Biology 202
1999 Second Web Reports
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Human Eye Model As mentioned before, the visual pathway consists of an elaborate network of neurons. In order to discuss photosensitivity, first we must look at the manner in which information is processed in the eye and relayed to the brain with the use of photoreceptors.

The development of the eye is very unique: it develops in an inside to out manner(1). From the rostral to caudal portion, the retina consists of the epithelium layer, photoreceptors, horizontal cells, amacrine cells and the ganglion cells (the ganglion endings make up the optic nerve). The structure is considered inside/out because light needs to pass through several layers before converging on the photoreceptors. The role and properties of the pupil and photoreceptors are key structures in relation to albinism. In the human eye, the pupil is made of melanin. Melanin is a photoreceptive pigment; when present it protects the eyes from ultraviolet light(2). After light passes through the pupil, cornea and other structures it converges on the back of the eye known as the retina. The retina is comprised of various cell which aid in the conversion of peripheral light to electrical/chemical energy which will be sent to the brain.

At the back of the eye is the epithelium layer and the photoreceptors. One of the functions of the epithelium layer, which has melanin, is to absorb photons of light entering the eye. This is a way in which the eye prevents light from stimulating the retina more than once. Cats for example, do not have as much melanin in their epithelium layer which explains why light bounces from objects to a cat’s eye and back again. The next layer is the photoreceptors which consists of cones and rods. The processes of absorbing the light source from the environment are primarily done in the outer segment of the rods. The rod photopigment which absorbs the light is called rhodopsin. Rhodopsin has two parts: a protein called opsin and the molecule retinal which are connected. Retinal takes on two appearances, a bent form when bound with opsin and a straight form when in contact with light. This process of converting light energy to chemical energy is known as transduction. Signals are sent to the ganglion cells, the ‘final output neurons(4), whose axons (which make up the optic nerve) terminate in the visual cortex for processing.

Range of Light The human eye is able to withstand dim to bright light because of, but not limited to, the hyperpolarization of sodium channels (Na+) as well as the pupil’s ability to constrict and expand itself to accommodate for the level of light present. The Na+ channels hyperpolarize and depolarize in accordance to whether the environment is light or dark. For example, displacement from a dark to light environment like a sleeping state to a sunlit usually there is a brief moment of blindness where it is necessary to shut one’s eyes in order to adjust to the change in light intensity. When the eye is in a dark environment, cells adjust to the light level by opening sodium channels. Upon entering a well lit room, the eyes, which were adapted to the dark environment are suddenly bombarded with a bright light source that causes the cell to maximally hyperpolarize. During hyperpolarization, as Na+ channels close the cell becomes unresponsive to incoming information. With the lack of incoming information from the hyperpolarized eye, behavior is temporarily disrupted and the individual is temporarily blinded.

From this simplified model of light adaptation, the brain is relying on properly functioning photoreceptors for reception and the ganglion cells for transmittance of electrical signals to continually receive information form the periphery. A faulty visual pathway can inductively affect human behavioral input and output if a mishap occurs.

Albinism Using the human vision model for structure and light adaptation we can hypothesize what might be occurring in the visual pathway of experimental albino rats. It is plausible to attempt a connection between humans and rats because of similar phenotypic appearance: albino rats have pale white fur, red eyes and a pale pink-colored tail; albino humans(5) have pale skin, hair and eyes. Since albino rats and humans share similar physical characteristics it may be the fact that they also have similar defaults in their visual pathway to result in photosensitivity.

Albino rats are incapable of taking in substantial amounts of light, which suggests that the problem may be in the rods’ capability to hyperpolarize sodium channels. As mentioned earlier, the hyperpolarization and depolarization of sodium helps the eye in adjusting to light levels. Through analyzing the roles of Ca+ (calcium) and cGMP (cyclic guanosine monophosphate) in vision, a conjecture can be instituted.

In a dark room both Na+ and Ca2+ enter cGMP gated channels within the eye. Calcium ions have a regulatory effect on cGMP: Ca2+ blocks the synthetic enzyme which create cGMP called guanylate cyclase. As Ca2+ continuously enters the cell it prevents cGMP production. As you enter a sunlit room, the photoreceptors in the retina are bombarded with light. This abundant amount of light onto the photoreceptors breaks down all the cGMP channels and hyperpolarizes the cell maximally (3) ( recall that this is temporary blindness). Photosensitivity in humans is controlled with the use of melanin. Albino rats lack melanin, in their pupils and epithelium layer, which aids in the ability to withstand sunlight. In addition to insufficient amounts of photopigment, albino rats may have a problem in retinal interaction. The interaction amongst the rods, cones and varying channels are the structures which give us the ability to decipher ranges of light. Humans use rods for dim light and cones for bright light. Taking the human model, two main differences between an albino rat and ‘normal’ human’s visual system is the amount of melanin and the proper functioning between rods, cones and their interaction with specific channels.

It is interesting to note this role reversal of ‘human model’ to observe animal behavior because it demonstrates that species other than the Homo Sapiens have a similar network of neurons which perform basically the same function in vision. Trying to connect albino rats and humans exemplifies the manner in which behavioral output is affected by a faulty connection or an insufficient amount of a specific photopigment. Even though abundant research has not been done on the amount of neurotransmitters in the visual system of albino rats, this comparison seems to suggest that humans and rats are interrelated. This demonstrates that two mammals do not need to be equivalent in behavioral productivity but have a common use in an evolutionary pathway.

WWW Sources

2)Albinism

4) Visual Responses of Ganglion Cells

5)Albinism’ (a picture of an albino child)

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