A. An overview of the function of the eye:
Light enters the pupil, is focused and inverted by the cornea and lens, and is projected onto the back of the eye. At the back of the eye lies the retina, seven layers of alternating cells and processes which convert a light signal into a neural signal (“signal transduction”). The actual photoreceptors are the rods and cones, but the cells that transmit to the brain are the ganglion cells. The axons of these ganglion cells make up the optic nerve, the single route by which information leaves the eye.
B. Structures at the anterior pole of the eye:
|Moving parts of the eye:1. The iris is really a shutter that can be closed down to regulate the amount of light entering the eye. This process is controlled by two muscles with distinct innervation:– the pupillary sphincter muscle constricts the pupil like a purse-string, and is under the control of the parasympathetic system. Therefore it is innervated by fibers from the oculomotor nerve which originate in the Edinger-Westphal nucleus of the midbrain.
– the pupillary dilator muscle is composed of radial fibers which pull the pupil open, and is controlled by the sympathetic system. Therefore it is innervated by post-ganglionic sympathetics from the superior cervical ganglion. Remember that the pre-ganglionics come from T1.
2. The lens is a naturally elastic structure. If it had its way, it would round up into a more spherical shape. Under normal conditions, however, an array of radial fibers – the zonule fibers – hold the lens stretched out into a more disc-like shape. This shape allows for far-focusing. What happens when you need to near-focus? At this point the ciliary body, a hoop-like structure that supports the zonule fibers, comes into play. Imagine a spiderweb built into the opening of a drawstring purse, suspending a disk in the opening. When the purse is open, the spiderweb is taut. If you pull the drawstring, however, the web will go slack and collapse on itself. The ciliary body is the drawstring purse, in this analogy. The ciliary muscle within it is the drawstring. When the ciliary muscle contracts (this is also under parasympathetic control), the zonule fibers go slack, the suspended lens is released from their tension, and it is free to round up. This change is necessary for near-focusing. The entire process of adjusting the focus to different distances is called accommodation.
C. The retina:
Trace the signal through the retina:
– Light enters from the GCL side first, and must penetrate all cell types before reaching the rods and cones.
– The outer segments of the rods and cones transduce the light and send the signal through the cell bodies of the ONL and out to their axons.
– In the OPL photoreceptor axons contact the dendrites of bipolar cells and horizontal cells. Horizontal cells are interneurons which aid in signal processing.
– The bipolar cells in the INL process input from photoreceptors and horizontal cells, and transmit the signal to their axons.
– In the IPL, bipolar axons contact ganglion cell dendrites and amacrine cells, another class of interneurons.
– The ganglion cells of the GCL send their axons through the OFL to the optic disk to make up the optic nerve. They travel all the way to the lateral geniculate nucleus.
D. Specializations of the retina:
|The fovea defines the center of the retina, and is the region of highest visual acuity. The fovea is directed towards whatever object you wish to study most closely – this sentence, at the moment. In the fovea there are almost exclusively cones, and they are at their highest density.|
|The ratio of ganglion cells : photoreceptors is about 2 :1 here, the highest in the eye. In addition, at the fovea all of the other cell types squeeze out of the way to allow the most light to hit the cones. This makes the fovea visible microscopically. The blood vessels also skirt a wide margin around the fovea. The area in and around the fovea has a pale yellow pigmentation that is visible through an ophthalmoscope, and is called the macula.|
The ganglion cell axons all leave the eyeball at one location, the optic disk. At the optic disk all photoreceptors and accessory cells are pushed aside so the axons can penetrate the choroid and the sclera. This creates a hole in our vision, the blind spot. Normally each eye covers for the blind spot of the other, and the brain fills in missing information with whatever pattern surrounds the hole. Therefore we are not conscious of the blind spot.
Photoreceptors are not distributed evenly throughout the retina. Most cones lie in the fovea, whereas peripheral vision is dominated by rods. Overall, rods greatly outnumber cones. Review the characteristics of rods (black and white vision, very sensitive to low light) and cones (color vision, not so sensitive) and explain these phenomena:
1. To see a faint star, you cannot look directly at it, but must look slightly to the side.
2. A person with macular degeneration can become functionally blind, yet their night vision is not really affected. How would their color perception be?
E. Interesting anatomical facts:
– The cornea is continuous with the sclera, which in turn is continuous with the dura.
– The choroid, a highly vascular, highly pigmented layer between the sclera and the retina, is continuous with the ciliary body and the iris. Do not confuse it with the pigment epithelium.
– The pigment epithelium is a single cell layer thick, and comes from the outer layer of the original optic cup (a classic embryological “pushed-in ball”). In the mature retina it is pushed directly up next to the neural retina, which came from the inner layer of the optic cup. They are not fused together, however, and can separate along the old plane – a “separated” or “detached” retina.
F. Signal processing in the retina – the center-surround receptive field.
If you were to record from a photoreceptor, you would find that it was “ON” (hyperpolarized, paradoxically) whenever light shone on it. If you recorded from a ganglion cell instead, you would find that diffuse light did little to the cell. However, the cell would respond well to a small spot of light, a small ring of light, or a light-dark edge. We say that this cell has a center-surround receptive field – the center must be mainly light and the surround mainly dark, or vice versa. What happens between the outer segment and the ganglion cell? This complex receptive field is created by the interneurons of the retina: the bipolar cells and the horizontal cells, primarily.
Let’s trace a signal through:
|1. Light hyperpolarizes the cone (or rod). For simplicity’s sake, we will just say that turns ON the cone, and thereby excites the bipolar cell directly underneath. That bipolar cell then excites its ganglion cell.The same thing is happening to neighbor cells.
2. However, here’s the trick. The neighbor cones also excite horizontal cells. The horizontal cells send processes laterally and inhibit the center bipolar cell.So, what does diffuse light do? It excites the central bipolar cell, but also inhibits it via the neighbors. Result – the ganglion cell does not get excited. It continues to tick along at its normal, tonic rate.
3. A small spot of light, however, excites the bipolar cell but not its neighbors. There is no inhibition, so it is free to get really excited and excite the ganglion cell, which fires like crazy.
4. A ring of light excites only the neighbors. Now, the bipolar cell is strongly inhibited, with no excitation. In response to this strong silencing of the bipolar cell, the ganglion cell shuts down as well. It will not turn on again until the light is turned off, at which time you will see a rebound “off-response”.
This is an ON-center cell.
The reverse of this entire scenario can be created by reversing all the signals (which we can do with different receptors to the same neurotransmitter) – you then have an OFF-center cell.
This unique center-surround receptive field is also a property of lateral geniculate neurons. Things get even more complicated up in the cortex.
What is the point? Well, our entire visual system exists to see borders and contours. We see the world as a pattern of lines, even things as complex as a face. We judge colors and brightness by comparison, not by any absolute scale. (Don’t believe it? Put that teal scarf next to a blue shirt, you’ll call it green. On a green coat, you’ll call it blue.) This system of lateral inhibition in the retina is the first step towards sharpening contours and picking up on borders between light and dark. Diffuse light is ignored by the ganglion cell, but a sharp dot will really turn it on. Higher up in the cortex, all these dots will be combined into lines, which will be combined into curves, etc, etc.