unit 23 review
Vision is a complex process involving multiple structures in the eye and brain. Light enters through the cornea, is focused by the lens, and detected by photoreceptors in the retina. The resulting signals travel through the optic nerve to the brain for processing.
The eye's anatomy includes protective structures like the cornea and sclera, as well as internal components like the iris, lens, and retina. Common vision problems include refractive errors, cataracts, and glaucoma. Regular eye exams help maintain eye health and detect issues early.
Anatomy of the Eye
- Cornea: transparent, protective outer layer that refracts light as it enters the eye
- Iris: colored part of the eye that controls the amount of light entering through the pupil by contracting or dilating
- Lens: transparent, biconvex structure that focuses light onto the retina by changing shape (accommodation)
- Retina: innermost layer of the eye containing photoreceptors (rods and cones) that convert light into electrical signals
- Fovea: central region of the retina with the highest visual acuity due to a high concentration of cones
- Optic nerve: bundle of nerve fibers that transmits visual information from the retina to the brain
- Sclera: white, tough, outer protective layer of the eye that maintains its shape
- Choroid: vascular layer between the retina and sclera that provides oxygen and nutrients to the outer retina
Light and Vision Basics
- Light enters the eye through the cornea, which refracts the light rays
- Pupil size is adjusted by the iris to control the amount of light entering the eye
- Lens further refracts light to focus it onto the retina
- Photoreceptors in the retina (rods and cones) convert light into electrical signals
- Rods: responsible for low-light and peripheral vision, more sensitive to light but do not detect color
- Cones: responsible for color vision and fine detail, concentrated in the fovea
- Electrical signals from photoreceptors are processed by bipolar cells, ganglion cells, and other neurons in the retina
- Optic nerve transmits the processed visual information to the brain for further interpretation
Visual Pathway and Processing
- Optic nerves from each eye converge at the optic chiasm, where nasal retinal fibers cross to the opposite side of the brain
- Visual information travels through the optic tracts to the lateral geniculate nucleus (LGN) in the thalamus
- From the LGN, visual information is sent to the primary visual cortex (V1) in the occipital lobe via the optic radiations
- Primary visual cortex processes basic visual features such as edges, orientation, and motion
- Higher-order visual processing occurs in the extrastriate cortex (V2, V3, V4, and V5), which analyzes more complex features like form, color, and motion
- Dorsal stream (occipital to parietal lobe) processes spatial information and guides actions (where pathway)
- Ventral stream (occipital to temporal lobe) processes object recognition and identification (what pathway)
Eye Movements and Focusing
- Extraocular muscles (six for each eye) control eye movements to maintain fixation and track objects
- Medial and lateral rectus muscles: horizontal movements
- Superior and inferior rectus muscles: vertical movements and intorsion/extorsion
- Superior and inferior oblique muscles: vertical movements and intorsion/extorsion
- Saccades: rapid eye movements that shift gaze from one point to another
- Smooth pursuit: eye movements that track moving objects
- Vergence movements: eyes move in opposite directions to maintain binocular vision and depth perception (convergence and divergence)
- Accommodation: the lens changes shape to focus on objects at different distances
- Ciliary muscles contract to allow the lens to become more convex for near vision
- Ciliary muscles relax to flatten the lens for distant vision
- Pupillary light reflex: pupils constrict in bright light and dilate in dim light to regulate the amount of light entering the eye
Visual Acuity and Color Perception
- Visual acuity: the ability to discern fine details, measured using a Snellen chart (20/20 vision)
- Factors affecting visual acuity include the density of photoreceptors (particularly cones) in the fovea, the health of the visual pathway, and the refractive power of the eye
- Color vision is mediated by three types of cones, each sensitive to a different wavelength range (red, green, and blue)
- Color perception is processed in the extrastriate cortex (V4) and inferior temporal cortex
- Color blindness: genetic conditions that affect the ability to distinguish certain colors, most commonly red-green color blindness (deuteranomaly or protanomaly)
- Contrast sensitivity: the ability to detect differences in luminance between adjacent areas, important for edge detection and depth perception
Common Eye Disorders
- Refractive errors: conditions that affect the eye's ability to focus light on the retina
- Myopia (nearsightedness): light focuses in front of the retina, corrected with concave lenses
- Hyperopia (farsightedness): light focuses behind the retina, corrected with convex lenses
- Astigmatism: irregular curvature of the cornea or lens, causing distorted vision, corrected with cylindrical lenses
- Cataracts: clouding of the lens, leading to blurred vision and glare, treated with surgical lens replacement
- Glaucoma: increased intraocular pressure that damages the optic nerve, leading to vision loss, treated with eye drops, laser therapy, or surgery
- Age-related macular degeneration (AMD): deterioration of the macula (central retina), causing central vision loss, treated with antioxidants, anti-VEGF injections, or laser therapy
- Diabetic retinopathy: damage to retinal blood vessels due to diabetes, leading to vision loss, treated with laser therapy or vitrectomy
Protective Structures and Maintenance
- Eyelids: protect the eye from foreign objects, bright light, and drying, and distribute tear film across the cornea
- Eyelashes: protect the eye from dust and debris
- Lacrimal glands: produce tears that lubricate and nourish the eye, and contain antibodies to prevent infection
- Lacrimal ducts: drain excess tears into the nasal cavity
- Conjunctiva: thin, transparent membrane covering the sclera and inner eyelids, secretes mucus to lubricate the eye and prevent infection
- Blinking: spreads tear film across the cornea, removes debris, and prevents the eye from drying out
- Tear film: consists of three layers (mucin, aqueous, and lipid) that lubricate, nourish, and protect the cornea
Clinical Applications and Diagnostics
- Visual acuity testing: measures the eye's ability to discern fine details using a Snellen chart
- Refraction: determines the refractive error of the eye and the appropriate corrective lens prescription using a phoropter
- Tonometry: measures intraocular pressure to screen for glaucoma using an applanation or non-contact tonometer
- Fundoscopy: examines the retina, optic nerve, and blood vessels using an ophthalmoscope to detect disorders like AMD, diabetic retinopathy, and glaucoma
- Slit-lamp examination: evaluates the anterior segment of the eye (cornea, iris, lens) using a biomicroscope to detect cataracts, corneal abrasions, and other abnormalities
- Visual field testing: assesses peripheral vision to detect vision loss due to glaucoma, brain tumors, or stroke using perimetry
- Optical coherence tomography (OCT): provides high-resolution cross-sectional images of the retina to diagnose and monitor AMD, diabetic retinopathy, and glaucoma