What happens when light strikes the center of an ON (Ocular Nerve) center photoreceptor?

Visual Pathway Response to Light Stimulation in ON-Center Photoreceptors

When light strikes the center of an ON-center photoreceptor, an ON-center bipolar cell is activated as part of this pathway.

Retinal Visual Processing Pathway

The visual pathway processes light information through a complex series of cellular interactions in the retina. When understanding this pathway, it's important to recognize the sequence of events that occur when light strikes photoreceptors:

Photoreceptor Response to Light

1. When light strikes the center of an ON-center photoreceptor (cone), the photoreceptor becomes hyperpolarized (not depolarized)
   • Light causes a decrease in the release of glutamate from the photoreceptor
   • This is contrary to option D which incorrectly states the cone becomes depolarized 1

Bipolar Cell Activation

1. The decreased glutamate release from the photoreceptor leads to:
   • Depolarization of ON-center bipolar cells (correct answer - option B)
   • ON-center bipolar cells have metabotropic glutamate receptors that are inhibited by glutamate
   • When glutamate decreases due to light stimulation, these bipolar cells are released from inhibition and become activated 1

Horizontal Cell Function

1. Horizontal cells provide lateral inhibition in the retina:
   • They send inhibitory (not excitatory) feedback to surrounding photoreceptors
   • This creates center-surround antagonism in the visual field
   • Option A is incorrect because horizontal cells do not send excitatory feedback 1

Ganglion Cell Response

1. ON-center ganglion cells:
   • Receive excitatory input from ON-center bipolar cells
   • Become depolarized when light strikes the center of their receptive field
   • Option C is incorrect because OFF-center ganglion cells would be hyperpolarized (not depolarized) when light strikes the center of an ON-center receptive field 1

Key Mechanisms in the Visual Pathway

The intrinsically photosensitive retinal ganglion cells (ipRGCs) express melanopsin photopigment that is maximally sensitive to ~480 nm light. While these cells are important for non-image forming functions like circadian rhythm regulation, the question specifically addresses the classical visual pathway involving cone photoreceptors 1.

In the classical visual pathway:

• Light causes photoreceptors to hyperpolarize
• ON-center bipolar cells depolarize in response to decreased glutamate
• Horizontal cells provide lateral inhibition
• ON-center ganglion cells depolarize when their center is illuminated

Common Misconceptions

Several misconceptions about the visual pathway should be avoided:

1. Photoreceptor response: Photoreceptors hyperpolarize (not depolarize) in response to light
2. Horizontal cell function: Horizontal cells provide inhibitory (not excitatory) feedback
3. Ganglion cell types: ON-center and OFF-center ganglion cells have opposite responses to light stimulation

Understanding these distinctions is critical for correctly interpreting visual processing in the retina and avoiding errors in clinical interpretation of visual pathway disorders.