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.