Structure and Function of the Cephalopod Eye 2
Image 5: The Cephalopod Eye and Retina Structure (Image used by permission of Dr. Paul Patton-University of Illinois at UC; Patton, P. The Visual System II. http://soma.npa.uiuc.edu/courses/bio303/Ch11b.html. October 4, 2003. Taken and adapted from Land, M.F. 1981. Handbook of Sensory Physiology: “Optics and Vision in Invertebrates.” Ed. H. Autrum. Vol. 2, Book 6. Springer-Verlag, Berlin.
Image 6: A Microscopic view of the Human Retina. (Image used by permission of Robert Rodieck, from The First Steps in Seeing. Sinauer Associates: Massachusetts. 1998.)
Returning to the retina, it is obvious to see in the figures above, that the position of the photoreceptors in each eye is not the only difference between the two. The photoreceptors in the retina of the vertebrate eye consist of rods and cones. Rods are responsible for collecting dim intensities of light and are the main photoreceptors in night vision. Cones on the other hand, collect high intensity light and are responsible for producing color vision (Rodieck 1998, Campbell et al. 1999). Cephalopods, lack rods and cones, and instead have photoreceptors called rhabdomeres. There are between 20,000- 105,000 plus rhabdomeres per mm^2 in the cephalopod eye (Muntz 1991). Each photoreceptor consists of an outer segment and an inner segment that is divided by a basement membrane. The outer segment is long in length, equal in size, and points toward the lens (Wells 1978, Young 1971). The interior of each outer segment has a central core with an axis that runs down the middle of it. On the sides of each central axes, are a series of rectangular projections that are layered on top of each other and are at right angles to the central axes (Wells 1962 and 1978). This arrangement causes the functional unit of the retina to appear not as an individual rhabdomere, but as a larger structure made up of four rhabdomeres, a rhabdome (Young 1971).
Within each rhabdomeres are pigments that are structurally and functionally different than those found in the vertebrate's rods and cones. The main visual pigment in both eye types is a rhodopsin, but the amino acid structure of each animal's rhodopsin is different. Their secondary pigments are different as well. The secondary pigment of a cephalopod is retinochrome, which is located toward the basement membrane of the outer segment, while the vertebrate's secondary pigment, photopsins, is located within the cones of their retina (Campbell et al. 1999, Davson 1972, Wells 1978). Because cephalopods do not have any structures equivalent to the vertebrate cone and photpsin they can not detect colors and can only see in black-and-white. Interestingly though, the pigments in the rhabdomeres are not stationary and actually move in response to light. To aid in light reception, the pigments move toward the top of the photoreceptor cells when light is present and back towards the basement membrane when it is dark (Young 1971). Sandwiched between each photoreceptor of the cephalopod eye are supporting cells that rest on the basement member and contain pigments that help collect light as well. At the end of each photoreceptor's inner segments, is an axon, that unites with axons from other photoreceptor cells in the cephalopods choroid and enters into the sclera (Ali 1984, Wells 1978, Young 1971). These axons gather together to form the optic nerve, which leads to the optic lobes and then onto the brain (Ichikawa et al. 1994).
Based on the diagrams above, it also becomes evident that the retina of the cephalopod eye has far fewer parts than the retina of the vertebrate eye. In the vertebrate retina, besides photoreceptor cells, their are ganglion, bipolar, or amacrine cells (Campbell et al 1999, Davson 1972). However this does not mean that the retina of the vertebrate eye is any more advanced or functionally superior to the cephalopod retina. Both have the ability to collect light and transmit action potentials equally well. Cephalopods in fact, do not completely lack the cell types mentioned above. The different cells have instead been placed in the outer layers of the optic lobe (Wells 1978). Depending on the species, the cephalopod optic lobe has anywhere from four to five parts. There is an outer cell body layer, a neurophil layer, another cell layer, and then another neurophil layer followed by a cell body layer, or a central medulla (Ichikawa et al. 1994) .The optic lobe of the cephalopod is located directly behind the eye in the eye socket and is exceptionally large in size. It is so large in fact, that it makes up over half of the nervous tissue of the brain (Wells 1962). At this time, little is under stood about what happens to the action potential once it reaches the inner parts of the optic lobe.
Image 7 The interior sections of the cephalopod optic lobe. (Image used by permission of The Netherlands Journal of Zoology; from Ichikawa, M., G. Matsumoto, R. Williamson. 1994. Neuronal Circuits in Cephalopod Vision. Netherlands Journal of Zoology. 44(3-4): 272-283.
Development of the Vertebrate and Cephalopod Eye The Cephalopod Eye 1
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