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.