Ans. (d) PhotoreceptorsRef: Ganongs Review of Medical Physiology 25th Ed; Page No-180* It is a controversial question.* The retina consists of several layers, named from inside out as follows Pigmented epithelium, photosensors (rods and cones), horizontal cells, bipolar cells, amacrine cells, and ganglion cells.* Photoreceptors (Rods and cones): First order neuron* Bipolar cells: Second order neuron* Ganglion cells: Third order neuron* The space between neurosensory retina and pigment epitheliums is known as subretinal space.Neural arrangement in RetinaRetina cellCharacteristicsRods* There are 120 million rods in each human eye* Sensitive to low-intensity light; night vision* Dark adaptation: It adapts later* On activation, produce hyperpolarizing potential change.* Lower visual acuity; Predominant in extra foveal regions (Not present in fovea)* No colour vision* Pigment in rods is called rhodopsin or visual purple.Cones* There are 6 million cones in each human eye* Sensitive to high-intensity light; day vision* Dark adaptation: It adapts first* On activation, produce hyperpolarizing potential change.* Higher visual acuity; Present in fovea* Pigment-lodopsinBipolar cells* Receives signs from rods and cones and make connection with ganglion cells.* On activation, produce hyperpolarizing or depolarizing potential changes.Ganglion cells* They are the only output cell of retina.* Axons of ganglion cells form the optic nerve.* They are only retinal cell capable of producing action potential.* The convergence ration for bipolar cells to ganglion cells: 105:1Horizontal cells* An inhibitory interneuron,* On activation, produce hyperpolarizing potential changes.* Connects rods and cones.* Sharpens the signal by lateral inhabitation.Amacrine cells* An inhibitory interneuron.* They are the purely depolarizing cells of retina.* Connects ganglion cells.* Sharpens the signal by lateral inhabitation.Muller cells* They are the supporting glial cells* They have no role in visual processing.Features of Photoreceptor* Each rod and cone cells have a light-sensitive outer segment, which is connected to an inner segment by a thin connecting part.* The inner segment rich in mitochondria and establishes synaptic contact with the neighboring cells.* Synaptic terminal stores the neurotransmitter glutamate for signaling.* The outer segment of the rod cells contains membranous disks, and the plasma membrane of the outer segment of the cones is folded.* Visual pigments are stored in these disks and folds.Photoreceptor mechanism-Photo transduction* In retinal rods and cones, light-absorbing visual pigments and a variety of enzymes and transmitters mediate the conversion of light stimuli into electrical stimuli; this is called photoelectric transduction.* Vitamin A is an important component of visual pigment rhodopsin.* Rhodopsin is composed of integral membrane protein opsin and the aldehyde 11-cis-retinal. The latter is bound to a lysine residue of opsin which is embedded in this protein; it is stably kept in place by weak interactions with two other amino acid residues.* In response of light, there is an isoform conversion of Vitamin-A, which happens in acyclic form which as initially identified by Wald. It is also called as Wald visual cycle for which George Wald rewarded with Noble prize in 1964.* Photic stimuli trigger a primary photochemical reaction in rhodopsin in which 11-cis-retinal is converted to all-trans- retinal.* Even without continued photic stimulation, the reaction yields bathorhodopsin, the intermediates lumirhodopsin and metarhodopsin I, and finally metarhodopsin II.* Metarhodopsin II activates a G protein called transducing (Gt), which in turn activates a phosphodiesterase.* The conversion of cyclic guanosine monophosphate (cGMP) to 5'-GMP catalyzes by Phosphodiesterase; so, cGMP levels decrease.* Due to decreased levels of cGMP leads to closure of Na+ channels - Hyperpolarization of the photoreceptor cell membrane (As inward Na+ current decreased).* When the photoreceptor is hyperpolarized, there is decreased release of glutamate, an excitatory neurotransmitter. There are two types of glutamate receptors on bipolar and horizontal cells, which determine whether the cell is excited or inhibited.