Bipolar cells are specialized neurons within the eye’s retina that form a bridge, transmitting visual information from photoreceptors—rods and cones—to the brain. They perform the initial processing of light signals before they are sent for further interpretation.
Location and Structure of Bipolar Cells
Bipolar cells reside in the retina’s inner nuclear layer. This location places them between the photoreceptors in the outer nuclear layer and the ganglion cells in the ganglion cell layer.
The name “bipolar cell” derives from its shape, featuring a cell body with two processes extending from opposite ends. One process, the dendritic tree, receives inputs from photoreceptors and horizontal cells. The other, the axon, extends into the inner plexiform layer to communicate with amacrine and ganglion cells.
Different types of bipolar cells exist. Midget bipolars in the fovea may receive input from a single cone, a connection that provides high-acuity vision for tasks like reading. In the peripheral retina, multiple photoreceptors converge onto one bipolar cell, which improves light detection in dim conditions but results in less detail.
How Bipolar Cells Transmit Visual Signals
The transmission of visual signals begins with photoreceptors. In complete darkness, photoreceptor cells continuously release the neurotransmitter glutamate. When light strikes the photoreceptors, it triggers a chemical cascade that reduces the amount of glutamate they release.
Unlike many neurons that fire all-or-nothing action potentials, bipolar cells use graded potentials. This means the cell’s electrical response is proportional to the strength of the input it receives. A small change in light intensity causes a small change in the cell’s membrane voltage, while a large light change causes a larger voltage change.
This graded signal is then transmitted to the ganglion cells. The amount of neurotransmitter the bipolar cell releases is directly related to its membrane voltage. This refined signal is passed to the ganglion cells, which generate action potentials that travel out of the eye along the optic nerve to the brain.
The ON and OFF Pathways
The visual system processes light increases and decreases through two parallel pathways that start at the bipolar cells: the ON and OFF pathways. This division is defined by how bipolar cells respond to glutamate from photoreceptors. It allows the visual system to simultaneously track both light and dark areas in a visual scene.
ON bipolar cells are depolarized, or “turned on,” by light. This occurs because light striking a photoreceptor causes it to release less glutamate. ON bipolar cells have metabotropic glutamate receptors (mGluR6) that cause the cell to become active in the absence of glutamate, signaling when an area becomes brighter than its background.
OFF bipolar cells are depolarized, or “turned on,” by darkness, which corresponds to an increased release of glutamate. They possess ionotropic glutamate receptors that are excitatory, so the cell becomes active when glutamate binds to them. These two cell types synapse in different sub-layers of the inner plexiform layer, keeping the light and dark signals separate for the next stage of processing.
Contribution to Visual Perception
The separation of visual information into ON and OFF channels is the first stage in detecting contrast. The ability to perceive edges and define the boundaries of objects begins with the distinct signaling of these bipolar cells.
Signals from bipolar cells, modulated by horizontal and amacrine cells, create the receptive fields of ganglion cells. A ganglion cell’s receptive field is the retinal area that triggers a response in that cell when stimulated by light. The center-surround organization of these fields is a direct result of inputs from the ON and OFF bipolar cell pathways.
The patterned activity of ganglion cells, shaped by bipolar cells, is what the brain interprets as an image. Information about light, dark, and contrast is transmitted along the optic nerve to the brain’s visual centers. Here, this data is used to build complex features like shape, motion, and color.