Vertebrates (having vertebrae) have single-lens eyes that are ‘camera-like’. The human eye is capable of interpreting a myriad of colors, objects and being able to respond to just one photon (unit of retinal illumination) of light.
Nerves carry signals (sensations, muscle movements) from the peripheral nervous system to the brain via the spinal cord. The types of nerves are:
- Peripheral nerves: found throughout the body
- Spinal nerves: arise from the spinal cord and control the limbs and trunk
- Cranial nerves: found in the brain and control the nerves of sense organs
The Nerves of the Eye
The optic nerve (cranial nerve II) is the nerve that serves the eyes and transmits to the brain. The rod and cone cells of the retinal surface transmit images that are focused on the retina. The retina of a human eye contains approximately 125 million rod cells and 6 million cone cells. These cells are photoreceptors (specialized cells that are sensitive to light) and were named due to their shapes.
Rod cells are more sensitive to light then cone cells, but are unable to distinguish colors. Rod cells assist humans to see at night. Cone cells aren’t used as much for seeing at night because it takes more light to stimulate them. Cone cells assist in distinguishing colors.
Images that are focused on the retina then travel to the optic nerves of each eye, and both optic nerves meet at the optic chiasm (band of fibers), before they move on to the thalamus, and finally the visual cortex where they will be processed.
There are three cranial nerves that serve the eye by controlling the six eye muscles responsible for moving the eye. These are:
- Oculomotor nerve (cranial nerve III): controls inferior oblique, superior, medial and inferior rectus muscles. It is controls the levator palpebrae superioris muscle that controls the eyelid.
- Troclear nerve (cranial nerve IV): controls the superior oblique muscles
- Abducens nerve (cranial nerve VI): controls the lateral rectus muscles
The Optic Path of the Human Eye
Both the left and the right eye have a slightly different visual field, but these fields overlap. The overlapping and the differences in what the left and right eye see, is how humans can tell distance and recognize an objects 3D form.
The images are translated into nerve impulses, and then travel along nerve fibers to the optic nerves, optic chiasm and the thalamus. The nuclei in the thalamus process this information, transmit it to the visual cortex, which then combines the two images (from left and right eyes) and interprets them. Images received on the retina are upside down, however, the brain corrects this so humans see things the right way up.
Resource and further reading:
Campbell and Reece, 2002, 'Sensory and Motor Mechanisms', in Biology 6th edition, ed. Benjamin Cummings, San Francisco, chap 49.
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