Chapter 8: Somatosensory Neurotransmission: Touch, Pain, & Temperature

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Okay, let's unpack this.

Welcome back to the Deep Dive, where we take a stack of dense sources, drill down into the core mechanisms, and extract the high -yield knowledge you need.

Today, we're tackling something absolutely fundamental to neurophysiology,

some nanosensory neurotransmission.

Right.

And our source material, mostly from Ganong's review, it really maps out the entire pipeline, doesn't it?

It does.

It shows how the physical world, you know, light tap, a burn, anything, gets converted into this complex electrical reality inside our brains.

So our mission today, and it's a big one, is to follow that signal.

We're going from a receptor in the skin all the way up to the cortex.

But I think the real goal, the core of this is understanding how the body separates the what and where from the ouch.

The suffering, yeah.

How it separates discriminative touch from the emotional experience of pain.

And it does that using two totally separate neurological highways that often seem to work in opposition.

And that separation is, I think, the key theme for this whole Deep Dive.

The somatosensory system itself is huge.

I mean, it covers touch, your sense of body position, temperature, pain, even itch.

And that's before you even get to vision or hearing.

Exactly.

But it all starts with a specialization of the receptors.

That's what makes the whole thing work.

Before we jump into those, though, we have to talk about the paradox of pain.

It seems so counterintuitive.

Why would we have this incredibly sophisticated system dedicated to making us feel agony?

Well, because it's the body's oldest, most essential alarm system.

I mean, acute pain is protective.

It makes you pull your hand off the hot stove.

It's a withdrawal reflex.

Exactly.

It forces you to rest so you can heal, and it creates this powerful negative emotional memory so you don't make the same mistake again.

But here's the really interesting part and where it all goes wrong.

When it becomes chronic.

Yes.

When the system fails and switches from being a protective alarm to this persistent chronic state that doesn't respond to normal drugs,

the system itself is the disease.

And understanding how that happens at a molecular level,

that's the key to changing how we manage pain.

So to understand the system, we have to start at the very beginning, at the front lines, you could say,

the sensory receptor toolbox in the skin.

That's where the physical world first becomes an electrical message.

Right.

So the first step is always transduction, taking physical energy and turning it into a nerve signal.

For touch and pressure, we have a whole suite of these things called cutaneous mechanoreceptors.

And the sources lay out four main types.

The key idea here is specialization.

They all work together, but they each have a very distinct job creating this high resolution, constantly updating map of what's touching you.

Okay, so let's walk through the four major players.

We can start with the ones closest to the surface.

Meissner corpuscles.

What do they do?

Meissner corpuscles are found just under the epidermis, and they're mostly in what we call glabrous or non -hairy skin.

So fingertips, palms.

Exactly.

Places you use for fine touch.

Their structure makes them really good at detecting initial contact and movement.

They're tuned to slow vibration or tapping specifically in that five to 40 Hertz range.

They're basically your contact has been made sensors.

And then right near them, you've got Merkel cells, which seem to do the exact opposite job.

They really do.

Merkel cells are in the same general area, but while Meissner's are dynamic and fast adapting, Merkel cells are all about persistence.

They're tonic or slowly adapting.

Meaning they keep firing.

So if you press your finger on a table and hold it there, it's the Merkel cells that are telling your brain about the sustained pressure, the texture, the shape.

They provide the continuous signal.

Okay, so now we go a little deeper into the skin for the other two.

Rafini corpuscles.

What's their specialty?

Rafini corpuscles are in the dermis, deeper down, and you find them in both hairy and non -hairy skin.

They're shaped in a way that makes them perfect for sensing skin stretch.

So that would be important for knowing your joint position, right?

Proprioception.

Absolutely critical for proprioception.

They tell the brain about the overall tension and distortion of your skin, which is vital feedback for coordinating movement.

And that brings us to the most famous one, the Pacinian corpuscle.

It's like the textbook example of a receptor.

It is, and for good reason.

First, it's huge.

You can actually see it without a microscope.

It's up to two millimeters long, and it has this beautiful structure like a sliced onion with all these concentric layers of tissue around the nerve ending.

And what's its job functionally?

It's the deepest and the fastest detector of the bunch.

It's tuned specifically for high frequency vibration.

We're talking 60 to 500 Hertz and deep, sudden pressure.

It feels the flutter, the buzz.

It's amazing that we can distinguish between a light tap, a steady push, a stretch and a vibration all at the same time.

But that information is useless if it's slow.

What kind of nerves are these mechanoreceptors using?

They are using the absolute superhighways of the nervous system.

These are the heavily myelinated A -alpha and A -beta fibers.

Myelin means speed.

Myelin means incredible speed.

The A -alpha fibers, which handle a lot of the proprioceptive data, can conduct at 70 to 120 meters per second.

The A -beta fibers for touch are a bit slower, maybe 40 to 75 meters per second, but still incredibly fast.

Which is why you feel a touch on your toe almost instantly.

Exactly.

That high speed gives you the instantaneous high fidelity information you need to interact with the world.

Okay, so let's pivot now from those fast, precise pathways to the body's alarm system.

The nociceptors.

These are all about detecting harm.

How are they different structurally?

They're much, much simpler.

There's no elaborate onion -like capsule.

Nociceptors are basically just the bare, unmyelinated nerve endings of sensory neurons.

They don't need a complex structure.

They just need to be exposed to whatever is causing the damage.

And they specialize in different kinds of damage.

Precisely.

We group them by their adequate stimulus.

So you have mechanical nociceptors for things like a pinprick or crushing pressure.

You have thermal nociceptors that fire at extreme temperatures.

Like above 45 Celsius or...

Or severe cold below 20 Celsius, yeah.

Then you have chemical nociceptors that respond to irritants or molecules released by damaged cells, things like bradykinin or high levels of acid.

And finally, you've got polymodal ones that can respond to a mix of all three.

And this brings us to one of the most compelling parts of the whole system.

The dual sensation of pain.

When you stub your toe, you get that immediate sharp crack.

And then a second later, the deep throbbing ache starts.

Why?

That is the perfect real -world example of how function is separated by the speed of the nerve fiber.

That initial sharp crack is what we call first pain or fast pain.

And that's carried on which fibers?

That's mediated by the thinly myelinated A delta fibers.

Now, they're slower than the touch fibers conducting at about 12 to 35 meters per second, but they're still fast enough to give you that immediate sharp, well localized warning.

They tell you where and how bad right away.

And they mostly use glutamate as their neurotransmitter.

Okay.

So the A delta fibers give you the warning shot.

What about the misery that comes after?

That's the second pain, the slow burning throbbing ache.

And that is carried by the slowest fibers in the whole sensory system, the unmyelinated C fibers.

They just crawl along at maybe 0 .5 to 2 meters per second.

Which explains the delay.

It perfectly explains the delay.

And functionally, they carry that diffuse, poorly localized and intensely unpleasant emotional feeling of suffering.

And here's a crucial molecular detail.

C fibers release both glutamate and a neuropeptide called substance P.

We're going to see that substance P is a key player in sensitizing the whole system later on.

So the difference between A delta and C fibers is like the difference between seeing the lightning and hearing the thunder.

That's a perfect analogy.

One is the sharp, immediate flash.

The other is the slow rolling rumble that follows.

Before we dig into the molecules, we should probably touch on itch or pruritus.

The sources link it pretty closely to pain.

They do.

Itch is its own specialized sensation, and it's often triggered by chemicals like from mast cells or various kinins like bradykinin.

But the connection to pain is really fascinating when you think about how you stop an itch.

You scratch it.

You scratch it.

And what you're doing is activating all those big, fast conducting A alpha and A beta fibers, the touch pathways.

And that fast input rushes into the spinal cord and actually inhibits the transmission of the slow itch signal.

It's basically a real world application of the gate control theory, which we'll get to.

You're using a fast signal to block a slow one.

Okay, let's go down to the molecular level.

How does something like heat or acid actually start the electrical signal in these nerve endings?

This is where the TRP channels come in.

Right.

The transient receptor potential channels.

These are basically the molecular sensors on the nosoceptor.

They're non -selective cation channels, which just means they're gates that, when opened, let positive ions flow into the nerve, causing it to depolarize.

And the most famous one is TRPV1?

The vanilla receptor, yes.

This is the one that's a great physiological party trick.

Because of chili peppers.

Exactly.

TRPV1 is activated by intense, noxious heat, anything above 43 to 45 degrees Celsius.

It's also activated by acids.

But it's also activated by capsaicin, the chemical that makes chili peppers hot.

Capsaicin chemically tricks the TRPV1 receptor into thinking it's being burned.

So your mouth isn't actually hot, it just thinks it is.

Your brain is getting the exact same signal it would get from a burn.

And what's interesting is that these TRPV1 channels aren't just on the nerve endings, they're also on skin cells, called keratinocytes, which adds another layer of communication.

Okay, what about other kinds of damage, like a strong mechanical force or something cold and irritating?

For that, the sources point to a different channel, TRPA1.

These are activated by strong mechanical stimuli and by noxious chemicals like the ones in wasabi or mustard oil.

And then, if the problem is just acidity, say from inflammation, the ASIC receptors, or acid ion channels, are the dominant players in that kind of pain.

But the activation isn't always direct, is it?

Sometimes the tissue damage itself releases a bunch of chemicals that then trigger the nerve.

That's a huge part of it.

When cells are damaged, they spill their contents.

They release a flood of ATP, for example, which acts on purinergic receptors on the nerve ending to cause pain.

And crucially, damaged tissue releases nerve growth factor, or NGF.

And NGF is more than just a trigger?

Oh, much more.

NGF binds to its receptor, TRKA.

And as we'll see, it plays a really sinister role in reprogramming the neuron and turning acute pain into a chronic pathological state.

And all of this together, the bradykinin, the histamine, the prostaglandins, that's the inflammatory soup that causes so much trouble.

It is.

The noxceptor endings are just studded with receptors for all these immune mediators.

And these chemicals don't just activate the nerve, they sensitize it.

They lower its which is the key mechanism behind hyperalgesia, that state of heightened pain.

Okay, let's wrap up this receptor tour with the thermoreceptors, the ones for temperatures that are not painful.

How do we tell the difference between cool and painfully cold?

We use different TRP channels.

For innocuous, non -painful cold, the main player is TRPM8.

This is also the menthol receptor, which is why mint feels cool.

Ah, so it's the same trick as capsaicin, but for cold.

Exactly.

The TRPM8 receptors are found on A delta and C fibers, and they fire most actively and range from about 40 degrees Celsius down to 24 degrees.

And what happens if it gets colder than that?

This is a really cool feature of the system.

Below about 10 degrees Celsius, the firing rate of these dedicated cold receptors drops off dramatically, and they basically go silent.

That cessation of firing is why extreme cold can feel numb or anesthetic.

The dedicated cold sensor is offline.

And any sensation you do feel is probably from the pain receptors kicking in.

Right, the noxious cold receptors.

Now, for warmth, it's a similar story with different channels.

Warm receptors are on C fibers, and they use TRPV and TRPV4.

Their firing rate increases as it gets warmer, up until about 45 degrees Celsius.

And at 45 degrees.

They go silent.

Because at that point, you've crossed the threshold into pain, and the dedicated thermal mastoceptors, the TRPV1 channels,

take over the job of screaming

danger.

Hashtag to generation and coding of the sensory signal.

Okay, so we've got the hardware down.

We know the receptors and the fibers.

Now we have to talk about the software.

How does a physical pressure get turned into a digital code the brain can actually read?

And for that, the Pacinian core puzzle is the absolute best model.

Why is it such a good model?

Because the whole transduction apparatus is contained within the receptor.

The myelinated nerve fiber, including the first place an action potential can fire, the first note of Ranvier, is right there inside that onion -like capsule.

It's a beautiful system for converting an analog force into a digital frequency.

Okay, so walk us through it step by step.

A mechanical force hits the core puzzle.

What happens first?

The first thing that happens is the mechanical energy physically deforms the nerve ending inside.

That deformation opens up some ion channels and positive ions flow in.

This creates a small, local, non -propagating depolarization.

We call it the graded receptor potential.

And graded means its size depends on the stimulus strength.

Exactly.

It's an analog signal.

A light touch creates a small potential.

A hard push creates a much larger one.

It's like a volume dial.

So it's still analog at this point.

When does it go digital?

It goes digital at step two.

When that graded potential is big enough to reach a critical threshold, about 10 millivolts, it triggers the all -or -nothing action potential.

And that happens at a specific spot.

At a very specific spot.

The first note of Ranvier.

This note is a converter.

It takes the size of that analog generator potential and converts it into a frequency of digital action potentials.

A small potential just over threshold might trigger one or two action potentials.

A huge potential will cause the neuron to fire a rapid -fire train of them, hundreds per second.

So magnitude becomes frequency.

And that idea is at the heart of sensory coding.

The brain has this rule book for interpreting every sensation, and it's based on four attributes.

Right.

Modality, location, intensity, and duration.

Let's break those down.

First, modality.

How does my brain know the difference between a light hitting my eye and a sound hitting my ear?

That's all about receptor specificity.

It's the principle of the adequate stimulus.

Each type of receptor is hard -wired and exquisitely tuned to one specific form of energy.

Rods and cones in your eye are for light.

Morrican receptors are for pressure.

But you can trigger them with the wrong kind of energy, can't you?

You can, but the threshold is absurdly high.

The classic example is if you press on your eyeball, you'll see flashes of light.

You're physically forcing the photoreceptors to fire with mechanical pressure.

Which is definitely not their adequate stimulus.

Not at all.

And the amount of pressure it takes is way, way more than what it would take to activate a touch receptor in your skin.

But the brain doesn't care how the signal started.

It just follows the labeled line.

If a signal comes in on the optic nerve, the brain says, that's light.

End of story.

Okay, attribute two, location.

How do we know where we've been touched with such precision?

Location is all about the receptive field.

That's the specific patch of skin that, when stimulated, will activate one single sensory axon and all of its little branches.

And because sensation is punctate, meaning it only comes from these discrete spots where receptors live, the brain can build a very detailed spatial map.

But the brain does more than just map it.

It sharpens the image.

It uses a process called lateral inhibition.

Lateral inhibition is absolutely vital for this.

The neurons at the very center of the stimulus, where the pressure is strongest,

fire like crazy.

But they also send out inhibitory signals to their neighbors, telling the neurons on the periphery to quiet down.

So it enhances the contrast.

It dramatically enhances the contrast between the center and the edge, which is what allows you to feel the sharp point of a pin instead of just a dull, blurry circle of pressure.

It's all about sharpening the edges.

And we can actually measure this ability clinically with the two -point discrimination test.

We can.

You use a set of calipers and you find the smallest distance between the two points, where a person can still feel them as separate.

And that distance tells you directly about the size of the receptive fields in that area of skin.

So on the fingertips, it's very small.

On the fingertips, it's tiny, maybe two millimeters.

Your tactile acuity is incredible there.

But on your back, the receptive fields are huge and far apart.

You might need 60 or 70 millimeters before you can tell it's two points and not one.

It's a fantastic and simple way to test the health of that dorsal column pathway.

The sources also link this high fidelity system to two other key senses,

stereognosis and palesthesia.

Right.

Stereognosis is just the fancy word for being able to identify an object by touch alone, like knowing it's a key in your pocket without looking.

Palesthesia is the sense of vibration, which is mostly handled by those deep Pacinian corpuscles.

And both of these depend on that fast, high -quality information.

They absolutely require it.

And that's why losing the ability to feel vibration from a tuning fork or not being able to identify a coin in your hand is such an important early warning sign for diseases that affect the dorsal columns, like a B12 deficiency or diabetic neuropathy.

Okay.

Third attribute, intensity.

We know what it is and where it is.

How does the brain know how hard the stimulus is?

It uses two main strategies.

The first one is what we already talked about, action potential frequency.

A stronger stimulus creates a bigger receptor potential, which makes the neuron fire faster.

The brain reads high frequency as high intensity.

And the second strategy is about spreading the signal out.

That's recruitment.

A very light touch might only activate a few very sensitive receptors in a small area.

But if you press harder, the force spreads out.

You start activating more sensory units over a wider area.

And you also start to recruit higher threshold receptors that didn't bother to fire for the light touch.

So the brain sees this massive increase in the total number of incoming signals and interprets that as a big increase in intensity.

Exactly.

More neurons firing and each one firing faster.

And the final attribute,

duration.

How does the brain know how long a sensation lasts?

This gets into receptor adaptation.

Adaptation is just the fact that a receptor's firing rate will slow down over time, even if the stimulus stays exactly the same.

And based on how quickly they do this, we put them into two categories.

The fast ones and the slow ones.

Right.

The rapidly adapting, or phasic, receptors.

These are your Meisner and Poissonian core puzzles.

They fire a big burst right when the stimulus starts and then they shut up.

They're perfect for detecting change movement, vibration, the start and end of a stimulus.

It's why you stop feeling your clothes a few minutes after you put them on.

You'd go crazy if you didn't.

And then there are the ones that don't shut up.

Those are the slowly adapting, or tonic, receptors.

That's your Merkel cells for sustained pressure, Ruffini endings for stretch, muscle spindles for posture, and most importantly, the NOS receptors for pain.

They need to keep firing for the whole duration because they're signaling something the brain needs to pay continuous attention to,

like maintaining your balance or, you know, not forgetting your hand is on a hot stove.

Hashtag tag 309.

The specific physiology of pain.

Okay, the physiology of pain is so important clinically that it really deserves its own section.

And the official definition of pain is interesting because it includes the mind.

It does.

The IASP defines pain as an unpleasant sensory and emotional experience.

And that emotional component is what distinguishes it from NOS deception.

What's the difference?

NOS deception is just the raw, unconscious neural activity.

It's the signal in the wire caused by a potentially harmful stimulus.

Pain requires consciousness.

It requires that emotional aversive reaction.

You can have NOS deception under anesthesia, but you can't have pain.

And when that system goes wrong, we get chronic pain.

How do we define the line between helpful acute pain and pathological chronic pain?

Acute pain is the good pain.

It has a clear cause, it's sudden, it's localized, and it goes away when the injury heals.

Chronic pain is the bad pain.

It sticks around long after the initial injury is gone, and it often doesn't respond well to normal painkillers.

It's not a symptom anymore.

It's a disease of the nervous system itself.

And chronic pain brings with it these two really bizarre and debilitating symptoms,

hyperalgesia and aledinia.

Right.

Hyperalgesia is when a stimulus that is already painful hurts way more than it should.

So a small pinprick feels like a major wound.

But aledinia is even stranger.

Aledinia is, I think, the most devastating part of neuropathic pain.

It's when a normally completely harmless stimulus, an innocuous one, causes severe pain.

So things like the touch of a bed sheet or a gentle breeze or a warm shower.

Exactly.

The classic example is a warm shower on a bad sunburn.

The warm water isn't damaging you, but the nerve endings have become so sensitized that they interpret that normal non -painful warmth as agony.

And that switch is caused by the biochemicals floating around the nerve endings.

Let's talk about that inflammatory soup.

What's in it that causes the sensitization?

When your cells get damaged, they burst open and they release potassium, which directly excites the nerve endings.

Immune cells show up and release histamine.

Platelets release serotonin.

And then your body starts making inflammatory chemicals like bradykinin and, most importantly, prostaglandins.

And prostaglandins are a key target for drugs.

The huge target.

Specifically, prostaglandin E2 is made by enzymes called cyclooxygenase or KEOX enzymes.

And what it does is it powerfully sensitizes the nosoceptors.

It doesn't necessarily make them fire on its own, but it dramatically lowers their thresholds so that other things can make them fire much more easily.

And that's exactly how drugs like aspirin and ibuprofen work.

That's their main mechanism of action.

They are COX inhibitors.

They block the production of prostaglandin E2, which reduces that peripheral sensitization and brings the pain level back down.

But the sources are really clear that for chronic pain, the changes go way beyond just what's happening at the site of injury.

The changes are structural, they're genetic, and they happen back in the neuron cell body.

This is where the problem becomes embedded in the system.

And a key molecule here is that nerve growth factor, NGF, that we mentioned earlier.

When it's released from damaged tissue, it gets picked up by the nerve ending and transported all the way back to the cell body in the dorsal root ganglion.

And what does it do there?

It acts like a switch.

It gets inside the nucleus and changes which genes are being turned on and off.

It cranks up the production of pain transmitters like substance P.

And even worse, it can cause a phenotypic switch, basically reprogramming neurons that weren't pain neurons before and turning them into hyper -exfitable pain neurons.

It's like the nervous system decides to permanently install a fire alarm in that part of the body.

That's a great way to put it.

And on top of that, you see the upregulation of a special kind of sodium channel called NAV1 .8.

This channel is pretty much only found in these sensory neurons.

And when you make more of them, it makes the neuron much, much easier to fire.

It becomes hyper -excitable.

And the wiring itself can physically change through a process called sprouting.

Correct.

After an injury, those normal fast -touch fibers, the A -beta fibers, can actually start to grow new connections or sprout.

And they'll form synapses on neurons in the spinal cord that are normally only supposed to receive pain signals.

So that's a physical explanation for allodinia.

It's a perfect explanation.

A normal light -touch signal comes in on that A -beta fiber.

But because of the new wiring, it gets shunted over to the pain pathway.

The brain receives a signal on the pain pathway and says, that's pain, even though the original stimulus was just a gentle touch.

And these changes aren't just in the first neuron.

The spinal cord itself can become sensitized in a process called windup.

Windup is a form of central sensitization.

If the C fibers bombard the dorsal horn of the spinal cord with enough glutamate, and substance P, they can over -activate the NMDA receptors on the next neuron in the chain.

This essentially makes that spinal cord neuron hyper -excitable.

It winds up its response so that later, even small signals coming in cause a huge amplified pain output.

It becomes a self -perpetuating cycle.

And even the local immune system in the CNS gets involved.

Yes, the microglia.

These are the resident immune cells of the brain and spinal cord.

They get activated by all these pain signals, and they start releasing their own pro -inflammatory chemicals, which just adds more fuel to the fire.

Targeting these activated microglia is a really hot area for new pain drug development.

Okay, let's shift focus to pain that doesn't come from the skin deep and visceral pain.

From your organs, how is that different?

It's a completely different character of pain.

Visceral pain is almost always purely localized.

You know your gut hurts, but you can't point to the exact spot.

It's often nauseating, and it comes with symptoms like sweating or changes in blood pressure.

And because our organs don't have a lot of those fast A delta fibers, you often don't get that sharp first pain.

It's mostly just that deep, dull, aching second pain.

And what are the receptors in our organs most sensitive to?

They're incredibly sensitive to distension to being stretched.

That's why the pain of a kidney stone or intestinal gas can be so excruciating.

It's the stretching of the wall of the organ.

But they're not very sensitive to being cut or burned, which is why during some surgeries, a surgeon can handle the intestines without causing sharp pain.

But if they pull on them, it's intensely painful.

And this all leads to the really interesting puzzle of referred pain.

Why does a heart attack cause pain down your left arm?

This is explained beautifully by the convergence projection theory.

What happens is that the sensory nerve fibers coming from a visceral organ like your heart, and the sensory fibers coming from a patch of skin like your left arm and shoulder, all converge and synapse on the exact same second -order neuron in the spinal cord.

So there's one neuron getting signals from two different places.

Exactly.

It's a shared pathway.

So when a pain signal comes up from the heart, it activates that shared neuron.

And how does the brain decide where the pain is coming from?

The brain makes an assumption.

It defaults to what it knows best.

Over your lifetime, that neuron has been activated thousands of times by signals from your arm bumps, cuts, scrapes.

It's only been activated a few times, if ever, by your heart.

So the brain projects the sensation back to the place it's used to hearing from, the somatic location.

It literally misinterprets the signal.

It's a wiring issue.

It's a wiring issue that provides an incredibly reliable diagnostic clue.

Hashtag brawnhiav,

central somatosensory pathways and localization.

Okay, now that the signal is generated and encoded, we need to follow it to the brain.

And the body uses two completely separate parallel superhighways to get the information there.

That's right.

And it keeps the signals for fine touch completely separate from the signals for pain and temperature until the very last stages of processing.

So let's start with the high -speed, high -fidelity highway.

The dorsal column medial lemniscal pathway, or DCML, what does it carry?

The DCML is for all the things that require precision and speed.

So that's discriminative touch, vibration, and proprioception.

Okay, let's trace its path.

The sensory fiber comes into the spinal cord.

And then what?

It immediately turns and enters the dorsal columns at the back of the spinal cord.

And the first critical point is that it ascends ipsilaterally.

It stays on the same side it entered.

It doesn't cross the midline yet.

Not yet.

It travels all the way up the spinal cord on the same side until it reaches the medulla at the base of the brain.

There, it finally synapses in one of two nuclei, the gracil or cuneate nucleus.

And this is where the second neuron takes over.

This is where the second -order neuron takes over.

And it's where the crucial event happens.

This second neuron immediately crosses the midline, or decusates.

It then travels up through the brainstem as a bundle of fibers called the medial lemniscus, all the way to a relay station in the thalamus called the VPL nucleus.

And from there, a third neuron goes up to the cortex.

Correct.

So the key takeaway for the DCML is it ascends on the same side and crosses high up in the brainstem.

Which means if you injure the dorsal columns in the spinal cord, the loss of touch and vibration will be on the same side as the injury.

Precisely.

It's an ipsilateral deficit below the level of the lesion.

Okay, now let's contrast that with the emergency route for pain and temperature,

the ventrolateral spinothalamic tract, or VST.

The VST does things completely differently.

The primary pain fiber comes into the spinal cord and it synapses immediately in the dorsal horn.

So it doesn't travel up first.

No traveling up.

And the second -order neuron does something radical.

It crosses the midline immediately, right there in the spinal cord, usually within a segment or two of where it entered.

Ah, so it crosses low and early?

It crosses low and early.

Then it travels up the opposite side, the contralateral side, and the ventrolateral part of the spinal cord, and it also ends up in that same VPL nucleus of the thalamus.

That is the single most important distinction in spinal cord anatomy.

It really is.

Touch pathway crosses high, pain pathway crosses low.

And we should also mention that some of these pain fibers take a detour, a spinal reticular pathway, that goes to the brain stem reticular formation.

That detour is really important for the emotional, effective component of pain.

So both pathways end up in the VPL nucleus of the thalamus, which then sends the signals on to their final destination, the somatosensory cortex.

Where is that?

That's in the primary somatosensory cortex, or S1, which is a strip of brain tissue located in the post -central gyrus, right behind the central sulcus in the parietal lobe.

And this is where we find that famous distorted little man, the homunculus.

Why does he look so strange?

The homunculus is distorted because the map isn't based on the physical size of the body part.

It's based on the density of receptors and the functional importance of that body part.

So it's all about how much we use it for sensation.

Exactly.

That's why the hands, the lips, the tongue.

Yeah.

They get a ridiculously huge amount of cortical real estate.

They are massive on the map.

But the entire back or the trunk, which doesn't do a lot of fine sensing, gets squeezed into this tiny little strip.

And let's connect this back to those two pain pathways.

When the pain signal gets to the cortex, what happens?

It essentially gets split into two experiences.

The main pathway that goes from the thalamus to S1, the primary cortex, that gives you the discriminative pain.

The what and where.

The what and where.

It tells you the location, the intensity, the quality of the pain.

But that other pathway, the one that went through the reticular formation and projects more broadly to the frontal lobe and the limbic system.

The emotional centers?

Yes, like the amygdala and the insular cortex.

That pathway mediates the motivational affective pain.

That's the suffering,

the unpleasantness, the emotional horror of it.

Which means you could theoretically separate the two.

You can.

A lesion in the primary somatosensory cortex might make it hard for you to tell exactly where the pain is, but you'll still find it incredibly unpleasant.

On the other hand, a lesion in the cingulate cortex might leave you able to describe the pain perfectly.

It's a sharp burning pain on my left hand, but you won't have that aversive emotional reaction to it.

You won't care about it.

Hashtag tag V.

Clinical impact.

CNS lesions and plasticity.

Okay, so since these two pathways cross at different levels, any injury to the nervous system is going to create a very specific, predictable pattern of sensory loss.

Which is incredibly useful for diagnosis.

Let's start with a clean lesion to the dorsal columns in the spinal cord.

What would you see?

You'd see an hypsolateral loss.

A loss on the same side as the injury of fine touch, vibration, and proprioception, but only below the level of the lesion.

Because those fibers hadn't crossed yet.

And what if you damaged the ventrolateral spinosalamic tract instead?

Now it's the opposite.

Because those fibers crossed, as soon as they came in, you would get a contralateral loss.

A loss on the opposite side of the body of pain and temperature sensation below the lesion.

The patterns are mirror images of each other.

What if the damage is higher up in the brain itself?

Well, if you damage the primary somatosensory cortex, you usually don't lose the ability to feel pain altogether, but you lose the ability to discriminate it well.

You can't localize it in time or space very accurately.

And then there's a really terrible rare condition called thalamic pain syndrome.

What causes that?

It's usually from a stroke that damages the thalamus.

And for reasons we don't fully understand, it can lead to this chronic, spontaneous, absolutely excruciating burning pain on the opposite side of the body.

And the ultimate clinical test of understanding these pathways is the brown secward syndrome.

Right.

This is the classic syndrome you get from a hemisection of the spinal cord, where exactly one half of it is damaged.

So what does that patient look like?

It's a very distinct pattern.

On the same side as the lesion, the ipsilateral side, they lose fine touch, vibration, and proprioception.

Because you've cut the dorsal column before it crossed.

They also have motor weakness on that same side, because you've also cut the descending motor tracks.

Okay, so same loss of touch and motor function.

What about pain?

Pain and temperature are lost on the opposite side of the body, the contralateral side.

Because the VST fibers that carry that information had already crossed over lower down in the cord and were traveling up on the side that got injured.

It's a perfect demonstration of the anatomy.

It is.

And there's one little detail.

The pain and temperature loss on the opposite side usually starts a segment or two below the actual level of the injury, because it takes the fibers a little bit of room to cross over completely.

All right, let's finish this section with one of the most amazing topics in all of neuroscience.

Cortical plasticity.

That map in our brain, the homunculus, it's not set in stone, is it?

Not at all.

It is incredibly dynamic.

The connections in our brain are constantly changing based on our experiences.

And the most dramatic and sometimes tragic example of this is phantom limb pain.

This is when an amputee feels pain in a limb that isn't there anymore.

Exactly.

Up to 80 % of amputees experience it.

And the current thinking is that it's caused by a massive reorganization in the central nervous system as it tries to deal with the sudden loss of all that sensory input.

What kind of reorganization?

Well, we see that the neurons in the thalamus that used to get signals from, say, the amputated hand, they're still active.

And in the cortex, the neighboring areas on the map start to invade the now silent territory.

So the area for the face might spread into the old hand area.

Precisely.

The cortical real estate for the face, which is right next to the hand on the homunculus map, will physically sprout new connections and take over the now unemployed hand cortex.

Which means if you touch the person's face.

They might feel the sensation in their missing phantom hand.

Because you're stimulating neurons that are physically located in the face area.

But the signal is arriving in a part of the brain that for its entire existence has only ever meant hand.

The brain reinterprets the input based on the old map.

It's profound evidence of how adaptable the brain is.

Hashtag take 6786.

Modulation of pain transmission and therapeutic approaches.

Okay, so since pain is such a powerful and important signal, it makes sense that the body would have ways to turn it down to modulate it.

Let's start right at the beginning in the spinal cord with the spinal gate control mechanism.

This is the theory that explains a very common sense behavior.

When you bang your shin, what's the first thing you do?

You grab it and rub it.

You rub it.

And it helps.

The gate control theory explains why.

When you rub the area, you're activating all those large, fast A alpha and beta McKenna receptors.

The touch fibers.

The fast pathways.

The fast pathways.

And when those signals arrive in the dorsal horn of the spinal cord, they activate inhibitory inner neurons that essentially close the gate on the slower pain signals coming in from the C fibers and A delta fibers.

The fast touch signal blocks the slow pain signal.

And this isn't just a theory.

It's the basis for a clinical treatment.

It is.

It's a whole rationale behind 10 SS or transcutaneous electrical nerve stimulation.

You put electrodes on the skin and use a low level of electricity to specifically activate those big A beta fibers, which then closes the pain gate without you needing to take any medication.

But the body's most powerful pain modulators are, of course, the opioids.

Both the ones we take, like morphine, and the ones we make, like endorphins.

How do they work?

They work at two key points in that first synapse in the dorsal horn.

First, they act presynaptically.

There are opioid receptors right on the axon terminal of the incoming pain fiber.

And what do they do there?

When an opioid binds there, it blocks calcium channels.

Less calcium coming into the terminal means less release of the pain, neurotransmitters, glutamate, and substance P.

It turns down the volume of the incoming signal.

OK, so it stops the signal from being sent.

What about the neuron that's supposed to receive the signal?

It works there too, postsynaptically.

There are also opioid receptors on the next neuron in the chain.

When an opioid binds there, it opens up potassium channels.

Potassium flows out of the cell, making the inside more negative.

It hyperpolarizes it.

It hyperpolarizes the cell, making it much harder to excite.

So even if some pain signal gets through, the receiving neuron is less likely to fire.

The combination of these two effects is a very powerful block on pain transmission.

We should probably make a quick clinical point here about the difference between tolerance and addiction.

It's a critical distinction.

The sources are very clear on this.

Tolerance is a purely physiological thing.

It's when your body adapts, and you need a higher dose to get the same effect.

Addiction is a psychological behavioral issue, the compulsive craving and use of a drug despite harm.

And the data shows that when opioids are used appropriately for legitimate chronic pain, psychological addiction is actually quite rare, especially in patients without a history of substance abuse.

Okay, now let's go up to the brain, to the command center for pain control,

the descending pain modulatory pathway.

And the absolute key structure here is a region in the midbrain called the periaqueductal gray, or PEG.

And what does the PEG do?

The PEG is like the CEO of pain control.

If you inject a tiny amount of opioid directly into the PEG, you get profound, body -wide analgesia.

It works by initiating top -down cascade.

The neurons in the PEG send signals down to two main areas in the brainstem.

Which are?

One is the nucleus raffi magnus, which sends serotonerotic neurons down to the spinal cord.

The other is the rostral ventromedial medulla, which sends down norectrotractor neurons that release norepinephrine.

So the brain sends serotonin and norepinephrine down to the spinal cord.

What do they do when they get there?

They powerfully inhibit the spinal cord neurons that are trying to transmit the pain signal.

And part of how they do this is by activating local interneurons that release our own endogenous opioids, like enkephalin.

So the brain uses this descending pathway to turn on its own internal morphine system right at the gate where the pain signal is trying to get in.

And this powerful descending system can explain some really amazing phenomena, like stress -induced analgesia.

It's the best explanation we have.

The soldier who is horribly wounded in battle, but feels no pain until they're safe.

That's likely the massive stress response flooding the system with norepinephrine, which activates this descending pathway and shuts down the pain signal to prioritize survival.

It's even thought that things like electroacupuncture might work by stimulating this pathway and causing a release of endogenous opioids.

So let's wrap up with the future of pain therapy.

We're moving beyond just systemic opioids and trying to target the specific pathological mechanisms we've been talking about.

The focus is definitely on reversing the sensitization.

So for the peripheral sensitization, you have things like capsaicin creams.

You put them on the skin, and at first they burn because they're activating the TRPV1 receptors.

But with chronic use, they desensitize those nerve endings and deplete their supply of substance P, effectively silencing them.

What about targeting that special sodium channel, NAV1 .8, that makes the pain neurons so jumpy?

That's the target for drugs like lidocaine patches or mexilotine.

By blocking the NAV1 .8 channel, they can selectively quiet down the hyper -excitable pain neurons without affecting other neurons as much.

And for the most severe, hard -to -treat pain.

For truly intractable pain, especially neuropathic pain, we have drugs like gabapentin, which is an anticonvulsant that targets calcium channels.

And for the most severe cases, you can even use zucconatide.

This is a drug derived from cone snail venom that has to be injected directly into the spinal fluid.

It blocks a specific type of calcium channel, and is incredibly potent at stopping neurotransmitter release.

And finally, a really exciting new direction is targeting the neuroinflammation, the activated microglia.

Yes, this involves developing cannabinoid agonists that specifically target the CB2 receptor.

CB2 receptors are mostly found on immune cells, including microglia, not on neurons in the brain that cause the high.

So the idea is you can activate these CB2 receptors to calm down the inflamed microglia and reduce neuropathic pain without the psychoactive side effects of targeting CB1.

It's a much more precise approach.

Hashtag tag outro.

What an incredible system.

I mean, so complex, so interconnected, we've gone from a simple touch on the skin all the way to the emotional experience of suffering.

I think the biggest takeaway has to be that functional separation.

You've got the DCML pathway,

fast, crosses high, handles fine touch.

Then you have the VST pathway,

slow, crosses low, handles pain and temperature.

That distinction is everything.

And the fact that pain itself is split, there's the where is it part process in one brain region and the how awful is it part process somewhere else entirely.

And clinically, the most important idea is that chronic pain is not just a long lasting acute pain.

It's a pathological reorganization of the nervous system.

It's a disease of sensitization with molecular changes like NAV 1 .8 upregulation and central windup.

Which leads us to our final provocative thought for you to think about.

As we continue to map out the exact molecular basis of this sensitization, the specific channels, the specific receptors on microglia,

we gain the ability to move beyond just trying to blunt the pain signal with drugs like opioids.

Instead, we can start developing therapies designed to fundamentally reverse the pathological state to reset the system.

And that shift from just managing chronic pain to potentially curing it, that is the ultimate promise of understanding this incredible system.

Thank you for joining us for this deep dive into somatosensory neurotransmission.

Until next time, keep digging into the details.