retinal signal processing LOCKED FINAL.pptx

This presentation done to help 1st year ophthalmology resident to understand the retinal processing

1. Signal processing in the outer and inner retina Supervisor Dr.Najah Al Quriashi Introduced by Ayad Ibrahim

2. Objective To know about the histophysiology of synaptic terminal To know major concept about retina To know about neurotransmitter in the retina To know about the cells of the vertical and lateral pathways To know about Muller cell and their activity To know about some clinical concept

3. Structure of Photoreceptor S ynaptic Termina l The rod spherule The rod spherule is sphere shaped. Contains mitochondria , glutamate containing vesicles , and synaptic ribbons positioned close to the presynaptic membrane. The rod spherule forms a synaptic triad with ON bipolar and H1 horizontal cells axons. The glutamate-containing vesicles fuse to either side of the ribbon prior to release. The cone pedicle The cone pedicle has a similar structure to the rod spherule but is larger, containing several triads with more ribbons and synaptic targets. It forms complexes with ON and OFF bipolar cells and H1 and H2 horizontal cells. Inter-photoreceptor junctions Gap junctions exist between photoreceptor cell terminals permitting electrical coupling . Rod-rod and cone-cone coupling are stronger than rod-cone coupling. Rod-rod coupling decreases visual resolution and, however, improves the gain of photoreceptors; it is important for rod function under dark-adapted condition .

4. Structure of Photoreceptor Synaptic Terminal GJ ……A low-resistance intercellular passage allowing direct chemical communication between adjacent cells through diffusion These allow intercellular communication and coordination

5. Inner Retinal Circuitry Key Concepts Rod and cone pathways The rod pathway I s tuned to scotopic visual information. It is sensitive at low levels of illumination and does not convey color. The cone pathway conveys photopic information . It is color sensitive and highly discriminative for fine acuity, requiring moderate to high illumination levels.

6. Inner Retinal Circuitry Key Concepts Parallel processing Parallel processing of visual information provides simultaneous analysis of various visual characteristics (e.g., line, shape, color, movement, and texture )

7. Inner Retinal Circuitry Key Concepts Convergence Convergence of signal involves many photoreceptors synapsing onto one bipolar cell and many bipolar cells synapsing onto one ganglion cell. Convergence is least at the fovea. convergence increases with retinal eccentricity. Convergence is greater for rod than cone pathways . It increases rod sensitivity at the expense of resolution.

8. Inner Retinal Circuitry Key Concepts

9. Inner Retinal Circuitry Key Concepts

10. Inner Retinal Circuitry Key Concepts ON/OFF channels Separate ON and OFF channels exist within the inner retinal neural network . These convey the appearance and disappearance of light, respectively . Cones contribute to both the ON and OFF channels. Rods primarily contribute to the ON channels their contribution to the OFF channels is indirect through influence of cone channels . The ON/OFF division is preserved from the level of bipolar cells to ganglion cells

11. Inner Retinal Circuitry Key Concepts Center-surround organization Receptive field is the influence area of a sensory neuron, it is circular in configuration. Stimulate Entire Retina ------- VF. Patch of the Visual Field ------- Receptive Field.

12. Inner Retinal Circuitry Key Concepts

13. Neurotransmitters and Receptors Glutamate Glutamate is the predominant excitatory neurotransmitter within the retina. It is used by photoreceptors, bipolar cells, and most ganglion cells. Ionotropic receptors = Off bipolar cell ----- +VE Metabotropic receptors = On bipolar cell ----- -VE

14. Neurotransmitters and Receptors GABA and glycine Gamma-aminobutyric acid ( GABA ) and glycine are the predominant inhibitory neurotransmitters . GABA is used by horizontal and wide-field amacrine cells, whereas glycine is used by small-field amacrine cells . These inhibit membrane depolarization by opening anionic (Cl − ) membrane channels.

15. Neurotransmitters and Receptors Dopamine Is a modulatory retinal neurotransmitter predominantly released by polyaxonal amacrine cells. On release dopamine traverses through the retina targeting every retinal cell type. Dopaminergic activation reduces cell-cell coupling of photoreceptors, horizontal, amacrine , and ganglion cells and increases transmission speeds . This converts the retina from a scotopic to photopic state .

16. Neurotransmitters and Receptors Acetylcholine  is a fast excitatory neurotransmitter ------starburst amacrine cells ---------targets directionally sensitive ganglion cells Nitric oxide (NO )  reduces cell-cell coupling ------------- facilitates retinal light adaptation. Neuropeptides  < somatostatin , substance P, and neurotensin > Co-transmitters------substance release at the time of release of NT to modify action of NT.

17. Action Potential and Graded Potential

18. Action Potential and Graded Potential Photoreceptor is a unique in that there are a steady current flow even in absence of light PR , HZ and BC respond to light with sustained graded potential rather than action potentials. The retina allowed to make discrimination in intensity and duration of visual stimuli that would not be possible with action potential. The release of NT at synapse is graduated in proportion to the amount of current flow and the response of post-synaptic cell is graded in turn. So the response is proportional to the light intensity.

19. Retinal cell Bipolar Cells Bipolar cells (BCs) carry signal from photoreceptor cells to ganglion cells and amacrine cells . They contain synaptic ribbons and use glutamate as their neurotransmitter, released on graded depolarization in response to photoreceptor and/or HC signal . BCs are the first visual pathway neuron with ON/OFF and CSARF organization. In the IPL, BCs form dyads (i.e., ribbon synapses which typically have two postsynaptic processes deriving from one amacrine and one ganglion cell, two ganglion cells, or two amacrine cells) . Type of Bipolar cell Cone Bipolar cell [ on and off]. Rod bipolar cell [ on only ].

20. Retinal cell OFF Chanel OFF BC-----Ionotropic glutamate [ depolarization of off BC by high Glutamate]. OFF BC pathway---- OFF ganglion cells , amacrine cells , and/or----- ON/OFF ganglion cells . ON Chanel Cone ON BC-----metabotropic glutamate receptors [ Hyperpolarization of On BC by high Glutamate]. Cone ON BC pathway----- ON ganglion cells, amacrine cells and/or ON/OFF ganglion cells . ----- ----- ------ ----- ----- ----- ----- ----- ----- ------ ------ ------ ----- ----- ----- Rod ON BC- --- metabotropic glutamate receptors [ Hyperpolarization of On BC by high Glutamate]. Rod BCs ------------- AII amacrine cells Direct pathway (a) Form electrical synapses with ON cone bipolar cells which in turn contact ON ganglion cells . (b) Make inhibitory chemical synapses onto OFF bipolar cells which in turn contact OFF ganglion cells. Indirect pathway cone spherule synapse indirectly with cone pedicle or indirectly with cone bipolar cell

21. Retinal cell

22. Retinal cell Center surround Organization Bipolar cell

23. Retinal cell

24. Types of RGC 1-P Cells 90%----- color , shape, details----acuity 2-M cells 5%---- large no. of connection responsible for motion-----Field 3- Non-M or P cells 5%----- responsible for light reflex. -------------------------------------------- Are the first cell that show action potential.

25. Retinal cell

26. Retinal Cell

27. Retinal Cell 1–3 % of GCs contain the photo-pigment melanopsin and can intrinsically sense light . Melanopsin uses a retinoid chromophore and has peak sensitivity in the blue region (480 nm ). They receive input from rod and cone pathways via bipolar and amacrine cells. Photosensitive GCs project to the hypothalamus and pretectal area ,controlling circadian rhythm and pupillary responses. Some project to the lateral geniculate nucleus and may be responsible for residual vision in degenerative outer retinal disease Resistance to pathological states (e.g., glaucoma , excitotoxicity)

28. Retinal Cell

29. Retinal Cell

30. Retinal Cell Contact a wide area of photoreceptor. Electrically coupled to one another by GJ. Each PR is in contact with at least one HC.

31. Glial Cell

32. ERG The a-wave The a-wave is generated by light-induced photoreceptor hyperpolarization , with some post- receptoral contributions from OFF bipolar cells. Cones respond more quickly than rods, so cone hyperpolarization gives earlier negative deflection The b-wave The b-wave is generated by light-induced membrane potential change in ON (depolarizing) bipolar and Müller cells . The c-wave The c-wave is an additional waveform reflecting RPE and Müller cell activity Electroretinogram duration The duration of the response is usually <150 ms. The implicit time (τ) is from stimulus onset to the peak of the b-wave.

33. Clinical Relevant with ERG Retinitis Pigmentosa Cone dystrophy Choroideremia Gyrate atrophy Etc ….

34. VEP The visual evoked potential ( VEP ) is a recording of electrical signal arising in the visual cortex in response to monocular visual stimulus. The P100 latency and amplitude can be affected by disease. D emyelination ------- < increased latency of the P100. I schemic injury- ----- < reduces amplitude of the P100 . Compressive ------ < reduces Both.

35. References