_ch_10c+lecture_presentation

Chapter Overview

• Focus on Vision & Sensory Systems

  • Importance of understanding the anatomy and functionality of the eye as part of the nervous system.

Learning Objectives

• Describe how light is focused on the retina.

• Explain how the iris regulates the amount of light entering the eye.

• Differentiate between rods and cones in the eye.

Anatomy of the Eye

• Three Layers:

  • Outermost Layer: Sclera and cornea.

  • Middle Layer: Choroid, ciliary body (zonular fibers, ciliary muscle), iris with pupil.

  • Inner Layer: Retina, photoreceptors (rods and cones), fovea, optic disk (blind spot).

• Fluid-Filled Chambers:

  • Anterior Segment: Anterior and posterior chambers.

  • Posterior Segment: Vitreous chamber filled with vitreous humor.

• Blind Spot:

  • Located at the optic disk where the optic nerve exits the eye, no photoreceptors present; not noticeable with both eyes open.

Light Characteristics

• Light Waves:

  • Electromagnetic waves exhibit properties of reflection and refraction.

  • Refraction: Light bends when transitioning between different media; depends on density and angle of entry.

Eye Functionality

• Refraction of Light:

  • Cornea: Fixed curvature providing significant refraction.

  • Lens: Adjustable curvature to fine-tune focus.

• Accommodation:

  • Process of changing lens curvature to focus on objects at varying distances.

  • Mechanism involves the ciliary muscle, controlling lens shape based on distance:

    • Far vision: Ciliary muscle relaxed, lens flat.

    • Near vision: Ciliary muscle contracted, lens rounded.

Clinical Defects in Vision

• Emmetropia: Normal vision where objects focus correctly on the retina.

• Myopia (Near-Sightedness):

  • The lens is too strong, making distant objects appear blurry.

  • Corrected with concave lenses.

• Hyperopia (Far-Sightedness):

  • The lens is too weak, making near objects appear blurry.

  • Corrected with convex lenses.

• Presbyopia: Age-related hardening of the lens leading to difficulty focusing on near objects.

• Cataracts: Opacity of the lens affecting clarity of vision.

• Glaucoma: Increased pressure in the eye leading to optic nerve damage.

• Astigmatism: Irregular curvature of the lens or cornea affecting focus.

Retina Structure

• Layers of the Retina:

  • Outer layer: Photoreceptors (rods and cones).

  • Middle layer: Bipolar cells and lateral modulation cells (horizontal and amacrine).

  • Inner layer: Ganglion cells.

• Fovea and Macula Lutea: Central part of the retina important for sharp vision.

  • Macular Degeneration: Includes dry and wet types affecting vision.

Phototransduction

• Process of converting light into nerve signals involving photopigments (retinal and opsin).

• Rods are responsible for black-and-white vision (rhodopsin), while cones contribute to color vision (S, M, L opsins).

Color Vision

• Types of Cones:

  • S-cones (blue), M-cones (green), L-cones (red).

• Color perception arises from the brain comparing the activity of different cones.

• Theories:

  • Opponent-process theory explains color perception (red-green, blue-yellow).

Adaptation to Light

• Bleaching and Dark Adaptation:

  • Rods become less sensitive in bright light (bleaching).

  • Transitioning to dark restores sensitivity (unbleaching).

Neural Processing

• Communication pathway:

  • Photoreceptors -> Bipolar Cells -> Ganglion Cells -> Optic Nerve.

• Convergence: Rods have high convergence allowing greater light sensitivity but lower acuity; cones in fovea provide higher acuity but lower sensitivity.

Visual Pathways

• Optic nerve leads to the lateral geniculate body of the thalamus and then to the visual cortex, processing visual information from the contralateral visual field.

Parallel Processing

• Various visual information types (color, shape, movement) processed in parallel pathways before being integrated in higher cortical areas.

Depth Perception

• Constructed by the brain using inputs from monocular and binocular visual fields to create a three-dimensional image.