How do bipolar cells relay signals from photoreceptors in the retina?

In the vertebrate retina, bipolar cells serve as a critical link in the neural pathway between photoreceptors and ganglion cells.1

Bipolar cells relay signals through the following mechanisms:

• Synaptic Connections: In the outer plexiform layer, bipolar cell dendrites form synapses with the spherules of rods and the pedicles of cones. Their axons subsequently connect to the dendrites of ganglion cells within the inner plexiform layer.2
• Signal Processing: Horizontal cells modulate the coupling between photoreceptors and bipolar cells, providing a control system that maintains bipolar cell activity despite fluctuations in light intensity and enhances visual contrast.1
• Functional Types: Bipolar cells are categorized as "on" or "off" types, responding respectively to increases or decreases in local light intensity. Structurally, they are classified as "midget" (connecting to a single cone) or "diffuse" (collecting signals from multiple rods or cones).12
• Neurotransmitter Regulation: Photoreceptors release the neurotransmitter glutamate in the dark. Light exposure hyperpolarizes the photoreceptor, inhibiting this release, which in turn signals the bipolar cells.1