A&P Visual Pathway 2022-23 Slides PDF

A&P 2022-23 Dr Sheila Rae The Visual Pathway Why do you need to know this? You had an introduction to the ‘visual field’ in FoO The visual field – the area in which we can see around ourselves – is routinely tested in optometric practice We will be covering visual field assessment and inte...

A&P 2022-23 Dr Sheila Rae The Visual Pathway Why do you need to know this? You had an introduction to the ‘visual field’ in FoO The visual field – the area in which we can see around ourselves – is routinely tested in optometric practice We will be covering visual field assessment and interpretation of visual field defects in CSFO1 in year 2 I will expect you to remember the anatomy that we have covered here! Visual Pathway Overview The visual pathway is the linking group of anatomical structures than transmit visual information from the retina to the primary visual cortex Damage at any point along the pathway will affect vision There is a very specific anatomical arrangement which changes along the pathway Allows the location (and cause) of visual field defects to be pinpointed based on their pattern  Visual Field Defect Localisation I know that this Px has a disease affecting the retina of the right eye, specifically the superior retina and affecting the inner layers of the retina I know that this Px has a disease affecting the temporal lobe of the left cerebral hemisphere, such as a stroke affecting the middle cerebral artery What should you know after this? Know in detail all the structures along the visual pathway (VP) Understand how the arrangement of nerve fibres changes along the course of the VP Know the pattern of related visual field defects if the VP is damaged along its course Be aware of associated structures that lie close to the VP Structure of the Key structures (front to back) Visual Pathway Optic nerve Optic chiasm Optic tract LGN Optic radiations Visual cortex Associated structures Pituitary gland Edinger Westphal nucleus Circle of Willis Synapses There are three neurons in a chain in the visual pathway There are two synapses Between the bipolar and ganglion cells in the retina At the LGN Optic Nerve: Revision Extends from the retina to the optic chiasm Optic nerve head (ONH) or optic disc is the collection point and exit point for the axons of the retinal ganglion cells ONH is what we see when looking in 3D e.g. with OCT or indirect ophthalmoscopy Optic disc is what we see when looking in 2D e.g. fundus photo or direct ophthalmoscopy Optic Nerve: Revision The axons travel across the retinal surface in the retinal nerve fibre layer (RNFL) then gather and pass into the optic nerve head The same fibres then continue their journey along the nerve to the optic chiasm Arrangement of Nerve Fibres in the Optic Nerve At the start of the optic nerve, fibres arranged based on retinal location Rearranged towards end of optic nerve with macular fibres in the centre Optic Chiasm Each eye (retina) forms an image of both the right and left sides of the visual field Right half on one side of the retina, left half on the other The visual field is therefore duplicated on the retinas of the R and L eyes and in the optic nerves At the optic chiasm, the visual information is sorted so all nerve fibres transmitting info from the right visual field end up together and all from the left visual field end up together Optic Chiasm The nerve fibres (neurons) that originate from the nasal side of each retina cross to the opposite side of the pathway Info from nasal retina of one eye joins with info from temporal retina of the other eye, and vice versa The crossing is called ‘decussation’ Around 53% of fibres cross in ‘neuro-typical’ individuals Varies in other species and in conditions such as albinism Optic Chiasm At the crossing point Inferior retinal peripheral fibres cross anteriorly Superior retinal peripheral fibres cross posteriorly Extension of the inferior and superior fibres into the contra-lateral optic nerve / tract has been suggested Knee of Willebrand Proven not to exist Optic Chiasm At the crossing point Inferior retinal peripheral fibres cross anteriorly Superior retinal peripheral fibres cross posteriorly Extension of the inferior and superior fibres into the contra-lateral optic nerve / tract has been suggested Knee of Willebrand Proven not to exist Optic Chiasm: Knee of Willebrand ‘Observed’ in dissected chiasms Extent of the ‘knee’ exaggerated Still commonly cited in text books and articles Optic Chiasm and Visual Field Defects Visual field defects can be groups together as Pre-chiasmal Chiasmal Post chiasmal Pre-chiasmal lesions will affect the visual field on the affected side A RE lesion would affect the RE visual field Post-chiasmal lesions will affect the visual field of BOTH eyes What will happen if a lesion is at the chiasm? Optic Chiasm and Visual Field Defects What will happen if a lesion is at the optic chiasm? Only point in the pathway where the nasal retinal fibres from each eye are in the same location A lesion at this location will affect the nasal retinal fibres of BOTH eyes therefore the temporal visual field of BOTH eyes Optic Chiasm Associated Structures Circle of Willis Connecting ring of arteries supplying the brain Aneurysm of the internal carotid artery can press against the chiasm Pituitary gland Sits below the chiasm Pituitary adenoma can press on the chiasm from below Optic Tract Extends from the posterior chiasm to the LGN Nerve fibres representing the right visual field are now together on the left side of the pathway, and vice versa Right nasal retina and left temporal retina together Rare site of lesions causing visual field defects Rearrangement of fibres within the tract so the macular fibres are now at the top Lateral Geniculate Nucleus 6 (LGN) 5 Can also be referred to at the lateral geniculate body 4 3 There are a pair, therefore nuclei They are nuclei, therefore are a site of a 2 synapse 1 Folded over, or horseshoe shape Arranged in six layers LGN Dorsal aspect of the LGN is the outside of the curve Ventral aspect of the LGN is the inside of the curve Lateral and medial horns  Optic tract LGN The nerve fibre arrangement from the optic tract is reflected in the LGN ‘Retinotopic’ arrangement Macular fibres represented in the LGN central areas Six distinct layers 2, 3 and 5 have uncrossed fibres, from the ipsilateral eye 1, 4 and 6 have crossed fibres, from the contralateral eye LGN Layers 1 and 2 have larger cells Magnocellular Contrast, low light, visual search Layers 3, 4, 5 and 6 have smaller cells Parvocellular Colour, detail Processed separately beyond the visual cortex Optic Radiations So called as the nerve fibres radiate out over a bigger area rather than being close together Extend into both the parietal and temporal lobes of the brain Potential for some parts of the radiations to be affected by a lesion whilst other parts remain intact Optic Radiations Meyer’s loop is the outside bundle of the radiating fibres Fibres from the inferior retina / superior visual field Inside of the loop has fibres from the superior retina / inferior visual field Lesions affecting the outside or inside of the loop only will affect one quarter of each visual field Quadrantanopia Optic Radiations Superior retina / inferior visual field in the parietal lobe Middle cerebral artery Inferior retina / superior visual field in the temporal lobe Posterior cerebral artery Posterior Part of the Optic Radiations Fibres from the superior and inferior retina now closer together Lesions tend to affect both Half field defects Hemianopia (Primary) Visual Cortex Nerve fibres that began at the synapse in the LGN terminate and synapse at the visual cortex Fibres project to different areas of the cortex depending on origin and purpose Retinotopic arrangement Macular fibres take up most space Impact of this? (Primary) Visual Cortex Visual field defects that are due to lesions closer to the visual cortex have some particular features Congruous- they are very similar in the two eyes Macular sparing – there is a half field loss (hemianopia) but the area around the macula is unaffected Thought to be because the macula covers a large area of the visual cortex, not all macular fibres may be damaged or there may be some redundancy Blood Supply to the Visual Cortex Common cause of visual field defects is cerebro vascular accident (CVA) Affected blood supply determines the type of defect Visual Field Defect Recap Pre-chiasmal Chiasmal Post-chiasmal (Primary) Visual Cortex Six layers Layer 1 is outermost / nearest the surface Layer 6 is deepest Input from LGN is to layer 4 Output from layers 1 to 3 to other cortical areas Output from layers 5 and 6 to thalamus and brainstem (Primary) Visual Cortex Arrangement from the LGN is maintained Input from LGN into layer 4 Layer 4 divided into sub-layers 4a, 4b, 4c Layer 4c further divided into 4cα and 4cβ Magno Parvo Magnocellular pathway inputs to 4cα Parvocellular pathway inputs to 4cβ (Primary) Visual Cortex Input originating from each eye is kept separate in layer 4c Similar to LGN Cells kept together in ‘ocular dominance columns’ Further divided into cells that respond to specific orientation Information originating from the same areas of the retina and specific to particular orientations are arrange in ‘hypercolumns’ Higher level processing: magno and parvocellular pathways