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Introduction
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What’s up!
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Let’s continue our Cranial nerve series.
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Cranial nerves are twelve pairs of nerves that exit the brain and the brainstem, and
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in this segment, we’ll talk detailed about the second cranial nerve, which is the Optic
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nerve.
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And we’ll do that by first making a quick scheme of the optic pathway to get an overview
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of it.
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Then we’ll cover the very basic of eye anatomy, and talk a little bit about the visual field.
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Afterwards we’ll go through the course of the visual pathway, and talk about the all
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the steps that the optic nerve go through in order to make us perceive and understand
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what we see.
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We’re also going to shortly go through the visual pathway collaterals.
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The optic nerve gives off collateral branches that aid with he pupillary light reflex, and
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the pathway of convergance.
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At the end we’ll talk a little bit about the clinical relevance around pathologies
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related with the optic pathway.
Optic Nerve Scheme
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So, I wanna start by saying that the olfactory and the optic nerves are not true nerves.
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They are just extensions of the diencephalon.
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They take the meninges with them as they extend towards the eyes.
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They are not covered by Schwann cells like the peripheral nervous system are.
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It’s covered by oligodendrocytes, like the Central nervous system is.
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Now.
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Another thing I want you know know about the optic nerve is that the optic nerve carries
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visual information to the brain via the optic tract.
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Let’s visualize this sentence.
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Our vision starts at the eye, specifically the retina.
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The visual pathway is what we call a four-neurone tract, meaning it consists of 4 different
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neurons.
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The first, second and third order neurons are located in the retina of the eyeball,
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I’ll show you a scheme later when we get to it.
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But it all starts with a visual stimuli from the three neurons in the retina.
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The visual stimuli is then sent through the optic tract, which goes through the optic
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canal of the spenoid bone, behind the eyes.
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The optic nerve then crosses with the contralateral optic nerve of the other eye to then continue
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as the optic tract.
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So half the fibres go to the contralateral side, and half the fibres continue in the
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ipsilateral side together with fibers from the other eye.
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We will go through this in detail in a minute.
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The optic tract then goes on and synapses with the fourth order neurones in the lateral
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geniculate body of the metathalamus.
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These neurones then take visual information from the retina to the primary visual area
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in the occipital lobe.
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Another thing that can happen, we won’t go in too much detail into this, but there
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are some collateral branches that go from the thalamus to the midbrain aswell, that
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are responsible for the pupillary light reflex, coordinated eye movements and contribute to
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motor control of the whole body.
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There are some collateral nerves going to the hypothalamus aswell, which have an important
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effect on autonomic functions and circadian rhythm.
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Our eyes control a lot of different functions whithin us.
Eye Anatomy
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Alright let’s now start with the very basic, beginning of the optic nerve.
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Our eyes.
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Let’s quickly do some eye anatomy.
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The cornea is a circular transparent layer that covers the pupil, iris and anterior chamber
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of the eye.
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Back here we got the lens, which is held in position by small ligamentous bands that extend
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from the ciliary process to the lens, called suspensory ligaments of the lens.
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We can see the posterior chamber, as well as the ciliary muscles that change the tension
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of the lens.
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Then we have the sclera, which is the fibrous layer.
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The sclera and the corena are both the fibrous layer of the eye.
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We got the choroid, which is the vascular layer containing a lot of blood vessels.
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Notice that the vascular layer consists of three parts that are continuous with each
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other.
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They are the choroid, ciliary body, and iris.
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Then we got the Retina, which is the nervous layer.
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Another thing we can see here is the vitreous body, or vitreous humor.
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This is a gelatinous structure that contribute to the refraction of light, focusing the light
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basically.
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It has a channel in the middle that extends from the optic disc to the posterior pole
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of the lens.
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This channel is called the hyaloid canal and it serves to transmit the hyaloid artery in
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fetal life, which supplies the lens in this period.
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Then we got a budle of optic nerve and vessels, which is where the optic disk is.
Retina
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But now what we’re going to do is have a little closer look into the Retina.
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So let’s go ahead and take a small section of the eye, and zoom in.
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What we can see here is the choroid, which contains a lot of blood vessels, and we can
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see the retina.
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The retina contains the neural component of the eye.
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So what happens is, when light reaches the back of the eye, it enters the cellular layers
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of the retina.
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The cells of the retina that detect and respond to light, known as photoreceptors, are located
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at the very back of the retina.
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There are two types of photoreceptors: rods and cones.
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Rods allow us to see in dim light, but don’t allow for the perception of color.
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Cones, on the other hand, allow us to perceive color under normal lighting conditions.
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Throughout most of the retina, rods outnumber cones.
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In one area called the fovea, however, there are no rods but many cones.
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The fovea represents the area of the retina that provides our highest acuity vision, and
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thus is at the center of our gaze.
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Alright now let’s focus on the Optic nerve.
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The visual pathway is what we call a four-neurone tract, meaning from start to end, it consists
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of 4 different neurons.
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The first, second and third order neurons are located in the retina of the eyeball.
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And when you look at this image, you can see a clear demarcation between these three neurons.
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The first order neurons are the Rods and cones.
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They’re receptor cells that respond directly to light stimulation.
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So when light hits photoreceptors, it interacts with a molecule called photopigment, which
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begins a chain of events that serves to propagate the visual signal.
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The signal is then transmitted to the second order neurons, called bipolar cells.
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Bipolar cells take in stimuli from the rods and cones to the ganglion cells.
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Ganglion cells are our third order neurons, and they’re the cells that form the optic
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nerve.
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The optic nerve is going to leave the eye in a large cluster at an area called the optic
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disc.
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Just a fun fact – The optic disc doesn’t contain any photoreceptors, and so it represents an
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area on the retina that can’t process visual information, creating a natural blind spot.
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However, we normally don’t notice our blind spot.
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The brain uses information from surrounding photoreceptors and the other eye to fill in
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the gaps in images that are processed by the retina.
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Alright.
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Now we understand where we’ll find the three first neruons of the optic pathway.
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They’re in the retina.
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First order neurons are photoreceptos, so rods and cones.
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Second order neurons are bipolar cells, and third order neurons are ganglion cells.
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Now before we go over and thalk about the rest of the visual pathway, I wanna first
Visual Field
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go through the visual field because you’ll understand a lot more of this pathway once
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you get this concept.
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When the head and eyes are maintained in a fixed position and one eye is closed, the
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area seen by that eye constitutes the visual field for that eye.
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Now, if the other eye is also opened, the area seen is more or less the same as was
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seen with one eye.
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So the visual fields of the two eyes overlap to a very great extent.
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Now.
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If an object is placed in the right half of the field of vision, its image is formed on
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the left half of the retina and vice versa.
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The two halves of the retina are usually referred to as nasal half, and temporal half.
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Or the medial half and lateral halves.
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We can also say that the region where the two visual fields overlap, they’re called
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the binocular field of vision because both eyes are involved.
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In the middle of the binocular field of vision, the light is focused towards both foveas,
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which is responsible for sharp central vision.
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Remember the fovea represents the area of the retina that provides our highest acuity
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vision, and therefore is at the center of our gaze which is necessary for reading, driving,
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and any activity where virual detail is important.
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The other important thing with the fovea, is that it kinda splits the fibers into two
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visual field fibers, here represented in red and blue.
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The lateral fibers of the left eye will go to the ipsilateral side.
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The medial fibers of the left eye will cross at the optic chiasm and cross to the contralateral
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side.
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Same thing with the other eye.
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The lateral fibers will continue on the ipsilateral side.
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The medial fibers of the right eye will cross at the optic chiasm and continue to the contralateral
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side.
Course of the Visual Pathway
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Alright.
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Now that we got this concept, let us finally go through the actual course of the visual
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pathway.
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And to do so, we’ll use this scheme since it shows the complete pathway from start to
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finish.
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Remember, the visual pathway is a four-neurone tract.
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I know I keep repeating this, but this is very very important.
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The first-order neurons are photosensitive cells called rods and cones.
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Visual stimuli are converted into neural impulses, the cells that react to light stimuli point
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towards the choroid and is embedded into the pigmented layer of the retina.
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Axons head towards the second-order neurons called bipolar cells, which continue into
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the third-order neurons called retinal ganglion cells.
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Axons converge to the optic disc and form the optic nerve.
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Optic nerves will continue posteriorly and converge into something called an optic chiasma
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or visual deccusation because this is where half the optic nerve fibers decussate.
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The optic chiasma is located on the superior surface of the sphenoid bone.
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What happens is, In the chiasma, the fibers from the medial half of each retina cross
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the midline and enter the optic tract of the opposite side, whereas the fibers from the
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lateral half of each retina pass posteriorly in the optic tract of the same side.
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So the way this works is that the right optic tract conveys information from the right halves
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of the retinae to from the left halves of the visual field.
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While the left optic tract conveys information from the left halves of the retinae to from
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the right halves of the visual field.
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Kinda makes sense doesn’t it?
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Alright so then, fibers of the optic tract terminate by synapsing with the fourth order
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neuron within the lateral geniculate bodies of the metathalamus, on either side of each
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thalami.
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This is where we’ll go a little deeper into this topic.
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The grey matter of the lateral geniculate body is split into six laminae.
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Fibres from the eye of the same side end in laminae 2, 3, and 5; while those from the
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opposite eye end in laminae 1, 4, and 6.
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Now.
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Remember I told you that that we got the temporal and the nasal parts of visual fields?
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The visual field is actually subdivided into 4 quadrants depending on where the fibers
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originate from within the eye.
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The fibers that come from the fovea, which, you know constitute around 90% of all axons
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leaving the eye since this is your point of focus, they’re called fovea fibers, or macular
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fibers.
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These fibers are going to end in the central and posterior part of the lateral geniculate
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body, and this area is relatively large.
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Fibres from the peripheral parts of the retina end in the anterior part of the lateral geniculate
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body.
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So, foveal fibers, in the central and posterior parts of the lateral geniculate bodies, and
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peripheral fibers, going to the anterior parts of the lateral geniculate bodies.
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So.
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The upper half of the retina is represented laterally and the lower half of the retina
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is represented medially.
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Now you probably don’t need to know this in detail, but this is just to show you that
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specific points on the retina project to specific points in the lateral geniculate body.
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Now, from the lateral geniculate bodies.
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What happens?
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Fibres leave as the optic radiation.
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The optic radiation ends in the primary visual area of the cerebral cortex which consists
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of the cuneus and the lingual gyrus of the occipital lobe.
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Also called Brodmann’s area 17.
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These fibers go to the primary visual area in two different ways.
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Fibers from the upper half of each retina, which remember is the lower field of vision.
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Fibers from the upper half of the retina, they’re going to go from the lateral geniculate
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bodies directly to the cuneus of the occipital lobe.
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Fibers from the lower half of the retina, which remember is the upper field of vision.
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Fibers from the lower half of the retina, they loop forward and downward into the temporal
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lobe to swerve around the inferior horn of lateral ventricle before turning backward
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to the occipital lobe, to terminate in the lower parts of the cuneus, called the lingual
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gyrus.
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This loop it makes before going to the occipital lobe is called Meyer’s loop.
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This is how a point-to-point relationship is maintained between the retinae and the
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visual cortex.
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One interesting thing to remember here is that the cortical area for the macula is larger
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than that for peripheral areas.
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The macular area has dual blood supply, which is the posterior cerebral artery and branches
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of middle cerebral artery.
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All this because remember the macular fibers are important for our central vision.
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The cortical area for the peripheral part of the retina is situated anterior to the
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area for the macula.
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So that was the course of the visual pathway.
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Let’s do that one more time.
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The retina has 1st, 2nd and 3rd order neurons.
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The first-order sensory neurons being photosensitive bipolar rods and cone cells.
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They synapse with second order neurons, which are bipolar cells.
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Third order neurons are retinal ganglion cells, where axons of the retinal ganglion cells
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converge to the optic disc and form the optic nerve.
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The right and the left optic nerves join to form the optic chiasma, where fibres from
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nasal half of retina cross to the opposite side and travel through the opposite optic
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tract to terminate in the opposite lateral geniculate body.
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The fibres from temporal half of each retina enter the optic tract of the same side to
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terminate in the ipsilateral geniculate body.
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The cell bodies of fourth-order sensory neurons are located in the lateral geniculate body.
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Their axons form the optic radiation, which projects to the visual cortex.
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Just a side note.
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Some sources don’t count Rods and cones as 1st order neurons, which leaves the bipolar
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cells being 1st order neuron, Ganglion cells being 2nd order neuron and the lateral geniculate
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bodies being 3rd order neurons.
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This is just some variations out there, but you know, keep this in mind in case your sources
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say so.
Visual Pathway Collaterals
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Alright.
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So that was the visual pathway.
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Now the visual pathway has some collateral nerves as well, meaning that it gives off
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nerves that go off to other parts outside the visual pathway.
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There are fibers whithin the optic tract that instead of traveling to the lateral geniculate
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nucleus, they diverge to the pretectal area in the midbrain, which is anterior ro the
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cerebral aqueduct.
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When they go to the pretectal area, the fibers then synapse the ipsilateral and contralateral
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oculomotor nerve nuclei, which is the cranial nerve number 3, and it also synapses with
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the Edinger-Westphal nuclei, which are accessory nuclei of cranial never number 3.
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These nuclei, along with other cortical influences, are responsible for several visual reflexes
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including accommodation, direct and consensual light reflex, and saccadic response of the
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eyes.
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Another thing that can happen, is that fibers can also bypass the lateral geniculate body
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to go synapse with nuclei of the superior colliculi of the midbrain.
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And from the superior colliculi, most fibers from here form the tectospinal tract, which
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remember it deccusate and then run through Tegmentum of Pons, medulla oblongata, anterior
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funiculus of the spinal cord, to synapse with motor nuclei of the anterior horn of the spinal
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cord to supply skeletal muscles involved in the reflectoric visual pathway.
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Meaning the tectospinal tract is responsible for reflectory movements according to unexpected
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visual irritation.
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Another thing that happens is, the superior colluculi is connected with MLF, or medial
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longitudinal fasciculus, which is responsible for coordination of the movements of eyeball,
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head and neck.
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And so not only the optic nerve is involved int his, the medial longitudinal fasciculus
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also communicates with the motor nuclei of the III, IV, VI and XI cranial nerves in order
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to coordinate the neck and eye movements.
Clinical Relevance
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So that’s that one.
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The last thing that I wanna mention here is clinical relevance.
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Why is it important to know all of this detailed pathway.
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The reason why is that based on the type of field defect, we can localize where the lesion
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is.
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Right?
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Lesions involving the whole optic nerve cause complete blindness on the affected side, that
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means damage at the right optic nerve causes complete loss of vision in the right eye.
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A lesion involving complete optic chiasm, will disturb the axons from the nasal field
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of both eyes giving visual field-bitemporal hemianopia.
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Cuz remember the nasal part of the retina shoots vision to the temporal visual field,
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and vise versa.
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Causes can be many, specially compression by pituitary adenomas and meningiomas.
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Lesion involving whole optic tract give homonymous hemianopsia.
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So if there’s a lesion in the left optic tract, it’ll cause right-sided homonymous
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hemianopsia, while a lesion in the right optic tract will cause left-sided homonymous hemianopsia.
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Leasons can be caused by demyelinating diseases and infarction.
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The lateral geniculate nucleus is the nucleus next to the thalamus that receives visual
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information from the retina and sends it to the visual cortex via optic radiations.
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A lesion of this nucleus produces the same outcome as lesions in the optic tract.
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Isolated lesions of the lateral geniculate nucleus are rare, but some pituitary adenomas
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can put pressure on this nucleus causing ti to degenerate.
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Now, lesions in the optic radiation.
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The optic radiation are axons from the neurons in the lateral geniculate nucleus to the primary
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visual cortex, right?
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If there’s an infarction caused by middle cerebral artery or the posterior cerebral
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artery, this may affect the optic radiations, and can cause quadrantanopias.
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If for instance there’s a lesion of the right temporal lobe damaging the meyer’s Loop
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of the optic radiation, that will lead to loss of the upper, outer quadrant of vision
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on the contralateral side, known as homonymous superior quadrantanopia, or pie in the sky
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disorder.
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If there’s a lesion in the visual cortex itself, what happens?
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The visual cortex is located in the occipital lobe.
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If there’s any partial or complete visual deficit that is cased by damage to the cisual
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cortex, we call that cortical blindness.
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It’s most of the times permanent as a result of stroke, head injury or gunshot injuries.
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Occipital cortex lesions tend to cause homonymous hemianopias of variable size, with or without
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macular involvement.
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It all depend on which parts of the brain is damaged.
Recap
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Alright let’s recap once again.
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Our vision starts at the eye, specifically the retina.
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The retina has 1st, 2nd and 3rd order neurons.
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The first-order sensory neurons being photosensitive bipolar rods and cone cells.
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They synapse with second order neurons, which are bipolar cells.
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Third order neurons are retinal ganglion cells, which takes their axons towards the optic
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disc and form the optic nerve.
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The right and the left optic nerves go through the optic canal, and then join to form the
21:58
optic chiasma, where fibres from nasal half of retina cross to the opposite side and travel
22:04
through the opposite optic tract to terminate in the opposite lateral geniculate body.
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The fibres from temporal half of each retina enter the optic tract of the same side to
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terminate in the ipsilateral geniculate body.
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The cell bodies of fourth-order sensory neurons are located in the lateral geniculate body.
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Their axons form the optic radiation, which projects to the visual cortex.
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So that was everything I had for the second cranial nerve.
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The next video is going to be about the third cranial nerve, the oculomotor Nerve.
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Alright guys so that pretty much covers the Optic nerve.
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Thank you so much for watching another one of my videos.
22:45
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22:50
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22:51
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Have fun ya’ll.
22:59
Peace.
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