Picture frames. We see them everywhere. We hang up pictures of all sorts of things on our walls, refrigerators etc. But how do we see them? Let us explore some aspects of the incredible body part which we usually take for granted, the eye.
What is the eye? The eye is what we see with obviously. But it is a very complex structure. Here is a diagram of the eye, with a focus on the accessory structures (a detailed diagram of the actual eye will come later):
The accessory structures, such as the eyebrows, are very important. The eyebrows help shade the eyes from the sun and prevent perspiration from trickling down from the forehead to the eyes. The eyelids meet at the lateral and medial commissures, as can be seen in the top picture. In the medial commissure, there is a fleshy elevation called the lacrimal caruncle. This structure has sebaceous (a type of gland which secretes sebum, an oily substance) and sweat glands which produce a whitish, oily secretion that can collect at the medial commissure. This happens especially during sleep, so this is what you are rubbing out of your eyes in the morning. The two muscles controlling the eyelids are the orbicularis oculi and levator palpabrae superioris. These muscles reflexively cause the eyelids to blink every 3-7 seconds. Blinking causes accessory structure secretions such as oil, mucus, and saline solution to be spread across the eyeball, preventing drying of the eyes. The eyelashes project from the eyelids and are rich with nerve endings. Anything which touches the eyelids triggers reflex blinking. The blinking reflex is probably the fastest reflex in the body.
The conjuctiva is a transparent mucous membrane. It does not cover the cornea. Inflammation of the conjunctiva is called conjunctivitis, and results in reddened, irritated eyes. Pinkeye, which is caused by bacteria or a virus, is highly contagious. The differences between viral and bacterial pinkeye can be found in the following link:
The eyes obviously produce tears. The lacrimal apparatus consists of the lacrimal glands and the pathways that drain excess secretion into the nasal cavity. Here is a diagram:
The glands continuously secrete a dilute saline solution, which is what tears are. The tears flow through several tiny excretory ducts into the conjunctiva of the eye, and then blinking carries the tears downward and across the eyeball. There are two tiny openings called the lacrimal puncta where the tears enter into the lacrimal canaliculi. From the canaliculi, the tears drain into the lacrimal sac and then into the nasolacrimal duct which empties into the nasal cavity. Lacrimal fluid contains antibodies, mucus, and lysozymes, so it protects and cleans the eye surface along with moistening and lubricating it.
The eye itself: Here is a diagram of the actual eye:
How do we see? The answer to this question is exceedingly complex and I cannot give a fully complete response. I will give as basic of a response as I can. But first we need to see the structure of the retina. This is a close-up of the posterior aspect of the eyeball:
Here is a close-up of the neural layer of the retina, where a lot of the important action happens:
Let us just focus on the rods and cones. Light passes through the cornea, aqueous humor, lens, and vitreous humor. It then passes through the entire thickness of the neural layer of the retina to excite photoreceptors in the pigmented layer. When light passes through different mediums, it is bent. This is why a spoon in a glass of water appears broken at the surface, because the light is bent. Once the light reaches the neural layer and signals pass to the rods and cones, the different functions and abilities of these two types of cells come into play. Rods are very sensitive and respond to very dim light, making them suited for night vision and peripheral vision. They have only one type of visual pigment, so inputs from the rods are only in gray tones. Cones need bright light but have one of three different pigments. The three different pigments are blue, green, and red. They overlap each other, which is how we perceive different colors, such as orange and yellow.
In the nervous system, everything happens through electrical signals and impulses. It is incredible what happens when you tap your foot on the ground. In literally milliseconds, a signal travels up your entire body to your brain, which interprets the tap. The mechanism of how the signal travels is for a different discussion. But the same thing has to happen with vision. A signal travels from the eye through the optic nerve to different centers in the brain, where what was seen is interpreted. Here is the basic pathway:
The optic nerves cross at the optic chiasma, and the fibers from the medial aspect of each eye cross over to the opposite side of the brain. The fibers then continue on as the optic tract. Because of this partial crossover, each tract contains fibers from the lateral aspect of the eye on the same side and fibers from the medial aspect of the opposite eye. The optic tracts send most of their axons (the part of the neuron which usually is the releaser of neurotransmitter) to synapse (the synapse is the tiny space in between two neurons) with neurons of the lateral geniculate nuclei, located in the thalalmus (one day I’ll explain the nervous system and the brain). Axons of the thalamic neurons project through the internal capsule to form the optic radiation. These fibers then pass into the primary visual cortex, located in the occipital lobe of the brain. Here the information is interpreted into what we call vision. Interestingly, I read in a book once that if you think about it, everything you see and feel is really in the past, because it takes some time (very, very little) for the information to reach the brain and be interpreted.
IMPORTANT: Vitamin A is crucial for vision. The key to photoreceptors being affected by light which ultimately translates into electrical signals is the molecule retinal. Retinal combines with the protein opsin to form four different types of pigments, and depending on the type of opsin which it binds to, retinal will absorb different wavelengths of light. Retinal is formed from Vitamin A. It is stored in the liver. Prolonged vitamin A deficiency is the most common cause of night blindness, or nyctalopia. The vitamin deficiency leads to rod degeneration, and it would be very dangerous to drive at night.
Most of the information in this section is via the following source: Marieb, Elaine N. and Hoehn, Katja, Human Anatomy and Physiology, 8th ed., San Francisco, Pearson Education Inc., 2010. Print
So the next time you look at a picture frame, appreciate the complexity of what is going on which enables you to properly see the picture.