated outer segment of a photoreceptor contains hundreds to thousands of flattened membranous plates, or discs. The names rod and cone refer to the outer segment’s shape. In a rod, each disc is an independent entity, and the outer segment forms an elongated cylinder. In a cone, the discs are infoldings of the plasma membrane, and the outer segment tapers to a blunt point. The discs of the outer segments of rods and cones have Achieve Page 194 of 368 ©2018