Human Retina - Introduction

Tutorial 23: Human Retina

Part 1: Image-Mapped Tutorial
Part 2: Matching Self-Test
Part 3: Multiple-Choice Self-Test

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The retina forms the inside lining of the back of the eye and is composed of light-sensitive neurons. The retina processes the light emitted from visual images via transduction (the conversion of energy from one form to another), and transmits this information to the brain for perceptual awareness of the images. The light must pass through all layers of the retina before it reaches the cells that systematically absorb the light energy. By the time light that has passed into the eye through the cornea reaches the retina, almost 90% has been scattered or otherwise lost.

The retina is organized into three primary layers, the photoreceptive layer, the bipolar cell layer, and the ganglion cell layer. The first layer, or photoreceptive layer, is made up of rods and cones. The rods and cones synapse with the bipolar cells in the second layer of the retina. Bipolar cells send appendages to communicate with both the first and third layers. The axons of the ganglion cells found in the third layer of the retina convey the visual information as encoded by the retina to the next synapse point in the visual pathway via the optic nerve. Amacrine and horizontal cells located in the bipolar cell layer contribute to the finer processing of visual information via lateral connections. These lateral connections modulate the transmission of information across the synaptic layers of the retina, between the first and second layers and between the second and third layers. This complex network of neurons takes the transduced visual information and processes it via encoding, compression, integration, and convergence.

The route of visual information within the retina is intuitively backward. After the light enters and reaches the back wall of the eyeball, it is deflected forward and is absorbed by the receptor cells, which point toward the back of the eyeball. Once absorbed by the receptors, the information travels a route toward the middle of the eye and away from its destination, the brain. As information is conveyed through the three primary layers of the retina, it moves toward the front of the eye. The axons of the ganglion cells of the third layer extend across the inner surface of the retina on their route to a hole at the rear of the eyeball called the optic disk. Ganglion cell axons exit the eye via the optic disk as the optic nerve (cranial nerve II).

The receptor cells or rods and cones detect light energy and absorb this energy for use by the nervous system. Rods and cones mediate different levels of vision, scotopic (high sensitivity, low acuity) and photopic (low sensitivity, high acuity), respectively. This contrast in function is called the duplexity theory. Sensitivity increases with the increased ability to detect objects in dim lighting. The high sensitivity vision provided by the rods is due to the high degree of convergence in the rod circuitry. The information from numerous rods cells converges on a single ganglion cell for further processing, whereas information from only one cone cell is processed and transmitted by a given ganglion cell.

The collection of rods and cones that convey signals to a particular ganglion cell of the retina and then ultimately to a neuron within the primary visual cortex, comprise that visual cell's receptive field. The portion of the visual field that a receptor cell responds to, will help define the receptive field of all the visual cells to which the receptor cell projects. When a collection of receptor cells converge on a given visual cell deeper in the visual pathway, the pattern of excitation increases in complexity. Although the receptive fields of visual cells found throughout the visual pathways come in a variety of complex shapes and sizes, they are typically arranged in two concentric circles with light falling on the center opposing the effect of light falling on the outside circle. This arrangement is called "center-surround". Much is known about the receptive fields of ganglion cells of the retina, which respond to the center surround arrangement. Sometimes the stimulation of receptor cells in the center of a particular visual field will result in the excitation of an impulse or depolarization of the ganglion cell to which it projects. For the same center-surround receptive field, stimulation of receptor cells in the surrounding circle of the visual field will result in the inhibition of an impulse or hyperpolarization of the ganglion cell. Other times the excitatory and inhibitory influence of the concentric circles in a visual field is reversed, so that stimulation of the center results in inhibition of the ganglion cell and stimulation of the peripheral circle results in excitation of the ganglion cell. The center-surround response is mediated by the lateral connections of the horizontal and amacrine cells found in the second primary layer of the retina.

Figure 23 illustrates the layers of cells that compose the retina. This tutorial describes the structural and functional relationships that underlie the reception and transduction of light into a neural code that is then processed by higher centers along the visual pathways.

Advanced

In more detailed descriptions of the retina, the three primary retinal layers are further divided into seven layers (Gray, 1918). For example, in one seven layer organization the following are distinguished: dark "nuclear/cell" layers containing cell bodies and white "plexiforn" layers containing axons and dendrites. The first layer contains the pigment end of the receptor cells. The second, outer nuclear layer (ONL) contains cell bodies of the receptor cells. The third, outer plexiform layer (OPL) contains the dendrites and axons of the receptor, horizontal, and bipolar cells. The fourth, inner nuclear layer (INL) contains the dendrites and axons of the horizontal, bipolar, and amacrine cells. The fifth, ganglion cell layer (GCL) contains the cell bodies of ganglion cells. Finally, the optic fiber layer (OFL) contains the axons of ganglion cells as they collect to form the optic nerve. The pigment epithelium, which lines the back of the eyeball behind the receptor cells, is composed of a single layer of cells. This layer is normally fused to the retina, but may separate causing the condition known as a "detached" retina.

Reference

Gray, Henry. (1918). The Organs of the Senses and the Common Integument, The Tunics of the Eye. Anatomy of the Human. Retrieved from World Wide Web: Bartleby.com - Great Books Online. (2000). http://www.bartleby.com/107/225.html

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