Anatomy & Physiology I and II

Anatomy & Physiology Study Guide

©2018 Achieve Test Prep Page 194 of 367 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 special organic compounds called visual pigments . The absorption of photons by visual pigments is the first key step in the process of photoreception—the detection of light. P otoreception The plasma membrane in the outer segment of the photoreceptor contains chemically gated sodium ion channels. Because the channels are open, the transmembrane potential is approximately –40 mV, rather than the –70 mV typical of resting neurons. At the –40 mV transmembrane potential, the photoreceptor is continuously releasing neurotransmitters (in this case, glutamate) across synapses in the inner segment. Sodium ions are constantly extracted from the cytoplasm by the inner segment. The transportation of sodium ions into the outer segment to the inner segment and out of the cell is known as the dark current. 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 C nes