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Vision
Learning objectives:
After completing this study unit, you will be able to:
- Understand how the optical system of the eye creates an image on the retina.
- Describe the structure and function of photoreceptors.
- Explain the steps of phototransduction.
- Compare dark and light adaptation.
- Trace the path of visual impulses from the retina to the visual cortex.
Watch a video
Vision is the ability to see the world around us, a process that starts with light rays falling on the eye. The rays undergo refraction as they pass through the eye, coming to focus at a focal point on the retina. To focus on nearby objects, the eye can adjust the curvature of the lens through a process called accommodation.
In the retina, light rays get absorbed by photoreceptor cells (rod and cone cells), which convert light energy into electrical signals through a process called phototransduction. A single photon of light induces conformational changes in their visual pigments, setting off a cascade of events that can influence the release of neurotransmitters (glutamate) by these cells.
Impulses from photoreceptor cells reach retinal ganglion cells via bipolar neurons. Action potentials generated in retinal ganglion cells are carried along their axons, which form the optic nerve (Cranial Nerve II).
Shift in illumination from light to dark and vice versa requires visual adaptation. After prolonged illumination, the eyes take time to adjust to darkness; this is termed dark adaptation. On the other hand, light adaptation is the process of adjustment to light after prolonged darkness.
The optic nerve carries visual information in the form of electrical signals from the eye. These impulses travel through the optic chiasm and the optic tract to reach the lateral geniculate nucleus of the thalamus, from where the optic radiation ensures visual information reaches the primary visual cortex in the occipital lobe.
Watch the following video to learn more about each step involved in the physiology of vision!
Explore concepts
Refraction and accommodation
Understand how the optical system of the eye focuses light on the retina with these images!
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Retina
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:watermark(/images/watermark_only.png,0,0,0):watermark(/images/logo_url.png,-10,-10,0):format(jpeg)/images/learnable/rod-bipolar-cell/WMnUt57glkxDsPvgxYOjCg_rod-bipolar-cell.jpg "Rod bipolar cell (Neuron bipolare bacillitopicum); Image: Irina Münstermann"){kind=logo}
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Pigmented layer of retina
Stratum pigmentosum retinae
1/8
Synonyms:
Simple cuboidal pigmented epithelium, Retinal pigmented epithelial layer
,
show more...
Phototransduction
Take a closer look at the structure of photoreceptors and the steps of phototransduction with this gallery!
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Visual adaptation
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Visual pathway
Trace the path of light step-by-step along the visual pathway with this gallery!
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:watermark(/images/watermark_only.png,0,0,0):watermark(/images/logo_url.png,-10,-10,0):format(jpeg)/images/learnable/optic-tract/GVwzSdeW1p1P3sk8IoEvPg_Optic_Tracts_2.png "Optic tract (Tractus opticus); Image: Paul Kim"){kind=logo}
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Optic nerve
Nervus opticus
1/7
Synonyms:
Cranial nerve II, CN II
,
show more...
Take a quiz
See how well you know the physiology of vision with this quiz!
Summary
| Visible light | Range of wavelengths in the electromagnetic spectrum visible to the human eye (380-750 nm) |
| Refraction in the eye | Bending of light rays as they pass from one medium to another of different density Refracted rays are brought to focus at a focal point on the retina |
| Accommodation | Change in refractive power of the eye (by changing the curvature of the lens) in order to bring light rays coming from objects at varying distances to focus on the retina |
| Refractive errors |
Presbyopia: Reduced ability of the lens to accommodate with increasing age Hyperopia (Farsightedness): Short eyeball, rays from near objects focus behind the retina Myopia (Nearsightedness): Long eyeball, rays from far objects focus in front of the retina |
| Retina |
Pigmented layer of retina Neural layer of retina: Photoreceptor cells (rod cells, cone cells), horizontal neurons, bipolar neurons (rod bipolar cells, cone bipolar cells), amacrine cells and retinal ganglion cells |
| Phototransduction | Conversion of light energy into electrical impulses by photoreceptor cells (cone cells and rod cells) Cone cells: Photopic (day) vision, high visual acuity, color vision (red, blue and green cones) Rod cells: Scotopic (night) vision |
| Visual adaptation | Adjustment of the eye to changing illumination Dark adaptation: Adjustment of the eye to dim light after prolonged illumination Light adaptation: Adjustment of the eye to bright light after prolonged darkness |
| Visual pathway |
Right visual field: Right nasal and left temporal retina Left visual field: Left nasal and right temporal retina Optic nerve: Axons of ganglion cells carry information from the right and left eyes Optic chiasm: Decussation of nasal fibers Optic tract: Contains nasal fibers of contralateral eye and temporal fibers of ipsilateral eye Lateral geniculate nucleus: Located in the thalamus Optic radiation: Carries information to the primary visual cortex Primary visual cortex, Visual association areas: Processing of visual information |
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