Mechanism of vision: In the dark photoreceptors release an inhibitory neurotransmitter that prevents the activation of ganglion cells by bipolar neurons. Light inhibits the photoreceptors from releasing the inhibitory transmitter. Without this inhibition, the bipolar neurons are able to stimulate the ganglion cells. The ganglion cells transmit action cortex analyses and compare the input with earlier stored experiences for understanding the input (image).
The photoreceptors contain many Na⁺ channels in their plasma membrane. In the dark these channels are open and Na⁺ ions enter inducing depolarization of photoreceptors and release of inhibitory neurotransmitters. In the light, the photopigments present in the receptors become converted from 11-cis-retinene to all –trans-retinene. The trans-retinene gets dissociated from opsin protein. This brings up the closing of Ca⁺² channels and opens up of Na⁺ channels and the potential gradient is formed. The photoreceptors do not release the inhibitory neurotransmitter hence, the bipolar neurons release excitatory neurotransmitters stimulating ganglion cells.