Library · Physiology
Vision
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Seeing begins with light entering the eye and ends with an electrical signal in the brain. The eye receives the light and converts it into energy, and that energy leaves the eye as an action potential, the electrical impulse of nerve cells, running along the optic nerve toward the brain.
The conversion happens in one molecule. It is called rhodopsin and it is made of two parts: a protein called scotopsin and retinal, a molecule the body gets from vitamin A. When a photon, the smallest unit of light, strikes retinal, retinal changes shape: it goes from the cis configuration to the trans configuration, that is, it rotates around one of its bonds. It all starts with a molecule that twists when light hits it.
Then comes the sorting. At the optic chiasm, the point where the two optic nerves cross, the nasal fibers pass to the opposite side and the temporal fibers stay on the same side. The result is that every pathway that continues backward carries information from both eyes, and it takes that information to the thalamus, the brain's relay station, and from there to the visual cortex.
In brief
Vision begins when the eye takes in light and turns it into action potentials that travel along the optic nerve to the brain.
At the center is rhodopsin, the protein scotopsin bound to retinal, which comes from vitamin A; a photon flips retinal from cis to trans.
It all starts with a molecule that twists when light hits it.
At the optic chiasm the nasal fibers cross and the temporal fibers do not, so each pathway carries information from both eyes to the thalamus and visual cortex.
Sources (1)
- National Center for Biotechnology Information, StatPearls PublishingPhysiology, Vision, StatPearlsPublished
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