The Nervous System: Sensory Systems - Vision

The Nervous System: Sensory Systems - Vision (10c)\n\n## Anatomy of the Eye\n- Three Layers:\n - Outermost: Sclera (white, protective) and Cornea (transparent, front).\n - Middle: Choroid (vascular), Ciliary Body (zonular fibers, ciliary muscle for lens), Iris (with pupil, regulates light entrance).\n - Inner: Retina (photoreceptors, fovea for sharp vision), Optic Disk (blind spot, where optic nerve exits).\n- Fluid-Filled Chambers:\n - Anterior Segment: Anterior and Posterior chambers (aqueous humor).\n - Posterior Segment (Vitreous Chamber): Vitreous humor (gel-like).\n- Blind Spot: Optic Disk lacks photoreceptors; not visible with both eyes open.\n\n## The Nature and Behavior of Light Waves\n- Light Energy: Part of the electromagnetic spectrum ($350-750$ nm is visible light).\n- Reflection: We detect light reflected off objects.\n- Refraction: Light bends when passing between media of different densities.\n - Cornea: Fixed curve, responsible for most refraction.\n - Lens: Adjustable curve, allows fine focusing.\n\n## Accommodation\n- Definition: Shifting focus from far to near by increasing lens curvature.\n- Mechanism (Near Vision):\n - Parasympathetic stimulation.\n - Ciliary muscle contracts (circular muscle).\n - Decreased tension on zonular fibers.\n - Lens becomes thicker/rounder.\n - Refractive index increases.\n- Far Vision: No parasympathetic stimulation, ciliary muscle relaxed, lens flat.\n\n## Clinical Defects in Vision\n- Emmetropia: Normal vision.\n- Myopia: Near-sightedness (eyeball too long or lens too strong); corrected with concave lens.\n- Hyperopia: Far-sightedness (eyeball too short or lens too weak); corrected with convex lens.\n- Presbyopia: Hardening of the lens, loss of accommodation for near vision (age-related).\n- Cataract: Opacity of the lens.\n- Glaucoma: Increased volume of aqueous humor, leading to optic nerve degeneration.\n- Astigmatism: Irregularities in cornea or lens shape.\n\n## Regulating Light Entry\n- Pupil Size: Regulated by the Iris.\n - Inner Circular Muscle (Constrictor): Parasympathetic control (constricts pupil).\n - Outer Radial Muscle (Dilator): Sympathetic control (dilates pupil).\n\n## Retina Structure and Cells\n- Neural Tissue Layers:\n - Outer: Photoreceptors (Rods and Cones).\n - Middle: Bipolar, Amacrine, Horizontal cells.\n - Inner: Ganglion cells (axons form optic nerve).\n- Key Areas:\n - Fovea: High concentration of cones, responsible for sharp central vision.\n - Macula Lutea: Area surrounding fovea, important for detailed vision.\n- Retinal Diseases: Macular degeneration (dry/wet), Retinitis pigmentosa, Diabetic retinopathy.\n\n## Phototransduction\n- Definition: Conversion of light energy into nerve signals.\n- Photopigments: Each has Retinal and an Opsin protein.\n - Rods: Contain Rhodopsin; for black-and-white (dim light) vision.\n - Cones: Contain L (red), M (green), S (blue) opsins; for color vision.\n- Mechanism (Light Presence): Retinal and opsin dissociate, activating Transducin (G protein), then Phosphodiesterase, which hydrolyzes cGMP, closing Na+ and Ca2+ channels, leading to hyperpolarization and reduced neurotransmitter release.\n- Circadian Rhythms: Special photoreceptors with Melanopsin link to brain's rhythm-generating center.\n\n## Neural Pathways for Vision\n- Path: Ganglion cells -> Optic Nerve (Cranial Nerve II) -> Optic Chiasm (nasal retina axons cross contralaterally, temporal ipsilaterally) -> Optic Tract -> Lateral Geniculate Body of Thalamus -> Optic Radiations -> Visual Cortex.\n- Processing: Right visual field processed by left cortex, and vice versa.\n\n## Parallel Processing and Depth Perception\n- Parallel Processing: Different visual information (color, shape, movement) transferred by specific neurons to primary visual cortex and integrated in higher cortical areas.\n- Depth Perception: Brain constructs three-dimensional image from two perspectives of the binocular visual field.