Retinal Anatomy

Retinal anatomy     Photoreceptors. o Detect light. o Rods: light/no light. o Cones: colour (red, green, and blue). Bipolar cells. o Synapse with rods/cones and ganglion cells to transmit signals. o Can receive input from horizontal cells and transmit signal to amacrine cells. o Work via graded potentials, not action potentials (i.e. variable, not fixed, amplitude and time-course). Ganglion cells. o Receive input from bipolar cells and transmit signals to the brain via their axons (i.e. optic nerve). Horizontal cells. o Modulate photoreceptor signals (e.g. dim vs. bright conditions).   Amacrine cells. o Regulatory cells, but their role is poorly understood. Muller cells. o Support cells for retinal neurons. Photoreceptors   Rhodopsin (opsin protein + retinal) is on the outer segment of photoreceptors, and can absorb photons of light to initiate an intracellular signal cascade. Rods and cones behave similarly, but not identically. Phototransduction  Molecular players: o Rhodopsin: visual pigment. o Transducin tetramer. o PDE: phosphodiesterase with 2 inactivating alpha subunits.  Mechanism of action: o When a photon hits the rhodopsin molecule, cis-retinal changes to trans-retinal, separates itself from opsin, which exposes opsin’s transducin binding site. o Opsin-transducin binding converts a transduce GDP to GTP. o Activated GTP/transducin monomer leaves and binds 1 PDE alpha subunit. o When a second photon hits, the second alpha subunit from PDE is removed. o De-inhibited PDE converts cGMP to GMP, reducing the intracellular cGMP pool. o cGMP-gated Na+ channels turn off (i.e. Na+ entry to cell inhibited by light), leading to hyperpolarisation of the cell. o Reduced glutamate release from synaptic body then alters neural activity. Cones  Fewer cones than rods; about 4.6 – 6 million (~5%).  Packed into the centre of the fovea.  Functional properties: o Provide high spatial acuity (cone:ganglion = 1-2:1). o Gives low signal amplification (single photon detection). o High temporal resolution (flicker resolution up to ~50Hz).  There are 3 different types of cones, which have different photosensitive pigments with different maximal light absorption. o Red (M cone) maximal light absorption at ~560nm. o Green (L cone) maximal light absorption at ~530nm. o Blue (S cone) maximal light absorption at ~430nm.  Thus, cones are responsible for vision in good light (photopic vision) and colour vision.  After cones detect light and decrease glutamate release to bipolar cells: o ON bipolar cells depolarise and a signal to ganglion cell is transduced. o OFF bipolar cells hyperpolarise and inhibit signal transduction.  Midget ganglion cells (β) project to the parvocellular layers of the lateral geniculate nucleus (LGN).  Parasol ganglion cells (α) project to the magnocellular layers of the LGN. Fovea  Ganglion and bipolar cells are pushed to sides to maximise light to cones.  There are more cones in this area than there are rods. Rods  Many more rods in the human eye than cones; about 90 – 120 million (~95%).  Widely distributed over the retina, but absent from the fovea.  Functional properties: o High signal amplification (rod:bipolar cell = 600:1). o High sensitivity to light. o Low temporal resolution (flicker resolution <12Hz).  All rods contain rod opsin, which have a photosensitive pigment maximally sensitive to absorb light of ~500nm.  Thus, rods provide vision in dim light (scotopic vision).  Rod synapse with rod ON bipolar cells that depolarise in response to light.  Rod bipolar cells do not directly contact ganglion cells, but rather, pass their signals through amacrine cells to cone ON/OFF bipolar cells, and then to ganglion cells. Why isn’t rod acuity much better than cone acuity?  The problem: o Rods outnumber cones by ~20:1. o More densely packed. o More sensitive to light.  The reason: o There are equal numbers of bipolar cells for rods and cones. o Rod bipolar cells need to filter a larger amount of detail compared to the cone bipolar cells (since there are more rods than cones). o Therefore, the signal transduced will be less than the original, more so for rods than cones. Contrast  Each ganglion cell bears a receptive field. o There are two different types of receptive fields; on-centre and off-centre.  Receptive fields are composed of inputs from many rods and cones.  Each receptive field is arranged into a central disk (the “centre”) and a concentric ring (the “surround”), each region responding oppositely to light (e.g. light in centre might increase the firing rate of a particular ganglion cell, whereas light in the surround will decrease it).  Centre-surround receptive field organisation allows ganglion cells to transmit information not merely about whether photoreceptors cells are exposed to light, but also about the differences in firing rates of cells.