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  • Voiceover: In this video,

  • I'm going to talk about the somatosensory tracts.

  • Somatosensory, which are the senses of the body,

  • and tracts, which are collections of axons

  • travelling together through the central nervous system.

  • The different types of somatosensory information

  • tend to travel in different pathways,

  • as they move through the central nervous system.

  • In general, the different types of somatosensation

  • break down into two big categories.

  • The first includes position sense,

  • vibration sense,

  • and fine touch sense,

  • or very precise touch sense information.

  • The other big grouping

  • of types of somatosensory information

  • that tend to travel together

  • includes the sense of pain, pain sense,

  • temperature sense,

  • and what we can call gross touch sense,

  • or less precise touch sense information.

  • Recall that somatosensory information

  • from most of the body is going to travel back

  • to the central nervous system

  • through nerves in the peripheral nervous system,

  • and then spinal nerves that'll enter the spinal cord.

  • For example, in this category,

  • let's say we have a receptor that carries

  • some position sense information.

  • Let me just draw an R here in the arm,

  • for receptor for position sense.

  • That information is going to travel in nerves

  • of the peripheral nervous system

  • and then spinal nerves to enter the spinal cord

  • and deliver that information into the spinal cord.

  • The same thing with this other big category

  • of types of somatosensation.

  • Let's say we had some sort of receptor here

  • down in the leg, I'll just write an R for receptor

  • that can detect noxious stimuli,

  • that can cause the experience of pain.

  • Then that information can travel

  • through nerves of the peripheral nervous system

  • and then spinal nerves to enter the spinal cord

  • and deliver that information

  • into the central nervous system.

  • The same is true for somatosensory information

  • from the face and other parts of the head,

  • but that information will enter usually the brain stem

  • through cranial nerves,

  • instead of entering the spinal cord

  • through spinal nerves.

  • For instance if we have a receptor

  • that can detect vibration somewhere here on the face,

  • that information can travel through a cranial nerve

  • into the brain stem.

  • If we had another receptor here in the face

  • that could detect say temperature,

  • one of these other types of somatosensation,

  • that also could travel through a cranial nerve

  • and enter the brain stem.

  • But now what happens once this information is delivered,

  • into either the spinal cord or into the brain stem-

  • So I've taken the brain and the spinal cord,

  • and I've drawn a little bigger illustration over here.

  • We're still looking at it from the front,

  • but I've kind of cut into the brain here,

  • so we're seeing inside of the brain,

  • and the inside of the spinal cord.

  • First lets consider this category

  • of different types of somatosensation,

  • the category that includes pain sense

  • and temperature sense and gross touch sense.

  • Let's say, like I've drawn here, we have

  • some of that coming in from the leg on one side.

  • Let me just draw an arrow, showing that information

  • entering the spinal cord down low here.

  • Inside the spinal cord,

  • neuron axons are going to carry that information

  • up to the brain, in one of these somatosensory tracts,

  • one of the tracts that's specific

  • to this category of types of somatosensation.

  • Let me just draw that,

  • but I'm going to leave out some of the details.

  • One important detail is

  • that it actually crosses to the other side,

  • and then goes up through the spinal cord,

  • and up through the brain stem,

  • and then comes to a place kind of deep down

  • in the cerebrum, what we'll talk about

  • a little bit more in another one of these videos.

  • Notice that it's on the other side,

  • it's actually coming into the cerebral hemisphere

  • on the other side, from the part of the body

  • that the receptor was on.

  • The same is going to be true,

  • for this other category of types of somatosensation,

  • that includes position and vibration

  • and fine touch sense.

  • Let's say, just like I drew some of that information

  • coming in from an arm on this side,

  • let me just draw an arrow here,

  • showing that that's also coming in the spinal cord,

  • but a little higher up on the spinal cord.

  • Now there'll be a different tract,

  • that's going to carry that information up,

  • and it's going to cross in a little different place,

  • a little higher up in the brain stem,

  • and then it's also going to keep going up,

  • and then it's going to come to the same place

  • deep down in one of the cerebral hemispheres,

  • on the other side from where

  • that information came in from the body.

  • The pain, temperature, and gross touch sense

  • from the face, and some other parts of the head,

  • can come into the brain stem through cranial nerves,

  • and then that information is actually going to take

  • a funny pathway that actually goes down first,

  • and then crosses, and then goes up

  • to about the same place,

  • or pretty near the place, that this information

  • from the rest of the body came from.

  • Lastly, position sense, vibration sense,

  • and fine touch sense from the face

  • and some other parts of the head,

  • will also come into the brain stem through cranial nerves,

  • and it will also cross over to the other side,

  • and go to about the same place

  • deep in the cerebral hemisphere.

  • In this place, deep in the cerebral hemisphere,

  • all these different types of somatosensory information

  • are going to come back together.

  • There going to be very close to each other now,

  • and then they're going to stay together pretty close

  • as they send that information on

  • to areas of the cerebral cortex,

  • areas on the outside of the cerebral hemisphere,

  • that will do more processing of that information.

  • Becuase the somatosensory tracts

  • carrying these types of sensory information

  • through the central nervous system

  • have this sort of anatomy, this sort of structure,

  • one of the big consequences of this

  • is that if there's some sort of injury

  • to one of the cerebral hemispheres-

  • Let me just draw a big orange arrow

  • through this cerebral hemisphere,

  • to indicate that there's an injury

  • or some kind of disease

  • has affected this side of the brain-

  • Then what we often see,

  • with an injury to one side of the brain,

  • is the other side of the body

  • can have somatosensory loss,

  • because all of these pathways

  • carrying somatosensory information

  • crossed from one side over to the other side.

  • If we're looking from the front

  • and this is the left cerebral hemisphere

  • that has an injury, we could see loss

  • of somatosensation, or abnormal somatosensation

  • on the right side of the face and the rest of the body,

  • depending on how much brain tissue is injured, and

  • and how much of these

  • somatosensory pathways are affected.

  • I think you could also see,

  • that since all these different pathways

  • have slightly different parts

  • of the central nervous system that they travel to,

  • abnormalities in some different parts of the brain stem

  • or the spinal cord could affect

  • some parts of somatosensation but not others,

  • depending on where the area of abnormality is.

  • We'll save a lot of the details of the exact anatomy

  • and the places these cross for other videos,

  • but I just wanted to introduce this idea

  • of these somatosensory tracts,

  • and why we often see injury to one side of the brain

  • causing somatosensory loss or abnormalities

  • on the other side of the body.

Voiceover: In this video,

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    伊玲 發佈於 2021 年 01 月 14 日
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