Week 5: Visual Pathways, Vision Loss Flashcards
Trace the visual pathway from the retina to the brain.
- Bipolar cells (Neuron 1) - rod/cone cells synapse at dendrites, axons contact retinal ganglion cells
- retinal ganglion cells (neuron 2) - dendrites synapse at bipolar cells, axons collect at optic disc and form optic nerve.
- optic chiasm: nasal retinal fibers cross, temporal retinal fibers remain uncrossed
- Lateral geniculate nucleus (neuron 3)-in thalamus, receive most fibers from optic tract of same side. Some axons go to brain stem for visual reflexes.
- lateral: fibers from lower retinal quadrants
- medial: upper retinal quadrants
- axons project to cortex as geniculocalcarine tract or optic radiations - optic radiations: axons from LGM pass through posterior limb of internal capsule (most posterior=retrolentiform part)
- Axons from lower retinal quads: fibers that travel through temporal lobe form loop of Meyer, anterior toward tip of inferior horn of lateral ventricle, then turn to reach occipital lobe (
- axons from upper retinal quads: travel directly posteriorly to occipital lobe through parietal lobe
Where is the primary visual cortex located?
-gyri within and next to calcarin sulcus on the medial surface of the occipital lobe
Axons that terminate in the upper calcimine sulcus are derived from which retinal quadrants?
upper retinal quadrants=lower quadrants of contralateral half of binocular visual field
Where is the macula and peripheral fields represented on the primary visual cortex?
- macula is most posterior
- peripheral fields are more anterior
Where is the visual association cortex? What is its function?
- partly medial and largely lateral surface of occipital lobe adjoining primary visual cortex
- receive afferents from primary visual cortex
- interpretation of form, color, depth, location ,motion
What is the “what” pathway for vision?
- ventral pathway from occipital association areas that goes down to temporal lobes
- perception of shape and color used for objective identification, e.g. face recognition
What is the “where” pathway for vision?
-dorsal pathway from occipital association area to multimodal association cortex to analyze motion and location of objects in the visual field
What is papilledema? How does it look clinically?
- edema of the optic disc resulting form an increase in intracranial pressure
- mild: seen as blurring of the edges of the disc
- severe: mushrooming of the disc and retina around it, obliteration of the vessels at the disc margin and hemorrhages
What do lesions in the the optic nerve result in?
- impaired vision in the corresponding eye
- horizontal meridian is respcted
What does a lesion of the optic chiasm result in?
- affects nasal retinal fibers because they cross=temporal visual fields of both eyes
- bitemporal hemianopsia-loss of vision in temporal visual fields of both eyes
- can occur due to pituitary adenoma that grows upward and compresses the chasm from below
What does a lesion of the optic tract result in?
- contralateral homonymous hemianopsia: field deficits in both eyes representing corresponding halves of two visual fields
- causes: tumor, infarct, demyelinating disease
What does a lesion of the optic radiations result in?
- contralateral homonymous hemianopsia
- associated with motor and sensory losses because of fibers running in posterior limb of internal capsule
- usually vascular lesions
Describe the visual loss from a lesion of the temporal lobe
- affects fibers that course from internal capsule to visual cortex through temporal lobe
- loss of vision in upper quadrants of the contralateral visual field
- superior quadrantanopsia: spares fixation and termed pie in the sky
Describe the visual loss from a lesion of the parietal lobe
- contralateral inferior quadrantanopsia
- causes: tumors, infarcts, occlusion of MCA
Describe the visual loss from a lesion of the cortical/subcortical lesions in the occipital lobe
- homonymous hemianopsia with macular sparing in the contralateral halves of the visual fields
- cause: vascular lesions of PCA