Recording-2025-02-11T17_39_24.329Z

Phototransduction Overview

• Phototransduction is the process of converting light stimuli (photons) into electrical signals in the retina.

• Three main types of cells involved: Photoreceptors, Bipolar cells, and Ganglion cells.

Photoreceptors

• Photoreceptors are the first cells to respond to light.

• They produce glutamate in the absence of light, making them depolarized when it's dark.

• When light hits photoreceptors, glutamate production decreases.

Bipolar Cells

• Bipolar cells receive glutamate from photoreceptors and play a critical role in processing visual signals.

• They do not generate action potentials; instead, they produce graded potentials.

• Two types of bipolar cells based on pathways:

  • On-pathway Bipolar Cells: Depolarize in the absence of glutamate.

  • Off-pathway Bipolar Cells: Hyperpolarize in the absence of glutamate.

Communication Between Cells

• In the presence of light, photoreceptors reduce glutamate release, impacting bipolar cells:

  • On Bipolar Cells: Increase glutamate response, signaling brightness.

  • Off Bipolar Cells: Decrease glutamate response, signaling darkness.

• This dual pathway allows the brain to interpret contrasting light information effectively.

Receptive Fields and Ganglion Cells

• Receptive fields consist of groups of bipolar cells connected to ganglion cells.

• Ganglion cells receive input from both types of bipolar cells (On and Off).

• Center-Surround Organization:

  • Certain orientations of bipolar cells enhance stimulus detection.

• When light hits edges, ganglion cells increase firing rates, highlighting objects within the visual field.

Visual Perception and Edges

• This selective enhancement allows the brain to identify edges of objects, which are considered more relevant than uniform backgrounds.

• Examples illustrate how edges are accentuated, aiding in object detection.

Color Vision

• Color vision arises from the activation of photopigments in cone photoreceptor cells.

• Human retinas have three types of cones, each sensitive to different wavelengths:

  • Short (S), Medium (M), Long (L) wavelengths.

• Cones exhibit fast adaptation, allowing the eye to adjust to changing light conditions.

Color Discrimination

• Ganglion cells integrate inputs from all three cone types to perceive color.

• Conditions like color blindness arise from anomalies in cone pigments, particularly red and green variations:

  • Protanomaly: Red appears more green and less bright.

  • Deuteranomaly: Green appears more red.

  • Protanopia/Deuteranopia: Complete inability to distinguish red and green.

• Complete color blindness, where only shades of gray are seen, is rare.

Summary of Color Blindness

• Color blindness affects approximately 8% of males and 0.5% of females.

• Variants vary in severity and can often go unnoticed in mild cases.

• Practical example provided for visualizing differences between normal vision and color-blind vision using numbers.