(From Dr. Glasser’s Lecture, 5 July 2000, by Brian Buschman)
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When a photoreceptor is stimulated it works vertically to stimulate the center and it works horizontally to stimulate the surround. To function horizontally the photoreceptor hyperpolarizes which depolarizes the horizontal cell which turns on (depolarizes) an adjacent photoreceptor. The depolarized photoreceptor stimulates the bipolar cell (via IPSP) to hyperpolarize and turn off which then turns off the ganglion cell. The polarities may be reversed depending on what type of bipolar cell we are working with. That is what causes the difference between a center on and a center off ganglion cell. The bipolar cells alternate IPSP and EPSP cells all the way around.
When you stimulate any one photoreceptor it is acting in both the center of one ganglion cell and in the surround of another. Our CNS uses feature extraction to get the info to area 17.
The only cell in this system that actually fires an AP is the ganglion cell and the amacrine cell which is not too important. All we need to know about the amacrine cell is that it acts like a horizontal cell and fires an AP. All the other cells in the retina depolarize but do not actually fire an AP.
The optic nerve goes to the optic chiasm to the optic tract to the LGN. No single cell in the LGN receives senses from both eyes. The LGN is made of six cell layers. Layers 1, 4, and 6 receive input from the contralateral side and 2, 3 and 5 from the ipsilateral side.
The LGN projects to area 17 which is the primary visual cortex. Remember that some fibers sneak from the LGN to the pretectum for the pupilary light reflex and some to the superior colliculi for the visual reflex.
The polvinar is a large thalamic nucli that sends visual input to the parietal association cortex. It is known to happen because people with occipital lobe problems that are legally blind are able to respond emotionally to pictures.
The retina can be talked about as the temporal retina and the nasal retina. The temporal retina looks at the opposite side while the nasal retina looks laterally. At the optic chiasm the nasal retinal fields cross and the temporal fields stay ipsilaterally. That means the right LGN receives the visual input from the left field.
There are two optic radiations on each side from the LGN to the banks of the calcarine fissure. The upper radiation carries info from the lower visual field as it passes through the temporal lobe. The lower radiation carries upper field input and has part called Myer’s Loop that passes through the parietal lobe. The upper radiation ends on the superior part of the calcarine and the inferior radiation on the inferior bank.
The posterior portion of the calcarine (area 17) receives input from the macula while the lower retina projects more anteriorly in area 17.
Scotomas are blind spots that are usually characteristic to damage to a particular part of the visual pathways. If you are examining for a scotoma be sure to test each eye separately. We have seven legions I am going to type about here. Remember that if there is a reciprocal pathway there is a reciprocal legion possible.
1) Optic nerve legions are characterized by a complete blindness out of one eye.
2) Optic chiasm legions will cause a loss of both of the nasal visual fields. This legion may be caused by enlargement of the pituitary and the patient may present with acractodactility. If we see a question with a patient with acractodactility and a binasal hemianopia we can recognize that the answer choice might be that the problem is an swelling of Rathkey’s pouch.
3) Contralateral homonymous hemianopia results from a legion of the optic tract.
4) LGN legion will result in complete loss of vision to the contralateral visual field.
5) Upper radiation legions will cause loss of the lower contralateral visual field. Contralateral homonymous inferior quadrantanopia.
6) Legions of both upper and lower calcarine cortex will give loss of the entire contralateral visual field except will allow you to still see the macula. Contralateral homonymous hemianopia with macula sparing.
7) Legion of the lower calcarine cortex will give contralateral superior homonymous quadrantanopia with macula sparing.
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