The outer segments of the photoreceptors are connected by a modified cilium to the inner segments, which contain a number of organelles, including numerous mitochondria. The inner segments are the sites where the visual pigment is synthesized before it is incorporated into the membranes of the outer segment. In rods, the pigment is inserted into new membranous discs, which are then displaced distally until they are eventually shed at the apex of the outer segment, where they undergo phagocytosis by cells of the pigmented
CHAPTER 8 The Special Senses •Fig. 8.4 Graph of a Plot of the Density of Cones and Rods as a Function of Retinal Eccentricity From the Fovea. Notethatconedensitypeaksatthefovea,roddensitypeaksatabout20degreeseccentricity,andnophotoreceptorsarefoundattheopticdisc,wheretheganglioncellaxonsleavetoformtheopticnerve.(DatafromCornsweetTN.Visual Perception. NewYork:AcademicPress;1970.) epithelium. This process determines the rod-like shape of the outer segments of rods. In cones, the visual pigment is inserted randomly into the membranous folds of the outer segment, and shedding, comparable to that seen in rods, does not take place.
Regional Variations in the Retina
The macula lutea is the area of central vision and is characterized by a slight thickening and a pale color. The thickness is due to the high concentration of photoreceptors and interneurons, which are needed for high-resolution vision. It is pale because both optic nerve fibers and blood vessels are routed around it.
The fovea, which is a depression in the macula lutea, is the region of the retina with the very highest visual resolution and, as noted previously, the light from the fixation point is focused on the fovea. (A major function of eye movements is to bring objects of interest into view on the fovea.) The retinal layers in the foveal region are unusual because several of them appear to be pushed aside into the surrounding macula. Consequently, light can reach the foveal photoreceptors without having to pass through the inner layers of the retina, and both image distortion and light loss are minimized. The fovea has cones with unusually long and thin outer segments, which allows for high packing density. In fact, cone density is maximal in the fovea, providing for high visual resolution, as well as high quality of the image (
Fig. 8.4
The optic disc, where ganglion cell axons leave the retina, lacks photoreceptors and therefore lacks photosensitivity. Thus it is a so-called blind spot in the visual surface of the retina (see
Figs. 8.4 8.9 ). A person is normally unaware of the blind spot because the corresponding part of the visual field can be seen by the contralateral eye and because of the psychological process in which incomplete visual images tend to be completed perceptually.
Asmentioned,theaxonsofretinalganglioncellscrosstheretinaintheopticfiberlayer(layer9)toentertheopticnerveattheopticdisc.Theseaxonsintheopticfiberlayerpassaroundthemaculaandfovea,asdothebloodvesselsthatsupplytheinnerlayersoftheretina.Theopticdisccanbevisualizedonphysicalexaminationwithanophthalmoscope. Thenormalopticdischasaslightdepressioninitscenter.Changesintheappearanceoftheopticdiscareimportantclinically.Forexample,thedepressionmaybeexaggeratedbylossofganglioncellaxons(optic atrophy), ortheopticdiscmayprotrudeintothevitreousspacebecauseofedema(papilledema) thatresultsfromincreasedintracranialpressure.