Anatomy of the Visual System
The visual system includes all the structures that capture, transmit, and process visual information:
1. The Eye (Sensory Organ)
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Cornea – Transparent, avascular structure; first refractive surface of the eye that bends light toward the lens.
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Anterior Chamber & Aqueous Humor – Clear fluid between cornea and lens that nourishes avascular cornea and lens, maintains intraocular pressure.
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Lens – Transparent, biconvex structure that fine-tunes focusing of light onto the retina (accommodation).
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Iris & Pupil – Iris regulates light entry through the pupil; pupil constricts (miosis, parasympathetic) or dilates (mydriasis, sympathetic).
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Vitreous Humor – Gel-like substance filling the posterior chamber, maintains shape and optical clarity.
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Retina – Light-sensitive layer containing:
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Photoreceptors (rods and cones):
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Rods: Sensitive to dim light, night vision, peripheral vision.
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Cones: Responsible for color vision and visual acuity.
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Bipolar cells – Relay signals from photoreceptors to ganglion cells.
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Ganglion cells – Their axons form the optic nerve.
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Macula lutea and fovea centralis – Central high-acuity vision area, rich in cones.
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Optic disc – Blind spot where optic nerve exits (no photoreceptors).
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2. Optic Nerve and Pathways
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Optic Nerve (CN II) – Formed by ganglion cell axons; transmits signals to the brain.
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Optic Chiasm – Nasal retinal fibers cross to the opposite side; temporal fibers remain uncrossed.
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Optic Tracts – Carry information from contralateral visual fields to the brain.
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Lateral Geniculate Nucleus (LGN) of Thalamus – Primary relay for visual signals.
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Optic Radiations – Pathways from LGN to primary visual cortex:
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Meyer’s loop (temporal lobe, superior visual field).
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Parietal pathway (inferior visual field).
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3. Visual Cortex
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Primary Visual Cortex (V1, occipital lobe) – Processes basic visual inputs (edges, contrast, orientation).
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Association Areas (V2, V3, V4, V5) – Higher-order processing: color, motion, depth, and object recognition.
Physiology of the Visual System
1. Optics & Image Formation
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Light is refracted by cornea and lens to focus on the retina.
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Accommodation: lens curvature adjusts (via ciliary muscle and zonular fibers) for near or distant vision.
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Pupil reflex controls light entry:
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Constriction in bright light (parasympathetic, CN III).
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Dilation in dim light (sympathetic).
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2. Phototransduction (Retina)
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In darkness, photoreceptors are depolarized and release glutamate.
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Light exposure → rhodopsin (rods) or cone opsins activate → triggers transducin → reduces cGMP → closes Na⁺ channels → hyperpolarization → decreased glutamate release.
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Signal transmitted to bipolar and ganglion cells → action potentials generated.
3. Neural Processing
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Ganglion cell axons → optic nerve → optic chiasm → optic tract → LGN → optic radiations → primary visual cortex.
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Each hemisphere processes the contralateral visual field:
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Left visual cortex → right visual field.
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Right visual cortex → left visual field.
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4. Binocular Vision and Depth Perception
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Overlap of visual fields from both eyes provides stereopsis (3D depth perception).
5. Higher-Order Processing
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Dorsal pathway ("where") – Motion and spatial location (parietal lobe).
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Ventral pathway ("what") – Object and face recognition, color (temporal lobe).
✅ Summary: The visual system consists of the eye (which refracts and converts light into neural signals), the optic nerve and pathways (which transmit signals), and the brain’s visual cortex (which interprets signals into perception). Physiology involves optics (focusing light), phototransduction (light to electrical signals), and neural processing (creating visual images and meaning).
Would you like me to create a diagram/flowchart summarizing this for quick study, or a table comparing anatomy vs physiology functions?
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The human visual system. (a) Visual perception begins in the eye, where the cornea and lens (1) project an inverted image of the world onto the retina (2), which converts incident photons into neural action potentials. (b) The retina consists of three layers of cells. The photoreceptors (PR), which are in contact with the retinal pigment epithelium (RPE), convert light into neural signals that propagate to the horizontal (HC), bipolar (BC) and amacrine cells (AC) of the inner nuclear layer. The axons of the retinal ganglion cells (RGCs) form the retinal nerve fiber layer (RNFL). They converge onto the optic disk (3), where they congregate to form the optic nerve (4), which relays neural signals to the brain. (c) Signals from the left and right visual fields of both eyes are combined at the optic chiasm (5). The lateral geniculate nucleus (6) relays the left visual field to the right visual cortex and the right visual field to the left visual cortex through neuron axons called the optic radiation. Higher visual processing finally takes place in the visual cortex (7), and further downstream in the brain.
The visual system comprises of
(1) the sensory organ (the eye)
and
(2) the part of the central nervous system which gives organisms the ability to process visual detail as sight
It enables the formation of several non-image photo response functions such as the pupillary light reflex (PLR) and circadian photoentrainment.