Okay. So let’s walk through some of the
different senses that we can employ
to engage with our environment.
So unfortunately for you, you’re
at your computer screen right now
and you don’t get to smell
how fresh I smell right now,
because I smell fantastic.
But if you were able
to smell me right now,
you’d be using your
sense of olfaction.
Now, we’re going to work through
some examples like that of what
parts of our brain are we using,
what are the different senses.
So one of them is
somatosensation, and this is
known as somatic senses of
touch and tactile perception.
So when you’re engaging
with actual objects,
you know, how is that --
how does that encoded in your brain
and where is that actually happening?
So sensory information is detected by
several different types of receptors.
So we can have mechanical, thermal.
You can have a whole bunch.
We’re going to go through a
whole bunch of them right now.
And what happens is we
detect that information,
that information needs
to get processed
and that happens in the
central nervous system.
So somatosensory information
is carried from a sensor
through a pseudounipolar nerve
and synapse in the spinal cord.
And then that, eventually, goes up
to the brain where it’s processed.
So, the nerves --
the nerve body or soma is located
in an exterior body outside
of the spinal cord called
the dorsal root ganglion,
and we’re going to show you
images to highlight that.
I think you probably studied over and over
all the different lobes of the brain.
And within that tutorial,
you might have heard the terms
somatosensory cortex or motor cortex.
And as you can see from
this diagram here,.
these are two bands that
are found within the brain
And this is where we believe
that the bulk of somatosensory
or information of our sensations
is encoded and processed.
this is a diagram of
the overall process
of you detecting something
and where it’s going.
So as you can see here from the
diagram, we have the hand.
And on your hand,
you have your skin.
And within your skin, you’ll have
the dermal layers and receptors.
And these receptors will
like I’m holding this clicker
in my hand right now.
How do I know that
I’m holding it?
The receptors can sense that feel
and the touch that’s happening,
and that information goes down.
Once activating the receptor,
it goes down the neurons.
We call that the pseudounipolar neuron
and it goes to the
dorsal root ganglion.
At that point, it goes into the spinal
cord and we’re showing a transection here.
So you'll notice in the gray,
something called the dorsal horn.
And the ganglion projects
into the dorsal horn.
It actually crosses the
midline of the spine
and then it goes on to travel up
to the central nervous system.
more specifically, the thalamus.
And so a couple of points I
want you to grasp out of this.
A, the process of the transmitting of
the signal via the dorsal root ganglion
and also, the fact that it enters
the dorsal horn and crosses.
So what’s happening at your right
hand goes across the spine
and to the left
side of your brain.
Okay. So we have this movement across,
bilaterally across the spinal cord.
So we’ve mentioned that here as the
info crosses to the other side
of the spinal cord and
goes up to the thalamus.
And it’s processed by the primary
somatosensory cortex which is located,
as indicated here, in the postcentral
gyrus of the parietal lobe.
these diagrams are quite deceiving because
I think it’s so clear and color coded.
But in reality, we know that,
you know, things aren’t
yellow and pink and green when
you open up somebody’s skull.
Everything looks one color and
it’s based on certain landmarks
and using sort of a human atlas of
you know where things typically are
if you’re in the area.
And you would know that there’s a
small strip that’s in relation
to something called the central
sulcus, which is our gyrus,
which is why things are called
pre and postcentral gyrus.
So they’re referring to that landmark and
that’s found up by the parietal lobe.
we’re going to show an image here.
And you look at this odd
looking character, right?
So this is what we
call a homunculus.
Now, what do you notice?
Take a close look at this guy.
What do you notice other than the fact
that he’s not wearing any clothes
and he looks a little bit weird?
You noticed that he has abnormally
huge hands and he has big lips.
So there are certain characteristics
on this little character.
And they’re meant to reflect
the significance that we place
on those areas of
So as humans, we’re
We use our hands to touch a
lot of different things;
a toothbrush, doorknob,
pen, other individuals.
And that information is really, really, really
sensitive and really, really important.
And therefore, you expect it to have
a nice representation in the brain.
As opposed to, say, a little floppy
skin on the back of your elbow.
You can pinch it.
You can squeeze it.
It doesn’t really do much.
You don’t really feel lots on it.
And so therefore, you would think in
the brain that you wouldn’t have a lot
of brain space allocated to understanding
and encoding that information.
So we say the degree of representation is
directly linked to somatic importance.
Same thing with your lips, you know,
kissing other individuals or eating.
A lot of information is encoded there
so you expect to have a lot of
representation in the brain.
So if you were take all the
different parts of your body
and figure out the amount of representation
that we actually have in the brain
and piece all that together, you get our
good little friend here Mr. Homunculus.
Now, let’s get into the
perception of pain.
So that is something that somatosensory
information would encapsulate and capture.
And that’s done through a process
that we call nociception.
So body’s ability to sense and process
pain is known as nociception.
And that’s done through
something called nociceptors.
And this can detect different
types of information;
So let’s break that down.
might be, for example,
you spilling a bottle of
crazy acid on your arm.
So that’s going to
really, really hurt.
Now why does that hurt?
Well, you have nociceptors
that can detect
that chemical information
and activate a response.
And once you put your hand on a
burning stove, what happens?
Very quickly, you move your hand.
Now, you got to appreciate, and
this is the science nerd in me,
but how quickly
all that happens?
So how do you know?
You’ve done this before.
You put your hand on something hot. How
quickly you’re able to move your hand?
That happens on the order
of milliseconds, right?
And thank God it does.
Otherwise, you’d have a lot of burn marks.
So you go and you touch that, and you
got to think about what’s happening.
The receptor is actually detecting
that thermal stimulation.
It’s sending the
signal up your arm,
across into the dorsal root
ganglion, across the dorsal horn,
up the spinal cord, to your
thalamus where it’s being processed
and you’re seeing a result in
reaction which is typically
some swearing and you
moving your hand away.
And all of that is happening
on the order of milliseconds.
So fairly, fairly impressive.
Now, three types of pain
receptors are found.
One, in the dermal layer, the
cutaneous layer of the skin.
You also can have a somatic
sensation in your joints and bones.
And you also can have visceral receptors
which are found in different body organs.
So different types of pain receptors,
they're all classified under nociceptors
and they can detect different
types of information
like chemical, thermal
So here’s a scenario exactly
that I just talked about
is a young girl reaching to the
stove and some water spills
and she’s going to sense that
burning handle or the burning water
that falls on her hand and that
would be through the nociceptors
and more specifically they’re
capturing thermal information.
Now, let’s walk through that process
that I quickly highlighted here.
So pain is carried to the DRG or the dorsal
root ganglion and on to the thalamus,
and that is where the
So pain perception and its qualities are
attributed in the somatosensory cortex.
Now this is a point I
want to highlight.
So when you are burning yourself,
and please don’t try this at
home, but if that happens,
your receptors aren’t
the severity of burn or what’s
happened or what to do.
They simply are being activated to the
stimulus being come in -- that’s coming in.
And so the processing and how you respond
to the pain and the aftereffects
of the pain are all linked to the more
cognitive function or the cerebral, the --
sorry, the central nervous system
is processing all this information.
And so, it’s the marrying the
two things of signal detection
along with actual
processing of information.