0:00 Hello, in this video we're going to look at the pathway of vision and we also 0:10 will look 0:11 at what happens when there are lesions that occur along this pathway and how it 0:15 affects 0:16 our vision. 0:18 So here is the occipital lobe, the eyeball, optic nerve, optic chiasm, lateral 0:24 geniculate 0:25 body and optic tract, also known as a genicotecric tract. 0:31 Here are our visual fields, we are looking at this whole visual pathway from 0:41 the top. 0:42 So the blue nerve on the left side here will pick up the left nasal field and 0:47 will carry 0:48 this visual information to the lateral geniculate body where it will synapse 0:54 with another nerve. 0:56 And this other nerve will then carry this information to the occipital lobe. 1:01 The blue nerve on the right eyeball here will capture images from the right 1:06 temporal field 1:07 and will carry this information down the optic nerve and will cross over at the 1:13 optic chiasm 1:13 and synapse with the neurons of the left lateral geniculate body. 1:18 The nerves will then carry the information to the occipital lobe. 1:23 This orange optic nerve on the left eyeball here will capture images from the 1:29 left temporal 1:30 visual field and cross over at the optic chiasm and synapse at the right 1:36 lateral geniculate 1:37 body where it will relay the information to the occipital to the optic tract 1:41 which then 1:42 will bring it to the occipital lobe. 1:45 Now similarly the orange optic nerve of the right eye will capture the image 1:51 from the right 1:53 nasal visual field. 1:55 It will not cross over but will remain, will relay this information down to the 2:01 lateral 2:02 geniculate body and then it will synapse with other nerves along the optic 2:06 tract and then 2:07 down to the occipital lobe. 2:09 I hope this pathway makes sense. 2:12 Now that we have an overview or an understanding of the visual pathway, what 2:17 happens to our 2:18 vision if we have lesions occurring along these different parts? 2:25 So let us have a look and compare normal to defects in visual fields. 2:33 If a lesion were to occur on the left optic nerve, A, what happens is that the 2:42 orange 2:43 and blue optic nerve on the left side cannot even bring the information down 2:49 and so we 2:49 get blindness in the left eye. 2:53 This is called the left anopia. 2:58 If a lesion occurs along the optic chiasm here in B and this can be due to a 3:05 pituitary 3:06 tumor, well the nerves that cross over that normally captures images on the 3:12 temporal visual 3:14 fields cannot, you know, send this information. 3:18 And so we get blindness in the temporal visual fields from both right and left. 3:24 And this is called by temporal heminopia. 3:29 If a lesion occurs here after the optic chiasm, we get loss of the visual 3:37 fields that these 3:39 blue nerves carried. 3:42 And which means we get loss of vision on the left nasal field and the right 3:47 temporal field. 3:49 This is called right homonymous heminopia. 3:55 And lastly D, if a lesion occurs on one of these nerve tracts, nerve tract 4:01 bundles, it 4:03 will cause what is called right homogeneous superior quadrant anopia. 4:12 Finally there can be other visual field problems that can occur due to an 4:20 underlying eye condition. 4:23 Let us cut a cross section of the eyeball and learn some important structures. 4:28 So here is the optic disc where the optic nerve and vessels of the eyeball pass 4:34 through. 4:35 Here is an area within the eyeball at the very back called the macula. 4:39 Now the macula contains what is known as a fovea, which is an area where a lot 4:47 of pigmented 4:48 cells are called cones. 4:52 And these pigmented cells of the retina, these guys allow for high resolution. 4:59 So when we damage the macula, we get loss of central vision. 5:06 We get loss of that high quality vision. 5:10 So drawing out normal to abnormal, here is normal. 5:15 And this central visual field is the macula. 5:19 In a condition known as central scumata or macular degeneration, you get loss 5:25 of this 5:25 central vision because of the damaged macula. 5:32 If everything else is lost but the macula is preserved, this is called const 5:37 ricted visual 5:37 fields. 5:39 This is what we see in glaucoma and retinar pigmentosa. 5:45 The macula is spared, as a result the patient has tunnel vision. 5:52 Now just to finish this diagram off, here is the midbrain, an area where the 5:57 optic nerve 5:58 also brings information to. 6:00 This area is primarily involved in reflexes to do with the eye, such as the 6:05 blink and 6:05 light reflex.
