Summary circuit diagrams for synaptic inputs to LED ganglion cells.

<p>Similar circuitry is envisaged for the OFF and ON inputs. All cone bipolar cells (CBC, green) in the diagram have the same sign response, i.e. OFF or ON. GABAergic amacrine cells are red, glycinergic amacrine cells are blue. Top: Centre responses are mediated by relatively sustained glutamatergic bipolar cell inputs (CBC1) and di-synaptic feed-forward glycinergic amacrine cell inputs (CBC2→AC1), which are more transient. Note that, despite the additional synapse, these glycinergic inputs activate rapidly enough to veto excitation, and delay spiking in the LED. The suppressive effect of strychnine on the excitatory input to the LED might be accounted for by a cascade of glycinergic and GABAergic synapses. Under control conditions, activation of a glycinergic amacrine cell (AC2) suppresses presynaptic inhibition from a GABAergic amacrine cell (AC3). The resulting dis-inhibition extends glutamate release from CBC1. Application of strychnine blocks this effect, thus allowing presynaptic inhibition from AC3 to suppress the excitatory input to the LED. This model requires that CBC3 drive tonic activity in AC3. Bottom: Surround inhibition is mediated by presynaptic GABAergic inhibition of the excitatory bipolar cells (AC4), and the feed-forward glycinergic amacrine cells (AC5). Different amacrine cells are shown since TTX partially suppresses the excitatory but not the inhibitory surround.