Special Senses - Vision

Special Senses ● Sensory receptors are within large, complex sensory organs in the head. ● Smell in olfactory organs ● Taste in taste buds ● Hearing and equilibrium in ears ● Sight in eyes VISION Sense of Sight ● Visual accessory organs ○ Eyelids ○ Lacrimal apparatus ○ Extrinsic eye muscles Eyelid ● Palpebra ○ Composed of four (4) layers: ■ Skin ■ Muscle ■ Connective tissue ■ Conjunctiva ● Orbicularis Oculi - closes eyelid ● Levator Palpebrae Superioris - opens eyelid ● Tarsal Glands - secrete oil onto eyelashes ● Conjunctiva - mucous membrane; lines eyelid and covers portion of eyeball Lacrimal Apparatus ● Lacrimal Gland ○ Lateral to eyes ○ Secretes tears ● Canaliculi ○ Collect tears ● Lacrimal Sac ○ Collects from canaliculi ● Nasolacrimal Duct ○ Collects from lacrimal sac ○ Empties tears into nasal cavity Extrinsic Eye Muscles 1. Superior Rectus→elevates & slightly rotates eye medially 2. Inferior Rectus→depresses & slightly rotates eye medially 3. Medial Rectus→Moves eye medially (adduction) 4. 5. 6. Lateral Rectus→Moves eye laterally (abduction) Superior Oblique→Depresses, abducts, & rotates eye inward Inferior Oblique→Elevates, abducts, & rotates eye outward. STRUCTURE OF THE EYE ● Hollow ● Spherical ● Wall has three layers ○ Outer fibrous tunic ○ Middle vascular tunic ○ Inner nervous tunic Outer Tunic ● Cornea ○ Anterior portion ○ Transparent ○ Light transmission ○ Light refraction ● Sclera ○ Posterior portion ○ Opaque ○ Protection Middle Tunic ● Iris ○ Anterior portion ○ Pigmented ○ Controls light intensity ● Ciliary body ○ Anterior portion ○ Pigmented ○ Hold lens ○ Moves lens for focusing ● Choroid coat ○ Provides blood supply ○ Pigments absorb extra light Anterior Portion of Eye ● Filled with aqueous humor Lens ● Transparent; biconvex; lies behind iris ● Lens fibers; elastic ● Held in place by suspensory ligaments of ciliary body Ciliary body ● Forms internal ring around the front of the eye ● Ciliary processes - radiating folds ● Ciliary muscles - contract and relax to move lens Accommodation ● Changing of lens shape to view objects Key Concepts of Accommodation: Near Vision • Ciliary muscles contract - Zonular fibers (suspensory ligaments) relax • Lens becomes more convex (thicker) to increase refractive power • Helps focus on close objects Distant Vision • Ciliary muscles relax → Zonular fibers tighten • Lens flattens (thinner) to decrease refractive power • Helps focus on distant objects Controlled By: • Parasympathetic Nervous System (via the oculomotor nerve CN III) Iris ● Composed of connective tissue and smooth muscles ● Pupil - a hole in the iris ○ Dim light stimulates radial muscles and pupil dilates ○ Bright light stimulates circular muscles and pupil constricts Aqueous Humor ● Fluid in anterior cavity of eye ● Secreted by epithelium on inner surface of the ciliary body ● Provides nutrients ● Maintains shape of anterior portion of eye ● Leaves cavity through Canal of Schlemm ● Maintains intraocular pressure (IOP) ● Provides nutrients & removes waste for lens & cornea ● Helps refract light for clear vision Key Aspects of IOP: • Normal Range: Typically 10-21 mmHg. • Increased IOP: Can result from blocked drainage, leading to glaucoma, optic nerve damage, and vision loss. • Decreased IOP: Can occur due to fluid leakage or hypotony, potentially causing retinal detachment or vision distortion. Flow Pathway: • Posterior chamber → Flows through pupil → Anterior chamber • Drains via Trabecular meshwork → Canal of Schlemm → Venous system Inner Tunic ● Retina ● Contains visual receptors ● Continuous with optic nerve ● Ends just behind margin of the ciliary body ● Compose of several layers ● Macula lutea - yellowish spot in retina ● Fovea lutea - center of macula lutea; produces sharpest vision ● Optic disc - blind spot; contains no visual receptors ● Vitreous humor - thick gel that holds retina flat against choroid coat Posterior Cavity ● Contains vitreous humor - thick gel that holds retina flat against choroid coat Major Groups of Retinal Neurons ● Receptor cells, bipolar cells, ganglion cells provide pathway for impulses triggered by photoreceptors to reach the optic nerve ● Horizontal cells and amacrine cells - modify impulses Feature Cones Rods Shape Short, blunt projections Long, thin projections Light-sensitive pigments Erythrolabe (red), Chlorolabe (green), Cyanolabe (blue) Rhodopsin (dark adaptation pigment) Sensitivity to light Less sensitive About 100 times more sensitive to light Vision in light Provide vision in bright light Provide vision in dim light Image sharpness Produce sharp images Produce outlines of objects Color vision Provide color vision (red, green, blue) Do not provide color vision (black & white) Location in retina Concentrated in the central retina (fovea) Found mostly in the peripheral retina Number of cells Fewer in number (~6 million) More in number (~120 million) Function Detect fine details and color Detect motion and shapes in low light Adaptation Quick adaptation to light conditions Adapt slowly to changing light conditions Visual Pigments Pigments on cones • Each set contains different light-sensitive pigment • Each set is sensitive to different wavelengths • Color perceived depends on which sets of cones are stimulated • Erythrolabe – responds to red • Chlorolabe – responds to green • Cyanolabe – responds to blue Pigment on rods • Rhodopsin o Light-sensitive pigment in rods o Decomposes in presence of light o Triggers a complex series of reactions that initiate nerve impulses o Impulses travel along optic nerve Rods and Cones Visual Nerve Pathway Light enters the eye ↓ Retina (Photoreceptors: Rods & Cones) ↓ Optic Nerve (CN II) ↓ Optic Chiasm (Nasal fibers cross, Temporal fibers stay ipsilateral) ↓ Optic Tracts ↓ Lateral Geniculate Nucleus (Thalamus) ↓ Optic Radiations ↓ Primary Visual Cortex (Occipital Lobe, V1) ↓ Higher Processing Dorsal “where” - spatial, motion Ventral “what: - object recognition, color Visual Pathway Steps: 1. Light enters the eye → Focused onto the retina 2. Retinal photoreceptors (rods & cones) convert light into neural signals 3. Optic Nerve (CN II) transmits signals from each eye 4. Optic Chiasm: • Nasal fibers from each retina cross to the opposite side • Temporal fibers remain on the same side 5. Optic Tracts carry visual information to the Lateral Geniculate Nucleus (LGN) of the Thalamus 6. Optic Radiations transmit signals to the Primary Visual Cortex (Occipital Lobe, V1) 7. Higher Processing: • Dorsal Pathway ("Where" stream): Spatial awareness, motion • Ventral Pathway ("What" stream): Object recognition, color 1. Vestibulo-Ocular Reflex – coordinates eye movements with head movements to maintain stable vision Integration with proprioception • Proprioceptors in muscles and joints send position and movement signals to the cerebellum. • Vestibular, proprioceptive, and visual inputs combine to maintain balance, posture, and coordination. • Dysfunction in the vestibular system can lead to vertigo, dizziness, and balance disorders. Clinical Application Disorder Descripti on Cause Example Psychological Impact Benign Paroxysmal Positional Vertigo (BPPV) Brief Episodes of vertigo triggered by head movement s Displaced otolith crystals in semicircul ar canals Anxiety, fear of movement, panic attacks Meniere’s Disease Recurrent vertigo, tinnitus, hearing loss, ear fullness eExcess endolymp h fluid in the inner ear Depression, stress, social withdrawal Vestibular Neuritis Sudden, severe vertigo with imbalance Viral infection affecting the vestibular nerve Anxiety, difficulty concentrating, frustration Vestibular Migraine Episodic vertigo linked to migraines Neurologic al dysfunctio n, genetic factors Mood swings. Irritability, cognitive fatigue