This is the reason why retinal detachments never extend beyond the ora serrata. The retina is firmly attached to the choroid at the ora serrata. In the extreme periphery of the retina there are no cones and only a few rods. The junction of the periphery of the retina and the ciliary body is called the ora serrata. Damage to these structures results in night blindness but with retention of good visual acuity for objects straight ahead. The rods are distributed in the periphery of the retina (not in the macula). Damage to this area can severely reduce the ability to see directly ahead. The cones form a concentrated area in the retina known as the fovea, which lies in the center of the macula lutea. Color vision is totally dependent on the integrity of the cones. ![]() Cones enable us to see small visual angles with great acuity. The cones number only about 6 million, whereas the rods number 125 million. The rods function best in dim light the cones function best under daylight conditions. The retinal receptors are divided into two main populations: the rods and the cones. The retina, which contains all the sensory receptors for the transmission of light, is really part of the brain. In The Ophthalmic Assistant (Ninth Edition), 2013 Retina In particular, the salamander retina has very large cells that ease recording. 4 We will use the mammalian retina as a model but many pioneering studies have been conducted using fish and amphibian retina. It has been characterized by Dowling as an approachable part of the brain. The retina is a ready-made brain slice which is important because there are few barriers to prevent penetration of drugs or antibodies. Light provides a rich, yet well controlled, input and it is the correct physiological stimulus to the retina. There are many experimental advantages in using retina, not least the use of light as a stimulus. It may in fact be useful to think of the retina in two parts 3: the sensory retina, concerned with phototransduction by rods and cones and the neural retina, consisting of more typical neurons which carry out the first steps in processing this visual information. An example would be the arrestin knockout mouse which appears healthy but has slow, prolonged responses to visual stimuli. Therefore, the litters may be healthy not withstanding a phenotype with a visual deficit or even blindness. Transgenic animals with mutations in the phototransduction cascade are not stillborn because usually, these proteins are only expressed in retina. ![]() Even the exception of the photoreceptors provides an experimental advantage. As we shall see, many neurons use glutamate as a neurotransmitter, sometimes in novel ways. To all intents and purposes, it is the same receptor, a common building block of the CNS. Sequencing of the GluR1 receptor from a mouse retina library reveals homology with the receptor from mouse cerebellum and rat brain. As in other parts of the CNS, glutamate is the major excitatory neurotransmitter in the retina. ![]() In other words, it is a piece of brain, made from the same components, apart from the specialized structures required for photoreceptors. The retina is part of the central nervous system (CNS), embryologically derived from the neural tube. Massey, in Retina (Fourth Edition), 2006 THE RETINA IS A PIECE OF BRAIN 123-5).Īpart from the aqueous outflow via the trabecular meshwork, there are the following outflow channels from the eye: uveoscleral outflow, vitreoretinal-choroidal outflow, sclerochoroidomeningeal outflow, and transscleral outflow. The neural retina, by offering resistance to flow, is pushed against the pigment epithelium 2 ( Fig. It is now accepted that fluid movement from the vitreous across the retina into the choriocapillaris is a major force keeping the retina attached. Much more significant forces are generated as a consequence of the dynamics of fluid flow within the retina.Ĭlinical and experimental evidence has shown that more force was needed to peel the retina in vivo than postmortem. The vitreous does not lend mechanical support to the retina – a fact confirmed clinically by the experience that few posterior vitreous detachments lead to retinal detachments. Anatomic adhesion is considered to play only a small part. ![]() The retina remains attached to the choroid because of a variety of mechanisms: these are discussed in detail in Chapter 116. Tasman, in Retina (Fourth Edition), 2006 Mechanisms of retinal attachment
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