Retinal bipolar cell

A retinal interneuron relaying photoreceptor signals toward ganglion-cell circuits. ON and OFF types differ in how changes in photoreceptor output affect their responses.

An OFF retinal bipolar cell is a retinal bipolar cell whose central receptive response shows depolarization for an appropriate decrease in illumination. Its input synapse preserves the sign of a cone photoreceptor's voltage response through excitatory AMPA receptors or kainate receptors, rather than the sign-inverting arrangement of an ON retinal bipolar cell.^[ref]

In a 2000 experiment, simultaneous recordings from connected cone cells and OFF retinal bipolar cells, together with receptor-selective manipulations, distinguished receptor mechanisms in different morphological OFF types. Differences in recovery from receptor desensitization filtered the same cone signal into different temporal components.^[ref]

Thus two OFF types can agree on whether a light decrement excites them while differing in response timing. Response sign, temporal filtering and neuron identity are related measurements, not interchangeable labels. The result from the studied retina should not be taken as an identical receptor assignment for every OFF subtype in every species.

An ON retinal bipolar cell is a retinal bipolar cell whose central receptive response shows depolarization when illumination increases appropriately. At the input synapse, this reverses the response sign of a photoreceptor, which undergoes hyperpolarization in response to light and releases less glutamate.^[ref]

The postsynaptic glutamate-sensing receptor mGluR6 controls a current requiring TRPM1. In a 2009 study, TRPM1-deficient mice retained the rod-related electroretinogram a-wave but lacked the b-wave; direct recordings of bipolar responses provided evidence for a postsynaptic signaling defect.^[ref]

A local sign model makes the circuit transformation visible. If a small light increment $\delta L$ changes glutamate release by $\delta G=-a\delta L$ and the ON response obeys $\delta V=-b\delta G$, with $a,b>0$, then $\delta V=ab\delta L>0$. These are illustrative gains, not fitted measurements. The label describes response polarity under specified stimulation, not continuous firing or identical behaviour in every ON subtype.

Parallel retinal pathways emphasizing increments and decrements in stimulation. Their sign differences depend on synaptic transformations of photoreceptor output; an ON pathway is not simply a photoreceptor that depolarizes to light.