Neuro Visual PDF

Embryology of the Eye Surface Ectoderm: Cornea and lens Neural Crest Cells: Sclera and choroid Neuroectoderm: Iris, Optic Nerve and retina Mesoderm: Vasculature and fibers of the eye Diencephalon Derived from the prosencephalon (forebrain) Adult brain structures: ○ Thalamus ○ Hypothalamus ○ Epithala...

Embryology of the Eye Surface Ectoderm: Cornea and lens Neural Crest Cells: Sclera and choroid Neuroectoderm: Iris, Optic Nerve and retina Mesoderm: Vasculature and fibers of the eye Diencephalon Derived from the prosencephalon (forebrain) Adult brain structures: ○ Thalamus ○ Hypothalamus ○ Epithalamus ○ Subthalamus ○ PART of the neural retina (CNS) ○ Optic Nerve ○ Optic Chiasm ○ Optic Tract Structures of the Eye Macula Lutea ○ Found on both left and right sides of the eye ○ Can be seen laterally when you look in the eye Fovea Centralis ○ Surrounded by Macula Lutea ○ Site of clearest vision - lots of cones and absent rods ○ Separated into nasal and temporal halves Ora Serrata ○ Transition between non photosensitive region and photosensitive region Optic Nerve CN II ○ Surrounded by cranial meninges ○ Oligodendrocytes provide myelination to CN II and CN III The myelination is provided by the CNS Multiple sclerosis affects the CNS If Myelination was provided by PNS: schwann cells ○ The optic nerve leaves the eye through optical canal Fibrous Sheath ○ Surrounds the fibers of the optic nerve ○ Continuation of the dura mater (continuation of the brain) Central Artery of the Retina ○ Branch of ophthalmic artery ○ ○ ○ ○ ○ Central artery emerges on the posterior pole of the eyeball after traveling with the optic nerve (CN II) in the dural sheath Does not anastomose with anything Vascularizes the eye and nourishes inner retinal layers Does not nourish photoreceptor layers (supplied by choriocapillaris from choroid) If edema happens in the area of the central artery- it will compress the arteries and veins which will lead to ischemia and vision loss Optic Disc ○ No receptors in the optic disk because of the vessels and fibers of optic nerve are coming together in this region These fibers are located in the fovea centralis ○ Optic Disk has a blind spot Small area in the visual field No receptors as the receptors are in the fovea centralis ○ Optic nerve comes together at the optic disk Fundus ○ Back of the eye Papilledema ○ Abnormal increase in CSF pressure in the subarachnoid space near the optic nerve ○ Edema of retina due to swelling of the optic disc - due to the lack of venous return of the retina which leads to fluid accumulation ○ Tell us increase intracranial pressure but does not tell us the etiology of the edema Retina Nourished via choroid because the retina is avascular Receives the external visual stimuli. ○ The posterior pole of the eyeball is connected with the optic nerve (CN II), which conveys the information from the retina to the brain. ○ After the processing in the cerebral cortex, the visual stimuli become visual information Has two layers that are not fully attached to each other ○ Therefore there is a possibility for retinal detachment 1) Retinal Pigment Epithelium (RPE) Deepest layer Firmly attached to the choroid- this is important for nourishment as they have no blood supply Tight junction between cells Transport between choroidal capillaries and neural retina is mediated by transport across RPE 2) Neural Retina Important for vision, conveys visual message Derives from diencephalon Blood retinal barrier Needs the RPE- one layer cannot be without the other one Structure of the Retina Direction of light (orange) Direction of information (purple) Has 10 histological layers in vitreous humor ○ 3 FUNCTIONAL LAYERS: Cones and Rods, Bipolar Cells, Ganglion Cells Ganglion Cell ○ 2nd order neurons in visual pathway ○ Multipolar cells - synapse with bipolar and amacrine cells ○ Bridge between photoreceptors and lateral geniculate nucleus of the thalamus Send output to LGN and it goes to visual cortex ○ Most superficial layer ○ Axons will form the optic nerve - axons are unmyelinated Axons of ganglion cells comprise optic nerve ○ First to come in contact with light ○ Respond to various stimulus Ex: on/off cells according to illumination Monochromatic vs color Transient vs. sustained stimuli Receptive fields (small vs large) Amacrine Cell ○ Help with processing/modulating information ○ Synapse with dendrites of ganglion cells and bipolar cells ○ Stimulated bipolar cells, inturn they will stimulate ganglion cells ○ Indirect connection between ganglion and bipolar cells Bipolar Cell ○ First order neurons in visual pathway - collect information from photoreceptor and give information to ganglion cells ○ Give information to the ganglion cells Horizontal Cell ○ Help with processing/ modulating information ○ Located around apices of rods and cones -synapse with rods and cones Cone & Rod (Receptors/Transducers) ○ Do not come in contact with light first ○ No axons, instead sends their information to the bipolar cells that will give information to axons ○ Located in the deepest layer of neural retina ○ Respond to electrical impulses and action potentials ○ Cone cells are more superficially located and are sensitive to different types of light ○ Rods are longer and are located on the pigmented layer Pigmented epithelium ○ Attaches firmly to choroid (not neural retina) ○ Serves to absorb light that passes through the retina and prevent it from reflecting back to the neurosensory layer. ○ Choroid ○ Vascular layer of the eye Firmly attached to retina and loosely attached to sclera Sclera Vision according to the intensity of light Starlight (no moon) ○ Only rods Moonlight (stars and moon) ○ Rods and Cones More than moonlight ○ Only cones Rods and Cones- Photoreceptors in neuroepithelial Absorbs light and converts from into electrical signal Rods- Achromatic & Night Vision High sensitivity in night vision Low density light (scotopic) - therefore responsible for vision in dim light Located across the retina except the center of fovea - density of rods increase away from of fovea Cannot distinguish wavelength- monochromatic (black and white) Cylinder shaped and most abundant ○ Rod outer segments are cylindrical in shape, consisting of flat, lobulated, membranous discs. Cones - Colour & Day Vision Cone outer segments are generally shorter than that of rods Present in fovea - Higher acuity (fine detail) of vision than cones Contain visual pigment formed by a protein + Vitamin A Colour Sensitive - Trichromatic- according to wavelength ○ Iodopsin pigments: Red (most numerous), green, and blue (least numerous) (RED>GREEN>BLUE) Blue has the highest sensitivity and is found OUTSIDE of fovea centralis (the medulla) Red and Green are packed INSIDE of fovea centralis For color vision two types of cones have to be stimulated (there are overlapping) ○ Different hues are seen by the different composition of the cone cells Color Blindness (Achromatopsia) ○ Inability to differentiate color- commonly red and green ○ Related to X chromosomes ○ Affects cone cells ○ More common in males than females Heterochromia- two different eye colors Anisocoria- refers to two different diameters of pupil ○ Visible in bright light ○ Could be normal ○ Could be caused by scopolamine patches (prevents nausea and vomiting) ○ Horner syndrome - interruption of sympathetic innervation Constricted Pupil, dry eye Visual Pathway Background information: Information is received from the visual field of the contralateral eye ○ What is up in the visual field will be low in the visual cortex (& vice versa) ○ Left visual cortex processes information from the right half of visual field ○ Right visual cortex processes information from the left half of visual field How come we see two things together? Because we have two hemispheres that communicate to each other via corpus callosum splenium 1) Light enters the eye cornea and projects on to the retina that has the photoreceptors: ○ Cones: Operate in bright-light conditions Responsible for visual acuity and color perception ○ Rods: Operate in dim-light conditions Responsible for low-light vision 2) Signal from the retinal photoreceptors go to the bipolar cells (1st order neuron) → then into the ganglion cells (2nd order neuron) 3) Axons from the ganglion cells converge and form the optic nerve (CN II) ○ No photoreceptor cells located here ○ Optic nerve emerges from posterior pole of the eye, travel through the superior orbital fissure and meet with contralateral optic nerve @ optic chiasm ○ Carries fibers of the retina 4) Fibers cross the optic chiasm (located in middle cranial fossa) ○ ○ ○ Bodies of neurons are in the retina- there are no bodies or synapse in optic chiasm Optic chiasm is point of union and decussation of bilateral CN II Fibers from the nasal aspect of each retina crossover (or decussate) to the contralateral optic tract, while fibers from the temporal retina remain ipsilateral 5) Fiber then go into the optic tract - stimulated by contralateral visual fields ○ Optic nerve bring information from both sides of the visual field ○ Fibers from optic tract-optic chiasm get information from the contralateral side of the visual field Nasal fibers from left eye cross over and join temporal fibers of right eye → Right optic tract Nasal fibers from the left eye and right temporal fiber will get information from left visual field Nasal fibers from right eye cross over to join temporal fibers left eye → left optic tract Nasal fibers of right eye and temporal fibers of left eye - get information from the right half of visual field Visual input from the right visual field will travel (decussation) with the left optic tract Visual input from the left visual field will travel with (decussation) right optic tract ***Summary*** Right visual field→ Right Nasal Fiber w. Left Temporal Fiber → Left Optic Tract Left visual field → Left Nasal Fiber w. Right Temporal Fiber → Right Optic Tract ○ Optic Tract will pass through pulvinar nucleus and synapse with LGN 6) Fibers then go to the ipsilateral LGN (Lateral geniculate nucleus/body of thalamus)runs on either side of diencephalon ○ Part of the thalamus - Relay station ○ 3rd order sensory neurons ○ Synapse happens here and project fibers into the visual field ○ Left visual field: represented by the right optic tract and LGN ○ Right visual field: represented by the left optic tract and LGN 7) Optic radiation ○ Fibers from LGN to visual cortex ○ Carries visual information from the contralateral visual field (same as the optic tract carrying contralateral visual field info) Recall: Right visual field→ Right Nasal Fiber w. Left Temporal Fiber → Left Optic Tract Left visual field → Left Nasal Fiber w. Right Temporal Fiber → Right Optic Tract ○ Lower (inferior) optic radiation- carry information from superior visual field Located on the lateral side of optic radiations ○ ○ Travel anteriorly and laterally around temporal horn of lateral ventricle then posteriorly meyer's loop to reach visual cortex Located in the inferior portion of the occipital lobe - it carries information from the superior retina (lower visual field) to contralateral primary cortex Pass through inferior bank of calcarine Upper (superior) optic radiation- carry information from inferior visual field Located on the medial side of optic radiation Travel through parietal lobe to reach visual cortex Pass through internal capsule and superior salcarine sulcus of cuneus Side note: Optic radiation receives blood supply from the middle cerebral artery and posterior cerebral artery. Occipital pole ○ Fovea is giving the occipital pole information ○ Posterior pole of the eyeball is connected with the optic nerve (CN II) 8) Primary Visual Cortex ○ Optic radiations head posteriorly to its ipsilateral primary visual cortex ○ Primary visual cortex receive information form contralateral visual field Summary: retina → optic nerve → optic chiasm → optic tract → lateral geniculate body → optic radiation to primary visual cortex Lateral ventricles ○ Fibers don't go into the lateral ventricles- they go around it/disperse Meyers loops ○ Group of fibers going to the lower part surround the lateral ventricles ○ Part of the inferior optic radiation that is receiving information form the superior retinal field Superior retinal quadrants (inferior visual field) ○ If upper part is damaged- we lose the lower contralateral side of the visual field ○ If lower part is damaged- we lose the upper contralateral side of the visual field Other destinations of fibers from the retina Not all fibers go to the LGN, can go into: ○ Superior colliculus- orient movement of head and eye ○ Hypothalamus - circadian rhythms ○ Epithalamus ○ Pretectal area (in the midbrain this is where bodies of the cranial nerves are located) Synapse with CN III and edinger westphal for accommodation and pupillary light reflex Visual Deficits GOOD LINK: https://geekymedics.com/visual-pathway-and-visual-field-defects/ Damage in pre-chiasmal lesion: Ipsilateral monocular visual defect Damage in post-chiasmal lesion: Homonymous contralateral visual field defect Damages in retina or optic nerve: Ipsilateral Damages in optic chiasm: optic tract or visual cortex: Contralateral Central Scotoma ○ Lesion in fovea (Central Retina) ○ Ipsilateral central vision loss ○ Abnormal blind spot can be located anywhere ○ When you close one eye the other one will not be affected ○ Cause can be retinal detachment Monocular Vision Loss ○ Lesion in Optic nerve ○ Only 1 eye loses visual (ipsilateral eye) ○ Damaged eye will not be able to perceive light stimulus but contralateral eye will be able to perceive light Pupil constriction will be seen in both eyes because that reflex is controlled by the brainstem and not visual pathway ○ Cause: Retinal artery occlusion Bitemporal Heianopia ○ Lesion in Optic Chiasm ○ Loss of temporal visual fields on both eyes - tunnel vision ○ Both eye (on temporal sides) affected because optic chiasm has the lesion You’ve damaged the nasal connections (the ones that decussate in the chiasm → lost peripheral vision in both eyes) ○ Cause:In the middle of optic chiasm the pituitary gland (stella terica) is located - if there is tumor in that area it can compress the optic chiasm and will cause tunnel vision Contralateral Homonymous ○ Lesion in optic tract or LGN or optic radiations or lower bank of calcarine fissure Right visual field → left optic tract. Can’t see stuff on the right if you damage your left optic tract ○ Visual deficit on contralateral side of lesion ○ Same part of visual field is affected in both eyes (temporal and nasal regions) ○ Cause: Middle cerebral artery occlusion, brain tumor, abscess in temporal lobe Contralateral Superior Quadrantanopia ○ Lesion in Temporal lobe and optic radiation ○ Lesion in inferior radiation or meyer's loop ○ Lesion in lower bank of calcarine fissure ○ Related to the same side visual field (superior ¼ of the visual field) Damage to the right lower radiations means we can see what's on the upper left. ○ Cause: tumor in occipital or temporal lobe Contralateral Inferior Quadrantanopia ○ Lesion in parietal lobe and optic radiation ○ Lesion in superior radiation/medial bundles ○ Lesion in superior bank of calcarine fissure ○ Related to the same side visual field (only inferior ¼ of the visual field affected) Lesion to your right upper radiations means we can’t see what’s on the lower left. ○ Middle cerebral artery affects this area ○ Cause: Tumor (occipital lobe) Contralateral Homonymous Hemianopia with macular sparing ○ Lesion in occipital cortex/primary visual cortex ○ Upper and lower bank of calcarine fissure affected (if PCA is occluded) ○ Partial Loss of vision in the same half of both eyes with the center macula spared ○ Macula is in the most posterior aspect of the hemisphere of the occipital lobe ○ Calcarine Sulcus: Posterior and middle cerebral artery (anterior portion) affects this area Receives blood supply from two area - if one gets the damaged the other one is still function ○ If you have macular sparing you will damage in the visual field What is the meaning of: Scotoma - blind spot or partial loss of vision in what is otherwise a perfectly normal visual field Scintillation- flickering in the visual field What is macular degeneration Macular is the area with the most numerous receptors Pt cannot see through the central retina, can only see through the peripheral vision