AUSTRALIAN NATUROPATHIC NETWORK
 Serving the community since 1998

I am a photon traveling through space and time at the speed of light on a bright day. I am a blue coloured photon with quite a lot of energy. the only problem is I have little control over my direction and, oops, splat! I am going into somebody’s eye ball.

First I pass through a glassy covering that slows me down a bit. This is the cornea which changes my direction and slightly aims me. I am now passing through some aqueus fluid that slows me down even more!

As I get nearer I notice several of my colleagues bouncing off the white sclera, and coloured iris, but I am directed straight for a big black hole in the middle of the iris called the pupil. The pupil is very small today – hey that’s right it is very bright.

Passing through the pupil I reach another transparent layer it changes my direction yet again. This must be the lens. Through the lens, and now into some jelly like stuff, the inside of the eyeball – the vitreous body. Oh no there is a big wall coming up and I am about to slam into it!

I passed through two layers of neurons before I stopped. They must have been the ganglion cell layer and bipolar cell layer of the neural portion of the retina. I noticed more of my colleagues missing the nerve endings all together and hitting a sheet of melanin covered epithelial cells (pigment epithelium).

As I come into contact with the nerve cells I notice some are shaped like rods and others like cones. There are many more rod like cells. I’ve hit a rod and I’m being absorbed by a partly bent coloured structure: a photopigment. This structure has two parts a glycoprotein (sugar and protein) called opsin, and a bit called retinal which is made from vitamin A. The retinal part straightened out when I arrived. The opsin in my rod is called rhodopsin. I’ve heard that cones have three different types of opsin. They must be used to sense different colours, but my rod only has the rhodopsin and must not be able to differentiate colours.

Now the straightened retinal is separating from the opsin. (I hope it gets fixed up and joined up again – in fact I am sure it does!) When the retinal straightened sodium ions that had been flowing into the outer segment of my rod were shut-off. The inside of my cell has become very negative (hyperpolarised)! This has caused glutamic acid, that was flowing from the rods synapse into the adjoining bipolar cell to stop as well, and now this has turned the bipolar cell on! So effectively the straightening of the retinal in my rod has turned the bipolar cell on!

Now that a receptor potential can pass, it is sent from my rods inner segment to the outer segment and neurotransmitters are sent to the bipolar cell. Quite a few rods connect to my bipolar cell, but I notice the cones are very exclusive in the their synapsing.

There are some horizontal cells joining the rod synapses together these are inhibiting bipolar cells next to my rod from getting excited. It won’t do getting everybody excited now – you’d end up with blurry vision. The bipolar cell is turning on the ganglion cell, and amacrine cells. The amacrine cells also send signals to the ganglion cells, indicating a change in the light levels.

The ganglion cell is passing me to the optic nerve (II). I seem to be veering sideways. Here come a whole lot of other impulses from the other side of me. It seems like some sort of cross over. This must be the optic chiasm – and I’m crossing over –some of the nerve fibres aren’t crossing over at all. Now I’m on the optic tract.

Hey it looks like the end of the road up ahead at the geniculate nucleus of the thalumus. Now I’m synapsing with another nerve the axons of which are the optic radiations. OK a bit further now and here I am at the occiptal lobes of the primary visual area of the cerebral cortex. Time to be seen!