Zoology 250 — Laboratory 10: Eye structure and EOGs
This laboratory consists to two parts. One part will involve dissection of an eye to become familiar with their structure, while the second part will be a study of eye movements that you may not be aware of. You should work in pairs to do this, with one half of your table doing the dissection while the other two people examine eye movements. Following this, you should switch places.
Eye Structure
The vertebrate eye is a remarkable, although somewhat quirky structure. It originally evolved from a small and simple photoreceptive structure on the outer surface of the body (much like the planarian ‘eye’ shown in figure 45-4 of Campbell). It is now a complex structure in animals like ourselves, but retains traces of this humble ancestry. The clearest example of this is the way neural information travels from the actual photoreceptors to the brain. It’s really like building a beautifully sensitive photoreceptor, then running all the wires and supporting structure out in front of the receptive surface where light comes from rather than out the back where it would not interfere with anything. More on this below.
The eye transduces electromagnetic signals in the visual range of the spectrum (light) into action potentials in the optic nerve. This is a difficult job for many reasons. First, light received from a large visual field passes from a low density medium that does not refract (bend) the light much into a high density medium that refracts it a great deal. This bending of the light is important since all light originating from a given point source in the outer world must converge on a single spot on the photoreceptive surface of the eye (the retina) in order for a focused image to be perceived. If these light rays are not brought back together in precisely the same places, a blurred image results. This is what happens with near- and far-sightedness.
External Anatomy
Most of the outer surface of the eye (~2/3) has a tough outer covering called the sclera. Approximately one third of this outer surface is instead covered by the transparent cornea. The cornea forms the clear front ‘window’ of the eye. It is made up of five layers of strong clear tissue. The first layer (epithelium) is made up of rapidly-replaced cells that allow for fast healing (24-48 hrs) of surface injuries. The last four layers add rigidity, provide a barrier against infection and keep the cornea clear.
The job of focusing images relies on more than one structure, but many people are surprised at which one accounts for most vision problems of this nature. It is the first structure to bend light and the one which bends light the most since this is where light moves from air into the much denser medium of the eye. Note on the diagram above where most of the light bending is occurring. Because the cornea accounts for most of this bending (~80%) and the light bending depends on the proper curvature of the eye, improper curvatures (too flat, too rounded) can cause vision problems. These problems can often be corrected by altering this curvature (e.g., radial keratotomy flattens the cornea).
Following the outer surface of the eye, find the optic nerve. This is a bundle of fibers traveling from the retina and contains approximately 1.2 million axons in humans. Note how this structure enters the eye as a very compact bundle. It then spreads out on the inside to form the retina.
The Iris is found at the front of the eye and gives the eye its color. It contains muscles which open or close the pupil (increase or decrease its diameter) in response to the brightness of surrounding light. Factoid: A blue iris in humans actually has a lack of pigment.
The pupil is essentially just a hole in the iris. It regulates the amount of light entering the eye.
Internal Anatomy
You will first explore the different layers of the eye away from the area where light enters, then open the eye further to examine the interior architecture and photoreceptive surface.
On the dorsal surface, use a razor or scalpel to cut a small window in the eye. You will find that it has three distinct layers. The sclera is formed of tough, dense connective tissue and fibrous. You have already seen this from the outside. The next layer is the vascular tunic or choroid. This is a spongy layer that contains blood vessels and helps to nourish the retina which lies above it. The retina is a fascinating structure and actually represents an outpocketing of the brain itself. The retina is the actual photoreceptive surface. As such, it is the site where electromagnetic energy in the form of light is converted into electrical signals in the form of nerve impulses. The retina contains specialized photoreceptors referred to as rods and cones. Rods confer black and white vision and are quite sensitive. Cones give us color vision and are less sensitive, but help form finer images. Cones are densest in a small area called the fovea. When we look directly at something, the light rays converge on the fovea.
You will now extend the cut you have started to divide the eye into an anterior and a posterior half. To do this, cut around the ‘equator’ of the eye with a small pair of scissors as shown by the dotted line above.
One important and unusual feature of vertebrate eyes is seen on the diagram above and should be visible on your dissected eye. The optic nerve enters the eye and the fibers then spread out to innervate the photoreceptors of the retina from the inside. There are also some blood vessels above the photoreceptive surface. What this means in a functional sense is that light actually has to pass through this layer of neurons and also some blood vessels. What it also means is that there can be no photoreceptors where the nerve leaves the eye. This is an area referred to as the optic disc. No photoreceptors = no visual information from this spot. This lack of input can actually be seen if you close one eye and focus on an ‘X’ while moving something else off the center of your vision (use the sheets with X’s and dots provided on your tables to ‘see’ your blind spot). Using a small penlight pressed to the side of your eye, you are also able to see the vasculature within your eye. Try this as well.
Immediately behind cornea is the Anterior Chamber. This is the space between the cornea and the iris. It is filled with filled with a fluid termed aqueous humor.
The lens is a clear biconvex structure (rounded surfaces on either side) behind the iris which works in conjunction with the cornea to focus light onto the retina. When the lens becomes opaque or cloudy, vision is affected and the condition is termed a cataract. The lens functions in focussing or accommodation. Through fine scale adjustments of lens shape, the bending of light passing through it is altered to ensure light from a single external source (e.g., the tip of your lab partner’s nose) converges to the same spot on the retina. Lens shape is altered by muscles of the ciliary body, a ring of tissue at the base of the iris and attached to the lens. Contraction of these muscles pulls the lens, flattening it and allowing us to focus on more distant objects. Relaxation of these muscles allows the eye to become more rounded and allowing accommodation to objects which are close. The ciliary body also produces the aqueous humor.
Behind the lens is another chamber filled with fluid. More specifically, it is filled with vitreous humor, which is more ‘jelly-like’ than the aqueous humor. Roughly 80% of the volume of the eye is filled with vitreous humor.
The
famous chemist John Dalton was color blind. He speculated that his color
blindness resulted from a blue-tinted vitreous humor and directed that
this hypothesis be tested upon his death by dissecting his eyes out and
having a look. He was wrong: his vitreous humor was clear like yours and
mine. It took about 200 years and some nifty molecular biology to really
figure out why he was color blind. More on this in lecture.
Part 2 — Eye Movements
You have looked at or will look at the structure of the eye here to begin learning how light is transduced into nerve impulses. In order to gain information, you also need to control what light reaches your retina. You do this in two different ways. The first of these is termed Voluntary Fixation. This entails directing your visual attention to a specific object and fixing it there. Once directed to a specific object, a second mechanism maintains attention on that object until the eye is directed elsewhere. This is termed Involuntary Fixation and is an unconscious process.
Direction of the eyes is accomplished by several extraocular muscles which control both their horizontal and vertical movements. Because the eyes shows electrical potential differences between the front and rear due to the nature of their electrical activity (they actually are a battery of sorts), we can measure their movements by measuring changes in this electrical activity at the skin surface. Such a recording is referred to as an electrooculogram or ‘EOG’. Using this measurement technique, you will examine both voluntary and involuntary eye movements. The involuntary eye movements fall into three categories. These are the following:
You will have specific directions for measuring the EOG with the Biopac apparatus in your lab. You will also receive a worksheet for your data and questions to hand in your laboratory section.