You may have heard the statistic that, on average, the brain comprises 2% of the human body mass, but uses up 20% of the total oxygen supply, and 25% of the glucose production. This makes the brain one of the most energetically demanding organs for its size. In particular, the retina is incredibly energetically demanding. The retina is a layer of neurons in back of each eyeball, and serves as the step where light is converted into neural signals. The rods and cones of the retina are photoreceptors responding to light, which then activate or deactivate the subsequent layers of cells.
Now, why might this structure consume so much energy? To start this discussion, we must first delve a bit into an evolutionary puzzle. In general, sensory receptors are depolarized when presented with the correct stimulus. Sufficient depolarization leads to an action potential transmitted to other neurons, which is how the nervous system communicates information. In vision, photoreceptors of invertebrates behave in this way when stimulated by light. Recording electrical signals from the retina is a newer practice in vertebrates than in invertebrates, so until the first recording from a fish in 1964, it was believed that vertebrate rods and cones are also depolarized in the presence of light. Much to the visual neuroscience community’s shock, this first recording by Tsuneo Tomita showed that in reality, the reverse happens in vertebrate photoreceptors.
When vertebrate rods and cones are in the dark, they are depolarized, and when light hits them, they are hyperpolarized. Hyperpolarization is the opposite of depolarization, and makes a neuron less likely to fire an action potential. Why this evolutionarily came about is still a mystery, but it does demand quite a lot of energy because the photoreceptors’ resting state (no light stimulus), is a constant flow of ions into the cell, termed the dark current. So that the dark current doesn’t go overboard, there is an active pumping of sodium ions out of the cell – an expensive process.
Disorders in energy metabolization can greatly affect our vision. For example, let’s consider diabetes and how it can affect your retina. When someone has diabetes and their blood-glucose levels are too high, the tiny and fragile blood vessels supplying nutrients to the retina can close, or start leaking. As such damage progresses, vision can become blurry, colors can start fading, and far enough along, one can get a total loss of vision.
Another, more rare, metabolic syndrome is Kearns-Sayre syndrome, which is caused by genetic mutations resulting in defects of cellular mitochondria all over the body, the organelles that convert oxygen and glucose to ATP (energy for cells). The most salient symptoms of Kearns-Sayre involve the eyes, such as drooping eyelids, paralysis of eye muscles, and retinitis pigmentosa, a progressive deterioration of the photoreceptors. This can result in loss of vision. Mitochondria are found in cells all over the body, so many organs are affected by Kearns-Sayre. Unsurprisngly, those that are most affected demand significant energy, and thus, are heavily reliant on mitochondria. This explains why even though Kearns-Sayre affects cells all over the body, the symptoms are manifested most readily in vision. Other symptoms include kidney problems, deafness, difficulty balancing, and abnormal heart rhythms.
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Hubel, D. (1988). Eye, Brain, and Vision. Scientific American Library.
Wong-Riley, M. (2010). Energy metabolism of the visual system. Eye and Brain, 2:99–116.