Test your understanding with 10 random multiple-choice questions from the question bank.
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What’s up, Meditay here; let’s continue the anatomy of the Central Nervous System.
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In this segment, we’ll cover the complete anatomy of Pons
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So the central nervous system consists of two parts:
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the encephalon and the spinal cord. The encephalon is then further divided into specific parts.
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We have the brainstem, which consists of the Medulla, Pons, and the Midbrain or the
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mesencephalon. We have the Cerebellum back here, then the diencephalon and the telencephalon.
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Our focus in this video is going to be Pons, which is here.
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So in this video, we’re first going to cover the external surfaces of the Pons. Basically,
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look at its topography and what structures you’ll find from an anterior view and a posterior view.
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Then we’re gonna slice up the Pons and look at the internal surface. Basically, see how the grey
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matter and white matter are arranged within it. Then I’ve made a little quiz at the end which
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might help you if you need to memorize. Alright, so we can start by replacing
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this picture with a more realistic one. Pons is located here, lying right above
Topography of Pons
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the medulla and in front of the Cerebellum. And down here, you can see the spinal cord.
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Now, if we remove a part of the cerebral cortex, you’ll be able to see the rest of Pons.
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As well as the mesencephalon, or the Midbrain, which is the most superior part of the brainstem.
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Alright. Now. Externally, your Pons has two surfaces. It has an anterior surface and
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a posterior surface. Let’s now cover the typical morphology of these two surfaces,
Anterior Surface of Pons
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starting with the anterior surface first. And we’ll do that by looking at the brainstem
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from this perspective; we’ll see this. So again, Pons is this prominent bulb you see here. In the
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midline of the anterior surface, there s a sulcus called the basilar sulcus. Laterally on either
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side, you’ll see that Pons turns backwards toward the Cerebellum. And that is because Pons forms
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the middle cerebellar peduncles on either side, which continue into the Cerebellum back there.
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That was the general morphology of the anterior surface of Pons,
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but there are some cranial nerves that leave the brainstem at this area. We
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have 12 cranial nerves in our body, and each serves its particular function in the brain,
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but from the anterior part of the Pons. You’ll see a nerve that goes out between the Pyramids
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of medulla oblongata and Pons. This nerve is the 6th cranial nerve, the nervus abducens,
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that go to the lateral extraocular muscles of the eyes to help with the abduction of the eyeballs.
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Then, between the Olives and the middle cerebellar peduncle, you’ll find cranial nerve number 7. The
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facial nerve, which provides motor innervation for the facial muscles, helps with facial
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expressions. And cranial nerve number 8, the vestibulocochlear nerve for hearing and balance.
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The last nerve is at the surface of Pons. And that is the 5th cranial nerve, the nervus Trigeminus.
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It’s a very large nerve that goes to your facial region
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to provide sensory innervation, motor innervation, as well as parasympathetic innervation for glands.
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So all of these nerves emerge from the anterior area of Pons.
Posterior Surface of Pons
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Now let’s turn this model around and look at the posterior surface. From the posterior view, you’ll
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see that the Cerebellum covers the whole posterior area of Pons. SO let’s go ahead and remove it.
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So Pons is here, above the medulla and below the Midbrain, or mesencephalon.
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Pons doesn’t really have any specific structure on its own. Rather it works with the medulla and the
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mesencephalon in forming certain structures. One of them is the Rhomboid Fossa. The rhomboid fossa
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has many nuclei for the cranial nerves and is a very important landmark for tracts and nuclei.
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I’ll cover this in detail in my next video. But for now, I want you to know that the upper part
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of the rhomboid fossa is formed by Pons, and the lower part is formed by the medulla. Cool.
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On either side of the rhomboid fossa, you’ll see the cerebellar peduncles, which contain
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fibers that run between the Cerebellum and all three parts of the brainstem. The inferior
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cerebellar peduncle contains fibers that go to the medulla. The middle cerebellar peduncle contains
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fibers that go to Pons. The superior cerebellar peduncle contains fibers that go to the Midbrain.
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So that was all for the posterior surface of Pons. Now let’s go over to the next segment of this
Internal Surface of Pons
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video, and look at the internal surface of Pons. But before we do that, I want you to keep in mind
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that grey matter always contains cell bodies, so all structures within the grey matter
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are nuclei. White matter contains nerve fibers that form tracts.
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Now let’s take Pons, and slice it up like this to look at the internal surface.
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So this is an outline of what Pons looks like. We’ll see the Basilar Sulcus here, so this is
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the anterior surface. We’ll also see the middle cerebellar peduncles and the Rhomboid Fossa back
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here. I hope it makes sense at this point. Before we dig into structures on the surface of Pons,
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we need to go through an important landmark that we use to separate Pons into two parts. So here we
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see the internal part of the ear. The inner ear we call it with the cochlea and the vestibulum.
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The cochlea, which is our primary organ of hearing, has a nerve called the cochlear
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nerve. The cochlear nerve will go directly towards Pons and then ascend towards the superior temporal
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gyrus as the Lateral Lemniscus. But some fibers cross to the other side and then ascend to the
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superior temporal gyrus as the lateral lemniscus. Now, why am I telling you this? Because as these
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fibers cross within Pons to the other side, they form the Trapezoid Body, as you see here.
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And the Trapezoid body divides the internal surface of Pons into two regions. The Dorsal part
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becomes the Tegmentum of Pons, and the Ventral Part becomes the Basilar Part. Let’s now go
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through the Grey matter in Ventral and Dorsal parts, then do the white matter, so we’ll start
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with the gray matter structures associated with the Basilar part first. The only grey
Gray Matter: Basilar Part
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matter in the Basilar part are the Pontine Nuclei These Pontine nuclei are a very significant part
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of Pons because, there’s gonna be a tract that originates outside of the primary motor cortex,
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from the frontal lobe of the cortex, called Frontopontine Tract.
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It will descend and synapse with the pontine nuclei, as you see here. Then once they synapse,
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these fibers will go to the Cerebellum through the middle cerebellar peduncles, as
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the Pontocerebellar tracts. So these fibers cross to the other side and then go to the Cerebellum.
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Once they’re at the Cerebellum, the tracts will then travel towards the Red nucleus
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of the Midbrain as the cerebellorubral tract and then descend as the rubrospinal tract to
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support voluntary movements. So the Pontine nuclei are a very important part of Pons.
Gray Matter: Tegmentum of Pons
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The grey matter of the Tegmentum consists of nuclei that belong to cranial nerves number
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6-8 in the Rhomboid Fossa. We’ll cover the rhomboid fossa in detail in a separate video,
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but for now, we’ll stick with the most important structures associated with Pons to really
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understand the anatomy of it. Next, we’ll see the reticular formation. You’ll find the reticular
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formation along all parts of the brainstem. Then between the fibers of the trapezoid body,
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you’ll find small nuclei called the nuclei of the Trapezoid body,
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which some of the fibers synapse with. So that was all the grey matter of Pons. Now
White Matter: Basilar Part
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let’s go ahead and cover the white matter of the Basilar Part and the Tegmentum of Pons.
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We’ll start with the basilar part. Here you’ll only find descending tracts or motor
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tracts. Remember earlier when we talked about the frontopontine tract and the pontocerebellar
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tracts? Where fibers from the frontal cortex go down to synapse with the pontine nuclei in Pons,
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and then fibers go from Pons to the Cerebellum as pontocerebellar tracts? Pontocerebellar tracts
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are descending tracts, so we’ll need to colour it as red. The other descending tracts you see
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here is the frontopontine tract, you’ll see it depending on at which level you cut Pons.
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But, the frontopontine tract is a part of a bigger bundle of tracts called the Corticopontine tract.
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The Frontopontine tract was just an example. If the tract originates from the occipital lobe,
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it’s called occipitopontine tract. If it originates from the temporal lobe,
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it’s called temporopontine tract. And so on. That’s why I specifically said frontal
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lobe earlier because the frontopontine tract comes from the frontal cortex.
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Notice that they all originate from different areas of the cerebral cortex,
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which are outside of the primary motor area of the brain. And because of that, these tracts
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are considered extrapyramidal tracts. Because they don’t originate from the pyramidal cells of
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the primary motor cortex. So when we use the term Corticopontine tract, you’re really talking about
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the frontopontine tract, occipitopontine tract, temporopontine tract, and Parietopontine tract.
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So you’ll find the corticopontine tract fibers here synapsing with the pontine nuclei. Awesome.
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The other descending tract is a pyramidal tract, called the Corticospinal tract. It’s a pyramidal
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tract because it comes from the pyramidal cells of the primary motor area in the cortex and descends
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down to the spinal cord to innervate skeletal muscles. Alongside the corticospinal tract, you’ll
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find corticonuclear tracts as well. It descends in the same areas as the corticospinal tract,
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but the corticonuclear tracts are responsible for the voluntary control of muscles located in the
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head and neck. So that was all the white matter of the basilar part—only descending tracts. Now let’s
White Matter: Tegmentum of Pons
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do the white matter in the Tegmentum of Pons. The first one is the medial lemniscus. Now we need to
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repeat a few things in order to remember this one. So here is the cross-section of the spinal cord
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and the medulla. Remember that there were fibers that came from the Lower parts of the body, which
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ascend as Gracile fascicle and sensory fibers that came from the upper parts of your body, which
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ascend as Cuneate fascicle? I use the G in Gracile Fascicle as Genitals to remember that it comes
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from the lower parts of the body. So these fibers receive conscious proprioceptive information,
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as well as sensory input from mechanoreceptors, and they ascend to the gracile and cuneate nuclei
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in the medulla. Then fibers there will leave as either the external arcuate fibers
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or internal arcuate fibers. The internal arcuate fibers cross to the other side, and then they will
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ascend as the medial lemniscus, which is what you see here in Pons. They will ascend and go
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to the primary somatosensory area in the cerebral cortex so that you’re aware of sensory touch and
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vibration, as well as the position of your body parts. So that is the Medial Lemniscus.
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Then we have the Spinal Lemniscus. Remember when we talked about the cross-section of the medulla,
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we had two spinothalamic tracts? One lateral and one anterior? They ascend together as the
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spinal lemniscus, and that’s what you see here in Pons. So it ascends to the cortex,
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to the primary somatosensory area as well. And they’re responsible for conscious sensory input
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of Pain temperature, Pressure, and touch. So that is this one.
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Next, we have the Trigemnical Lemniscus. The trigemnical lemniscus comes from the trigeminal
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ganglion, which is a part of the trigeminal nerve, the 5th cranial nerve. So Pons receives sensory
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input from this nerve, and then it crosses to the other side and ascent to the primary somatosensory
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area. So that is the trigeminal lemniscus. Then we have the Anterior spinocerebellar tract.
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Remember when we talked about the medulla, that we have posterior and anterior spinocerebellar
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tracts? The posterior spinocerebellar tract will go through the inferior cerebellar peduncle
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to go to the Cerebellum. But the anterior one will go through Pons and the Midbrain
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and then go to the Cerebellum through the superior cerebellar peduncle. And that’s why
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we see it here because it’s going up to the midbrain to eventually go to the cerebellum.
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Then, I wanna repeat something I mentioned earlier in this video. About the auditory nerve called the
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cochlear nerve that goes to the Pons and then ascends as the lateral lemniscus to go to the
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superior temporal gyrus, which is the primary auditory area of the brain. Some fibers cross
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and then ascend to the superior temporal gyrus. And the fibers that cross form the trapezoid
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body. But when they ascend after crossing, they also ascend as the lateral lemniscus, which is an
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ascending tract that we need to include as well. So that was all the ascending tracts in
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the Tegmentum of Pons. Remember, blue represents sensory or ascending tracts, and red represents
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descending or motor tracts. Now let’s do all the descending tracts in the Tegmentum of Pons.
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And the first one is the Tectospinal Tract The tectospinal tract is called tectospinal
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because these fibers come from the tectum of the Midbrain. So here is the Midbrain. Posteriorly,
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you’ll find the tectum. So the tectospinal tract descend from here. And these are fibers
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that are associated with coordinated eye and neck movements. And remember, since this tract
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originates from the brainstem it is referred to as an extrapyramidal tract. Hence, it unconsciously
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moves your neck muscles with your eyes. So when you look at something.
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Imagine you’re looking at a hamburger, you look at it, and you keep looking at it as it passes you,
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and your neck muscles unconsciously follow your eyes. That’s what this tectospinal
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tract is responsible for. And that is our first descending tract of the tegmentum.
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The next one is called the Rubrospinal tract. Remember we talked about the corticopontine tract,
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which are fibers that originate from anywhere outside of the primary motor area that descends to
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synapse with the pontine nuclei? These fibers will go to the Cerebellum as the pontocerebellar tract.
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From the Cerebellum, fibers will go to the Red nuclei of the Midbrain,
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as the cerebellorubral tract. And then they will descend as the rubrospinal tract. Notice that I
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said these fibers do not originate from the pyramidal cells of the primary motor cortex,
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that’s why they’re referred to as extrapyramidal tract. They don’t initiate movement,
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but they support and coordinates the voluntary movement. So that is the rubrospinal tract.
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Next, we have the reticulospinal tract, which is a part of the balance and posture system. They come
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from the reticular formation inside the brainstem. The Reticular system are responsible for Sleep,
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alertness, cardiovascular control, breathing and all of those vital things. But they’re also
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responsible for motor control like your balance and posture through the reticulospinal tract.
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So that is this one. Next, we have a vestibulospinal tract
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And to understand this one, we need to involve the inner ear again, because the inner ear has
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the vestibular system. The vestibular system has crystals within it that monitors your balance. And
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it transmits impulses to your brain, and then down to your muscles so that you can keep your balance.
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So whenever your head is tilted or you’re upside down, or you’re about to fall,
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all of that is monitored and controlled by your vestibular system. So it helps with balance and
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posture. Alright, let’s just clean up the labels and add a little color to differentiate them.
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There is one more tract that we need to mention, which is the medial longitudinal fasciculus,
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which descends and is present only in the cervical segments in the spinal cord.
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This tarct coordinated involuntary movements of the head neck and eyes through synapses between
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the cranial nerves 3, 4, 6, and 11. So that was all I had for the internal surface of the Pons.
QUIZ!
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I made this table for the nuclei and tracts we’ll find along the basilar part and the Tegmentum of
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Pons along with a little description of them. Now, this Is where this video gets scary.
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I am going to make all of these names disappear, and can you, from the beginning, tell me what is
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the name of number 1, what is the name of number 4, where does number 14 go and where does number
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12 go. If you can do that, then you’ve grasped the anatomy of the Pons fully. If you found this
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video helpful, please put a like, comment, share, whatever you find convenient to you.
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The next video is going to be about the Fourth Ventricle and the Rhomboid Fossa.
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