The physiology of the visual system describes how the eyes and brain work together to detect light, process images, and generate vision. Here’s a structured overview:
Cornea & Lens: Refract (bend) incoming light rays to focus them onto the retina.
Cornea provides ~70% of refraction.
Lens fine-tunes focus (accommodation).
Pupil (Iris): Regulates the amount of light entering the eye (like a camera aperture).
Photoreceptors:
Rods: Highly sensitive to light, responsible for night (scotopic) vision, no color perception.
Cones: Less sensitive to light, responsible for daylight (photopic) vision and color perception (red, green, blue types).
Bipolar Cells: Relay signals from photoreceptors to ganglion cells.
Horizontal & Amacrine Cells: Provide lateral connections → contrast enhancement, edge detection.
Ganglion Cells: Their axons form the optic nerve; transmit visual signals to the brain.
In darkness: photoreceptors are depolarized, continuously releasing glutamate.
Light exposure: activates rhodopsin (rods) or photopsins (cones) → triggers G-protein (transducin) → activates phosphodiesterase → lowers cGMP → closes Na⁺/Ca²⁺ channels → hyperpolarization → reduced glutamate release.
Signal is inverted: light decreases neurotransmitter release, which bipolar/ganglion cells interpret as "light present."
Optic Nerve → Optic Chiasm → Optic Tracts:
Nasal retinal fibers cross at the optic chiasm; temporal fibers remain uncrossed.
Ensures each cerebral hemisphere processes the contralateral visual field.
Lateral Geniculate Nucleus (LGN, thalamus): Major relay center for visual signals.
Optic Radiations → Primary Visual Cortex (V1, occipital lobe): Initial cortical processing of form, orientation, and movement.
Dorsal stream (“Where” pathway): Occipital → parietal cortex; motion, spatial location.
Ventral stream (“What” pathway): Occipital → temporal cortex; object recognition, color, faces.
Color Vision: Based on cone types (trichromacy). Color blindness results from absent/defective cones.
Dark/Light Adaptation: Rods regenerate rhodopsin slowly (night vision adaptation). Cones adapt quickly (daylight).
Binocular Vision & Depth Perception: Overlapping visual fields → stereopsis (3D perception).
Pupillary Reflexes: Light reflex (parasympathetic CN III) and accommodation reflex adjust for lighting and near vision.
✅ In summary:
The visual system converts light into electrical signals (retina), transmits them to the brain (optic pathways), and processes them into images and perception (visual cortex and association areas). It relies on precise coordination of optical focusing, phototransduction, neural transmission, and cortical interpretation.
Would you like me to make a diagram-style flowchart (from light entering the eye → to perception in the cortex) so it’s easier to visualize?