retina4_2022_23.ppt

Retina 4/7 1-2. Structure of the retina 3. Phototransduction cascade in photoreceptors: from photoactivation of visual pigments to hyperpolarisation of photoreceptor plasma membrane 4. Signal processing in the retina; colour vision 5. Shut-off of phototransduction; adaptation to light 6. Dark adapta...

Retina 4/7 1-2. Structure of the retina 3. Phototransduction cascade in photoreceptors: from photoactivation of visual pigments to hyperpolarisation of photoreceptor plasma membrane 4. Signal processing in the retina; colour vision 5. Shut-off of phototransduction; adaptation to light 6. Dark adaptation; retinoid cycle 7. Ageing of the retina Phototransduction in photoreceptors ROD CONE Activation of Rhodopsin Activation of Transducin Activation of PDE Degradation of cGMP → Closure of cGMP-gated channels → Na+↓ and Ca2+↓ → Hyperpolarization of plasma membrane Inhibition of the release of neurotransmitter (glutamate) Outer segment Inner segment Synaptic terminal pedicle spherule Phototransduction in photoreceptors … - Hyperpolarization of plasma membrane spreads to the synaptic terminal - voltage-dependent Ca2+ channels in the synaptic terminal close - ↓ influx of Ca2+ (Ca2+ needed for fusion of neurotransmitter-carrying vesicles with plasma membrane) - Inhibition of the release of neurotransmitter (glutamate) Synapses and Gap Junctions (chemical synapses and electrical synapses) Phototransduction CONE ROD Outer segment Phototransduction Inner segment Light Synaptic terminal CLASSES OF RETINAL NEURONS axon ganglion cell Neurotransmitters: amacrine cell glutamate GABA (γ-aminobutyric acid) and many others bipolar cell horizontal cell Hyperpolarisation or Depolarisation Gap junctions photoreceptor Light Activation of Rhodopsin Activation of Transducin (G protein) Activation of PDE Degradation of cyclic nucleotides (cGMP) Closure of cGMP-gated channels → Na+↓ and Ca2+↓ → Hyperpolarization of plasma membrane Neurotransmitter Activation of G protein-coupled receptor Activation of G protein Activation of an effector molecule (for example PDE, Adenylyl cyclase) Degradation or synthesis of cyclic nucleotides (for example cAMP) Closure of cyclic nucleotidegated channels or other ion channels → change in influx of ions → change in polarization of plasma membrane http://webvision.med.utah.edu/book/part-v-phototransduction-in-rods-and-cones/glutamate-and-glutamate-receptors-in-the-vertebrate-retina/ Omega-3 lipids Docosahexaenoate (DHA) - deficiency causes impairment of learning and memory Signal processing 1.25 million ganglion cells 125 million photoreceptors Rod synaptic connections Rod spherule: 2 ribbons associated with invaginated second-order neurites: -rod bipolar cell dendrites with glutamate receptors mGluR6, for which glutamate acts as an inhibitory neurotransmitter -(rb; ON BC depolarizes in response to onset of light) - horizontal cells (HC) neurite terminals + 3-5 gap junctions occur on a single rod spherule from neighbouring cone telodendria Cone synaptic connections Cone pedicle: ~ 30 ribbons associated with "triads" of invaginated processes ON BC and OFF BC Horizontal cells OFF BCs express two types of glutamate receptors (one responsive to AMPA, the other - to kainate); hyperpolarize in response to light onset + gap junctions (a single L or M cone pedicle can have gap junction contacts to as many as 10 neighbouring rods) Synaptic connections of photoreceptors and their responses to light “ON” “ON” “OFF” Bipolar cells: more than 10 types Rod bipolar “S-cone” bipolars Midget bipolars Diffuse bipolars Rod bipolars - connect up to 50 rods, connect to ~4 ganglion cells S-cone bipolars - exclusive to S cones Midget bipolars - join one cone with one ganglion cell, foveal Diffuse bipolars - widespread synaptic connections (up to 7 cones) and many ganglion cells, excitory or inhibitory, hyperpolarising or depolarising Bipolar cells - are in contact with all types off retinal neurons axon parasol ganglion cell diffuse bipolar cell 5-6 diffuse bipolar cells ~10 cones 20-25 cones http://webvision.med.utah.edu/book/part-v-phototransduction-in-rods-and-cones/bipolar-cell-pathways-in-the-vertebrate-retina/ http://www.sciencedirect.com/topics/biochemistry-genetics-and-molecular-biology/bipolar-neuron Horizontal Cells Review on horizontal cells in the retina: Thoreson WB and Mangel SC (2012) Lateral interactions in the outer retina. Prog. Ret. Eye Res. 31(5):407-41 HII Horizontal cells HI Synaptic connections with photoreceptors; hyperpolarize in response to light (express iGluRs); release inhibitory neurotransmitter GABA to provide feedback to cones; gap junctions three types, HI, HII and HIII multipolar, processes may be as long as 1 mm HI – small dendritic fields (75-150 m) and long axon (~300 m); dendritic arbour connects with S, M and L cones, axonal arbour connects with rods HII - dendritic arbour connects to S cone and axonal arbours connect with S, M and L cones HIII – larger dendritic fields and contact more cones than HI; dendritic arbour connects with L and M cones, axonal arbour connects with L and M cones Amacrine Cells many types (>20) which vary in size, morphology and function starburst - 40% of total amacrine population dopaminergic - 50/mm2 at fovea, 10 /mm2 at periphery, (neuromodulator dopamine) A1 - 21/mm2 receive from bipolar cells and other amacrine cells send messages to bipolar cells, amacrine cells and ganglion cells Amacrine Cells Many different neurotransmitters: GABA, glycine, dopamine, acetylcholine serotonin Retinal Ganglion Cells (RGCs) At least 25 different types Retinal ganglion cells (RGCs): generate action potential biplexiform blue-yellow midget parasol vary in diameter from 10-30 m, multipolar, 1.25 million present in the retina majority (70%) are midget, receive from midget bipolars parasol 8-10%, receive from diffuse bipolars coupling with amacrine cells one cell thick RGC layer except for the macula (in parafovea up to 8-10 nuclei thick) A small subset of RGCs known as intrinsically photorsensitive RGCs (ipRGCs) express pigment melanopsin and are responsible for photoentrainment and pupillary light reflex In order for an action potential to occur, the sum of excitatory and inhibitory postsynaptic potentials must be greater than a threshold value. RGC Retinal ganglion cell axon: Change in frequency in generation of action potential ON OFF ON-OFF Signal Processing in RGCs axons in the nerve fibre layer unmyelinated enveloped by extensions of Müller’s cells and astrocytes RGCs axons in the nerve fibre layer travel towards the optic disc and then in the optic nerve From the retina to the brain Retina prosthesis - retinitis pigmentosa - AMD Retinal prosthesis in clinical trials 2007 Retinal Prosthesis http://news.bbc.co.uk/2/hi/health/7919645.stm Retinal Prosthesis Current clinical trials of Argus II Phototransduction 125 million photoreceptors 1.25 million ganglion cells Different neurotransmitters Hyperpolarisation or Depolarisation Further developments of retinal prosthesis (animal models) migration of retinal cells into the perforated sub-retinal implant stimulation on subretinal side (in contrast to epiretinal side currently used) Colour Vision Colour Vision 3 types of cones each of them with different absorption spectrum: S cone, M cone, L cone → trichromatic colour vision S M L Visual Pigments: absorption of light S M L Can we see colours using rods only? cones rods Light Light Light trichromacy Colour Vision cones rods S M L Colour sensitivity depends on relative ratios of different types of cones: S-cones: - lowest density in the foveal pit (3-5% of the cones) - reach a maximum density of 15% on the foveal slope - 8% of the total cone population elsewhere Colour Vision Colour perception depends on spectral range of light reaching the retina (age-related yellowing of the lens) S M L Colour Vision 27 y.o. 68 y.o. 72 y.o. S M L Colour Vision Processing of the signal from cones in the retina and brain Mantis shrimp: 12 types of cones Can distinguish colours that differ by 25 nm Humans with only 3 types of cones can distinguish colours that differ by 5 nm Recommended Reading Retinal circuits: WebVision: https://www.ncbi.nlm.nih.gov/books/NBK11530/? term=webvision Oyster, C. W. (1999) The Human Eye Rodieck, R. W. (1998) The First Steps in Seeing Matthews, G. G. (2001) Neurobiology. Chapter on “Light responses of 2nd-order Retinal Neurons” (pp.: 332-350). For fun: https://www.pinterest.co.uk/pin/551761391817808106/ http://web.mit.edu/persci/people/adelson/ (Webpage of Prof. Edward H. Adelson with optical illusions and their explanations)