Biol+216+2020+Topic+7+The+Senses_Powerpoint+7.2+Vision

Topic Overview

  • Biol 216 - The Senses: Focus on Vision and the Retina.

Learning Goal

  • Objective: Understand the anatomical components, processes, and molecular basis of vision.

Components of the Eye

  • Sclera: White protective outer layer of the eye.

  • Choroid: Vascular layer between sclera and retina.

  • Retina: Layer of neural cells at the back of the eye with photoreceptors.

  • Ciliary Body: Includes ciliary muscle for lens shape control and produces aqueous humor.

  • Fovea: Center of macula, high density of cone cells for sharp vision.

  • Iris: Controls pupil diameter, regulates light entering the lens.

  • Pupil: Opening in the iris through which light enters.

  • Cornea: Transparent layer that refracts light; covers the anterior chamber.

  • Blind Spot: Area where optic nerve exits the retina, no photoreceptors present.

  • Optic Nerve: Transmits visual information to the brain.

  • Aqueous Humor: Fluid in the anterior and posterior chambers, maintains pressure and provides nutrients.

  • Vitreous Humor: Gel-like substance that maintains eye shape behind the lens.

The Vertebrate Eye

  • Cornea: Responsible for light admission and refraction.

  • Lens: Focuses light on the retina. Shape altered by ciliary muscles for accommodation.

  • Retina: Contains photoreceptor cells (rods and cones).

  • Macula: Involved in high-acuity vision, contains the fovea.

    • Fovea: High concentration of cone cells; necessary for detailed vision.

Photoreceptor Cells

  • Structure: Rods for low-light vision; cones for color detection.

  • Rhodopsin: Photopigment in rods; Iodopsin in cones.

Visual Processing Pathways

  • Direct Pathway: Photoreceptors ➔ Bipolar Cells ➔ Ganglion Cells.

  • Additional Cells: Horizontal cells and amacrine cells modulate the signals.

    • Horizontal Cells: Help adjust sensitivity to light levels.

    • Amacrine Cells: Dopamine-secreting neurons enhancing color sensitivity in light.

Mechanism of Phototransduction

  • In the dark: Rods release glutamate, depolarizing the bipolar cells.

  • In light: Phototransduction leads to hyperpolarization, altering neurotransmitter release, leading to ganglion cell activation.

  • Na+ Channels: In light, channels close due to cGMP decrease, causing hyperpolarization.

Receptive Fields & Neural Circuitry

  • Receptive Field: Area from which a ganglion cell receives input from photoreceptors via bipolar cells.

  • On-Center/Off-Center: Response patterns of bipolar cells dictate activation of ganglion cells based on light presence.

Adaptation Responses

  • Dark Adaptation: Increased rhodopsin sensitivity after entering dim light.

  • Light Adaptation: Breakdown of rhodopsin in bright light decreases sensitivity.

  • Pupil Reaction: Constriction in bright light; dilation in dim light.

Visual Signals Processing

  • Each eye's visual input is processed on the opposite side of the brain:

    • Optic Chiasm: Where optic nerve fibers cross over.

    • Lateral Geniculate Nucleus: Relay center for visual information.

  • Primary Visual Cortex: Final destination for visual processing, integrating visual field information.

Summary Points

  • Understand the eye's structure and function.

  • Key aspects of visual processing and signal pathways.

  • Adaptation mechanisms of photoreceptors to varying light conditions.