Chapter 11: Motor Control & Plasticity

Welcome to Last Minute Lecture.

This free chapter overview is designed to help students review and understand key concepts.

These summaries supplement not replaced the original textbook and may not be redistributed or resold.

For complete coverage, always consult the official text.

I want you to try something for me.

Right now, wherever you are, unless you're driving, please don't do this if you're driving.

But if you're safe,

just close your eyes.

Go on, it's worth it.

Okay.

Now, with your eyes still closed, take your right hand and just touch the tip of your nose.

And you did it.

Right.

It's probably the easiest thing in the world.

You don't miss.

You don't poke yourself in the eye.

Exactly.

You don't have to think about it.

Yeah.

Yeah.

You just know what your nose is.

You know where your hand is.

It feels automatic.

It feels like magic.

But today, we're going to talk about what happens when that magic, that automatic feeling

just vanishes.

We are digging into a case study that, honestly, it sounds like something from a science fiction story, but it's very real medical history.

And it's the perfect way to kick off our deep dive.

It is.

We're talking about the case of Ian Waterman.

And his story is our gateway into today's topic, which is motor control and plasticity.

We're doing a dump dive into Chapter 11 of Behavioral Neuroscience, the eighth edition.

And Ian's story is going to anchor this entire conversation because he lost something that, you know, we all just completely take for granted.

So let's set the scene a little.

Ian is 19 years old.

Just a kid, really.

He's young, healthy, working as a butcher's apprentice.

And then one day, he gets sick.

And at first, it seems like nothing serious.

Right.

It presented as a standard gastric flu, unpleasant, but not something you'd think would change your life forever.

But behind the scenes, something else was happening.

The virus triggered a very specific and very rare autoimmune reaction.

We hear about autoimmune issues all the time, but this one was just surgically precise in what it attacked.

It really was.

It didn't attack his brain.

It didn't attack his muscles.

So, you know, if you tested his muscle strength, Ian was still strong.

He could physically contract his muscles just fine.

So he wasn't paralyzed in the way we normally think of it.

The motor nerves, the wires carrying commands out from the brain to the muscles, they were OK.

They were totally intact.

But the virus, or the autoimmune response, it attacked the sensory neurons.

Specifically, it destroyed the large fiber sensory nerves that carry information from the body back to the brain.

OK, so let's be really clear about what he lost and what he kept.

He could still feel pain, for example.

Yes, exactly.

Pain and temperature sensation travel on different smaller nerve fibers.

Those were spared.

So if you burn him, he'd know it.

But what he lost was the sense of light touch.

And this is the crucial part.

He lost proprioception.

Proprioception.

It's often called the sixth sense.

And for good reason.

It's our body's internal GPS system.

That's a great way to put it.

It's the constant unending stream of data coming from millions of tiny sensors in your muscles, in your tendons, in your joints, that all tell your brain exactly where your body parts are in three -dimensional space.

It's how you can scratch an itch on your back without a mirror.

It's how you know your arm is raised right now, even with your eyes closed.

And Ian lost that completely from the neck down.

The text What You See Is What You Get, which covers his case, describes him as having no body sense.

He was, in effect, disembodied.

So just try to imagine that you wake up one morning, you can't feel the sheets on your skin, you can't feel your own body.

You try to sit up, but you have no idea where your torso is.

So you just flop over.

He described it as floating in bed.

He'd try to stand up, but without the feeling of his feet on the floor or the position of his knees, he'd just crumble into a heap.

And this is where it gets truly terrifying.

The detail about the lights.

Yes, the lights out scenario.

Because he had no internal sense of where his limbs were, he had to find a substitute.

And the only thing he had left was his vision.

He had to look at his arm to know where it was.

He had to watch his hand to guide it to pick up a cup.

And if the lights went out, if the room went dark, he'd collapse instantly, just fold.

Because his brain lost its only remaining reference point.

He was blind to his own body.

That brings us to the whole point of this deep dive, the so what of Ian's story.

We tend to think of movement as a one -way street.

The brain is the general, right?

It barks an order, sends it down the chain of command, and the muscles, the soldiers, obey.

But Ian's case just shatters that entire model.

Movement isn't a monologue from the brain.

It's a conversation.

It's a loop.

A constant back and forth.

The brain sends a command, lift arm, but it desperately needs that sensory feedback to know, okay, did the arm lift?

How high?

How fast?

Is it where I wanted it to go?

If you cut that feedback wire, that proprioceptive input, the motor output becomes almost useless.

And Ian, amazingly, learned to walk again.

But he had to, to basically hack his own nervous system.

He did.

He had to consciously create a new feedback loop using his eyes to replace the proprioception he lost.

So for him, walking is a series of deliberate, planned actions.

He has to think, okay, I am now lifting my right leg.

I'm watching it move forward.

I see that it has been placed on the floor.

Okay, now time to start the sequence for the left leg.

It requires immense cognitive effort.

For you and me, walking is completely subconscious.

We can walk and talk and drink coffee.

For Ian, every single step is a high stakes tactical operation.

So with Ian as our sort of guide through this strange world, we're going to map out this entire machine.

We're going from the absolute basics, the hardware of the muscles and the spine.

All the way up to the complex software in the brain's cortex that does all the planning.

And we're going to see how this system is built.

But also, and this is the really cool part, how incredibly plastic, how changeable it is.

So let's start at the beginning, section one, the behavioral view.

Before we had fMRI scams, before we even knew what a neurotransmitter was, how did scientists even begin to figure out how we move?

To do that, we have to travel back a bit.

So the late 19th and early 20th centuries, and the giant of this era is a British physiologist named Sir Charles Sherrington.

He is, in many ways, the father of modern neurophysiology.

And he did some experiments that you probably couldn't get them past an ethics board today.

He worked with what he called spinal animals.

He did.

And it sounds grim, I know, but it was absolutely essential for understanding the hierarchy of the nervous system.

A spinal animal, usually a cat or a dog in these early experiments, was an animal where the spinal cord had been surgically disconnected from the brain.

So the brain is completely cut off from the body.

In terms of voluntary movement, the animal is paralyzed from the neck down, right?

In terms of voluntary movement, yes.

The brain can't send any commands down to the body.

But Sherrington noticed something absolutely incredible.

The body wasn't just dead weight.

The spinal cord, on its own, was still doing things.

Without the brain's permission.

Without the brain's involvement at all.

For instance, if you gently pinched the animal's foot, the leg would withdraw in a coordinated way.

If you tickled its flank, it might make a scratching motion with its hind leg.

So it could generate these pretty complex -looking movements purely based on sensory input with no brain.

Exactly.

And this led him to his big idea.

This is the concept of the reflex.

The knee -jerk reflex at the doctor's office.

That's the classic one.

Sherrington defined a reflex as a simple, unvarying, unlearned response to a sensory stimulus.

You tap the knee, the leg kicks, it happens every time.

You don't learn it, you don't decide to do it.

So let's break that one down, because it's a perfect example.

What's actually happening there?

It's just a beautiful piece of biological engineering.

The little rubber hammer taps the patellar tendon, which briefly stretches the big quadriceps muscle on the front of your thigh.

Inside that muscle are tiny sensors that scream, warning!

We're being stretched too far, too fast!

That signal, that sensory information, shoots up a nerve into the spinal cord.

It goes all the way up to the brain.

Nope.

It doesn't have time.

It gets to the spinal cord and it takes an immediate U -turn.

It synapses directly onto a motor neuron that goes right back to the same quadriceps muscle and says, contract now, to protect the muscle from being overstretched.

So it's a local circuit.

Input, U -turn, output.

All inside the spine.

All inside the spine.

Your brain might find out about it a half second later, like, hey, what was that?

But the action is already over.

So Sherrington looked at this and thought, aha,

maybe that's everything.

That was his hypothesis, the reflex chain theory.

The idea that all complex behavior, even something like walking, was just a long sequential domino effect of reflexes.

The sensation of your foot hitting the ground triggers a reflex to bend your knee, which triggers a reflex to swing your leg forward, and so on.

But we know that's not the whole story, right?

I could decide right now to wave my hand in the air.

There's no stimulus triggering that.

It's voluntary.

Exactly.

And even more importantly, we can plan movements in advance.

Think about a concert pianist playing a ridiculously fast passage.

Their fingers are moving so quickly, there literally isn't time for a sensory signal to go from the fingertip to the spine and back to trigger the next note.

They're not reacting, they're executing.

They are executing a pre -planned sequence.

This is what the text calls a motor plan or a motor program.

It's a complex set of muscle commands that is established in the brain before the action even starts.

The brain writes the whole musical score and then just hits play.

This brings us to a really useful comparison the text makes between two kinds of control systems.

And if you have any background in engineering or robotics, this will sound really familiar.

It's open loop versus closed loop.

And this is absolutely crucial for understanding what went wrong for Ian Waterman.

Let's start with closed loop because that's the one he lost.

Okay, closed loop.

I always think of this like the thermostat in your house.

That's a perfect analogy.

A closed loop system is all about accuracy.

It's a system that uses feedback to make corrections during the movement.

So my thermostat has a goal.

Keep the room at 70 degrees.

It has a sensor, the thermometer, that gives it feedback.

The room is currently 68 degrees.

That's an error signal.

Right.

And based on that error signal, it performs a correction, it turns on the furnace, then it keeps sampling the feedback until the error is gone, until it hits 70 degrees, and then it shuts the furnace off.

So it's a constant cycle.

Action dot feedback, correction dot new action.

Constantly.

This is what you're doing when you're trying to, say, thread a needle.

You're watching the thread.

You're watching the eye of the needle.

And you're making these tiny micro adjustments based on visual feedback.

But there's a tradeoff, isn't there?

The cost is speed.

That feedback loop takes time.

The signal has to go from your eyes to your brain, get processed, and then a new command has to go down to your hands.

If you need to move in a flash, closed loop is just too slow.

And that's where open loop comes in.

Open loop is designed for pure speed.

The technical term is ballistic, like a bullet from a gun.

I think of a baseball pitcher throwing a fastball.

Perfect example.

Or a martial artist throwing a punch.

You can't adjust a punch mid -flight.

You wind up, you plan the movement, and you just release.

And the moment that ball leaves the pitcher's fingertips, the motor plan is done.

The program has been executed.

It's a millisecond later the pitcher realizes, oh no, I aimed a little too high.

It's too late.

There is no feedback loop fast enough to catch that ball in midair and redirect it.

So to put it simply,

open loop is fire and forget.

And closed loop is fire, check, adjust, fire again.

And in a healthy person, we're constantly blending these two, we might use an open loop program to start a fast reach for a glass of water.

And then switch to a closed loop system for the final delicate part of actually grasping it without knocking it over.

And Port Ian Waterman, he lost his internal automatic closed loop system proprioception.

So he's forced to live his life in a slow, deliberate,

manually operated closed loop using his eyes.

Which is why it's so exhausting for him.

He's manually doing the job that your spinal cord and cerebellum are supposed to be doing on total autopilot.

Okay, so that's the big picture behavioral level.

We have reflexes and plans.

We have these open and closed loops.

Now let's go deeper.

Let's go down to the engine room.

Let's look at the hardware itself, section two.

You can have the most brilliant software plan in the world, but if the engine is broken, the car doesn't go anywhere.

And in the body, the engine is, of course, the muscle.

And the book makes a really simple but profound point that I think most people don't think about.

Muscles are actually very limited in what they can do.

They are absolute one trick ponies.

A muscle can only do one thing, contract.

That's it.

It can shorten.

It can pull.

A muscle can't push.

Never.

Wait, hold on.

Explain that.

Right now, I'm pushing my chair back from the table.

I'm definitely pushing.

You are pushing the chair.

But let's look at the muscles in your arm that are doing it.

To straighten your elbow and push that chair, your triceps muscle, the one on the back of

is contracting.

It's shortening.

And in doing so, it's pulling on the bone of your forearm to extend the limb.

The force is always generated by pulling.

Ah, okay.

So because they can only pull, they have to work in teams to get anything done.

Precisely.

We call these teams antagonists.

They are muscles that work in opposition to each other across a joint.

The classic example is the bicep and the tricep.

Exactly.

To bend your elbow to do a bicep curl, your biceps muscle contracts.

It's the agonist for that movement.

But for that to happen, the triceps on the back must relax and lengthen.

It's the antagonist.

And if they both contract at the same time, with full force, your arm would lock up, completely rigid, which is something you do intentionally sometimes, like if you're bracing for an impact.

But for smooth movement, you need that perfect reciprocal coordination.

One turns on, the other turns off.

And the text also mentions synergists.

Those are the helpers.

They're muscles that work together with the main mover, the agonist.

They might help stabilize the joint or contribute some force to the movement.

So even a simple action like raising your hand is this incredible symphony of agonists firing,

antagonists relaxing, and a whole bunch of synergists keeping everything steady.

It's a whole orchestra.

Now let's zoom in even further.

Let's get down to the molecular level.

How does the nerve actually tell the muscle fiber, okay, now's the time to contract.

What is that spark?

This is one of the most well understood connections in the entire body.

It's called the neuromuscular junction, or the NMJ.

This is the exact point where the tip of the motor neuron, the nerve cell coming from the spine, meets the muscle fiber itself.

That's right.

And they don't quite touch.

There's a tiny, tiny gap called the synapse.

When the electrical signal, the action potential, comes racing down the nerve, it hits the very end of the axon.

And it has to jump the gap.

It can't.

The electricity stops.

Instead, that electrical signal triggers the release of a chemical, a neurotransmitter.

And at the neuromuscular junction,

that chemical is always acetylcholine, or SC.

Acetylcholine.

This is a huge player all over the nervous system, but here it has one job.

One critical job.

The AC molecules float across that tiny gap.

They bind to special receptor proteins on the surface of the muscle fiber, and that binding action is what triggers the chemical cascade inside the muscle that causes its fibers to slide past each other and contract.

So nerve signal, chemical release, muscle contraction.

Every single time you move a muscle.

And because this is a chemical process, it's also vulnerable to other chemicals.

The text mentions some pretty scary toxins that target this exact spot.

Oh yeah.

The NMJ is a major target for poisons and venoms in the natural world.

For sure.

Take curar, for example, a plant extract used by indigenous peoples in the Amazon on their blowgun darts.

Curar is an antagonist for acetylcholine.

It gets into the synapse and it just sits on the AC receptors, blocking them.

So the brain is screaming, move.

The nerve is dutifully releasing all the acetylcholine it can.

But the muscle has its fingers in its ears, so to speak.

The receptors are blocked.

The signal never gets through.

And the result is paralysis.

Placid paralysis.

You just go limp.

And if it hits your diaphragm, the muscle you use to breathe, you suffocate.

And then you have toxins that do the exact opposite.

Nerve gases or black widow spider venom.

Right.

Those can cause a massive uncontrolled flood of acetylcholine or they can block the enzyme that's supposed to clean it up afterward.

So the muscle gets the contract signal and then it just keeps getting it over and over.

It never gets the stop signal.

Exactly.

The muscle goes into a rigid spastic contraction, a state of tetanus.

So you can see both extremes, too little signal or way too much, are fatal.

The system has to be balanced on a knife's edge.

Okay.

Let's follow that nerve back from the muscle, up the arm, and into the spinal cord.

We talked about Sherrington's spinal animals, but let's look at the actual anatomy.

The text talks about the roots of the spinal nerves.

This is the fundamental architecture, the highway system, of the spinal cord.

And it's organized into two main flows of traffic.

Dorsal and ventral.

I always need a little memory aid for this one.

Okay.

I think dorsal is the door.

The dorsal roots are at the back of the spinal cord.

This is the entrance.

All the sensory information, touch, pain, and importantly, proprioception, it all enters the spinal cord through the dorsal roots.

So Ian Waterman's problem, functionally, was like a massive permanent blockade at the dorsal door.

The information just couldn't get in.

That's a perfect way to think about it.

And then you have the ventral roots, which are at the front of the spinal cord.

This is the exit ramp.

The cell bodies of the motor neurons live in the ventral part of the cord, and they send their axons out through the ventral roots to the muscles.

So input in the back, output in the front, sensory in via the dorsal root, motor out via the ventral root.

And the beauty of the simple reflex like the knee jerk is that it all happens right there in the middle.

The signal comes in the back door, takes a shortcut across a single synapse, and goes right back out the front door.

The brain is completely bypassed.

Like a local circuit breaker designed for speed.

Okay.

So we've covered the hardware.

We have muscles.

We have the NMJ.

We have the basic wiring of the spinal cord.

Now we have to go on stairs to the C -suite, the boss,

the brain.

This is section three, cortical control.

This is where voluntary lives.

When you decide to pick up that cup of coffee, the initial command starts way up in your cerebral cortex, and the main highway for that command is called the pyramidal system.

Why pyramidal?

Is it shaped like a pyramid?

The tract of nerve fibers itself isn't, no.

But as this massive bundle of neurons travels down from the cortex towards the spinal cord, it has to pass through the brain stem.

And at a specific spot, the medulla, these bundles of fibers form two distinct wedge -shaped bumps on the surface that, to the early anatomists, look like little pyramids.

Scientists are very literal sometimes.

They really are.

And right at that spot, at those pyramids in the medulla, something incredibly important happens.

The decussation.

The great crossing over.

This is the famous fact that your left brain controls the right side of your body, and your right brain controls the left side.

This is where it happens.

The fibers coming down from the left motor cortex cross over the midline at the medulla and then continue down the right side of the spinal cord to control the right side muscles, and vice versa.

It's a contralateral system.

Which is exactly why, if someone has a stroke in the left hemisphere of their brain, you often see paralysis or weakness on the right side of their body.

Correct.

So where does this signal actually start?

The command originates in a strip of cortex that runs over the top of your head, kind of like a headband.

It's called the primary motor cortex, or M1 for short.

And this gives us one of the absolute most memorable, and frankly weirdest, images in all of neuroscience.

The motor homunculus.

The little man.

If you have the textbook, it's figure 11 .14.

If you don't, just imagine taking a drawing of a human body and mapping it onto that strip of brain.

But the proportions are all wrong.

They're monstrously wrong.

The hands are gigantic.

The thumb alone is the size of a leg.

The lips and the tongue are massive, just hanging off the side of the brain map.

But then the torso, the legs, the arms, they're these tiny, withered little things.

It looks like a bizarre caricature, but it's not a map of physical size.

Right.

It's a map of importance.

Yeah.

Or precision.

Exactly.

It's a concept called cortical magnification.

The size of the body part on that map corresponds directly to the number of neurons in the motor cortex that are dedicated to controlling it.

So think about your back.

Physically, it's a huge part of your body, but what can you really do with it?

You can bend, you can twist.

The movements are pretty crude, so it gets a tiny little patch of cortical real estate.

Your hands, your fingers, your thumb.

Oh, they're precision instruments.

A thumb can thread a needle, type a message on a phone, play a violin, perform microsurgery.

That incredible level of fine motor control requires millions and millions of neurons processing information.

So the thumb gets a giant mansion in the motor cortex.

And the lips and tongue?

Speech is arguably the most complex and precise motor task that humans perform.

We have to coordinate the lips, the tongue, the jaw, the larynx with millisecond precision just to produce a single word that takes an enormous amount of brain power.

So the homiculus is basically a picture of what the brain thinks is most important,

manipulating the world with our hands and communicating with our mouths.

That's the evolutionary story, yes.

And the coolest part about this map is that it's not fixed.

It's not hardwired at birth.

It's plastic.

It can change with experience.

This is the use it or lose it principle in action.

Or maybe use it and grow it.

The text talks about studies of musicians, professional violin players.

If you scan their brains, the area of the motor cortex that controls the fingers of their left hand, the hand that does all the complex rapid fingering on the strings.

That part of that is bigger.

It's significantly larger than in a non -musician.

And it's also larger than the area controlling their right hand, the one that just holds the bow.

So years of practice have literally terraformed their brain, physically expanding that territory.

That's amazing.

But M1, the primary motor cortex, it's just the final executive, right?

It's the one that sends the final command down the pyramidal tract.

It has managers that help it plan.

It does.

We call these the non -primary motor cortex.

They're areas that sit just in front of the primary motor strip.

The two main ones are the supplementary motor area, or SMA, and the premotor cortex.

And they're involved in the planning and sequencing of movement.

What's the difference between them?

It's a subtle distinction, but it seems to come down to internal versus external cues for movement.

Okay.

What do you mean by that?

The SMA seems to be crucial for movements that are generated internally.

So if you're just sitting there and you decide on your own to start tapping out a rhythm with your fingers, your SMA is firing like crazy before the movement even starts.

It's also very active when you mentally rehearse a complex sequence, like a dance routine, in your head without actually moving.

It's like the brain's internal rehearsal studio.

And the premotor cortex.

The premotor cortex is more involved when a movement is guided by external stimuli.

So if you see a red light turn green, the visual cue of the green light is what triggers your motor plan to press the accelerator.

If a ball is flying towards your face and you lift your hands to block it, that's a movement guided by an external object.

The premotor cortex helps map the outside world onto your motor system.

So SMA is, I have an idea, I want to do this.

And the premotor cortex is, the world is making me do this.

That's a great simple way to put it.

They both work together to create and polish a motor plan, which they then feed to the primary motor cortex, M1, which actually executes it by sending the signal down the line.

Okay, so we have the command center, the cortex, we have the superhighway, the pyramidal tract, we have the hardware down in the muscles.

But if that was all there was to it, our movements would be clunky, robotic,

we'd be jerky.

We need the systems that smooth everything out.

We need the modulators.

This is section four, the extrapyramidal system.

And extrapyramidal literally just means everything outside the pyramidal tract.

Exactly.

These are brain systems that don't send their own signals directly down to the spinal cord to make a muscle twitch.

Instead, they form these massive loops with the cortex to modulate, to fine -tune, to refine the motor signal.

And the two main players here are the basal ganglia and the cerebellum.

Let's start with the basal ganglia.

I've always thought of this system as being like the bouncer at a nightclub.

That is a surprisingly accurate metaphor.

The basal ganglia, which is a collection of deep brain structures like the caudate nucleus, the putamen, the globus pallidus, it really does act like a gatekeeper.

Your cortex at any given moment is bubbling with potential movements.

I could scratch my nose, I could stand up, I could take a sip of water, I could throw this pen across the room.

The job of the basal ganglia is to powerfully inhibit, to suppress, almost all of those potential actions.

It keeps the gate closed.

So it's basically saying, no, no, no, no.

Until the cortex makes a strong, concerted decision, okay, we are definitely taking a sip of water now.

And at that moment, the basal ganglia opens the gate for that one specific action to proceed while still holding all the others back.

So it's crucial for the initiation of movement.

And we really see how important this is when it breaks.

The text brings up Parkinson's disease.

Right.

In Parkinson's disease, the cells in a structure called the substantia negra, which feed the crucial neurotransmitter dopamine to the basal ganglia, they die off.

Without that dopamine, the basal ganglia's gate gets stuck in the closed position.

It becomes over -inhibitory.

And that's why a classic symptom is difficulty starting a movement, akinesia.

A patient might want to walk, their cortex is sending the command, but the body just won't go.

They freeze.

Exactly.

And you can see the flip side of that coin with a different condition, Huntington's disease.

In Huntington's, the gate is broken, but it's stuck open.

Right.

The inhibitory part of the circuit degenerates.

So the basal ganglia can't suppress all those unwanted motor plans.

And you get chorea, these constant involuntary dance -like writhing movements.

The brain can't filter out the noise.

So the basal ganglia is the gatekeeper.

It says go or no go.

Yeah.

Then we have the other big modulator, the cerebellum, the little brain tucked away at the back.

If the basal ganglia is the bouncer, the cerebellum is the quality control engineer or the conductor of the orchestra.

Its job is all about coordination, timing, and motor learning.

How does it do that?

It's an information processor.

It receives two streams of data simultaneously.

First it gets a copy of the motor plan from the cortex, a kind of carbon copy of the intended movement.

I intend to reach for that cup.

At the exact same time, it's getting a flood of sensory feedback proprioception from the arm itself telling it this is where the arm is actually right now and this is how fast it's moving.

So it compares the plan to the reality.

It compares the intention with the performance in real time.

If there's a mismatch, an error signal, it sends an immediate correction back up to the cortex to smooth out the movement.

This is why the classic roadside sobriety test, touching your finger to your nose, walking a straight line, is really a test of cerebellar function.

Alcohol hits the cerebellum hard and fast.

It absolutely does.

When your cerebellum is compromised, you lose that fine tuning.

Your movements become clumsy.

You overshoot your targets.

You stumble.

That condition is called ataxia.

Now this brings us to a specific piece of research in the textbook that I just found mind -blowing.

Figure 11 .28.

Because usually when we talk about motor learning,

like learning to ride a bike or play the piano, we talk about neurons.

We talk about synapses getting stronger or weaker.

Right.

The whole story of neuroscience for a hundred years has been about neurons.

But this study suggests we've been completely ignoring half the cells in the brain, the glia.

This is a really revolutionary study.

It's about a specific type of glial cell in the cerebellum called Bergmann glia.

Now for a century, we thought glial cells were just the support staff.

The word glia literally means glue.

We thought they just held the neurons in place, cleaned up their waste, and fed them.

Just the boring infrastructure.

But it turns out they are active participants in the conversation.

So here's the experiment.

Researchers took a group of mice, and using a really clever genetic technique called Cree recombinase, they were able to delete a specific gene in just one specific type of cell.

So they could do targeted molecular surgery.

Exactly.

And what they did was they deleted the gene for a type of glutamate receptor, an AMPA receptor, only in the Bergmann glial cells of the cerebellum.

The neurons were left perfectly normal.

So the neurons are fine, but the glue cells can't hear the main excitatory neurotransmitter glutamate anymore.

Correct.

So then they tested the mice.

First, they put them on a simple task, like just walking on a treadmill.

And the mice were totally fine.

Their basic, everyday motor coordination was intact.

But then they gave them a challenge.

Then they put them on what they called a complex wheel.

You can imagine it like a ladder that's been turned into a wheel.

But the rungs are all irregularly spaced.

You can't just run on autopilot.

You have to look and plan and adjust your steps.

It's a motor learning task.

You have to get better at it.

Right.

And the normal mice, they learned it pretty quickly.

They got smoother.

They made fewer mistakes.

But the mice with the modified glial cells, they never improved.

They kept slipping.

They kept making errors.

They couldn't master the complex task.

Wow.

So what does that actually tell us?

It tells us that the glial cells are essential for the plasticity that underlies difficult motor learning.

When you are struggling to learn a new hard skill, a new piece on the piano, a new golf swing, your glial cells are actively participating in reshaping the synapses to lock in that learning.

They aren't just the glue.

They are the engineers remodeling the entire structure.

That is a massive shift in how we think about the brain.

It's not just a neural computer.

There's this whole other layer of processing happening in the glia.

The whole system is much more collaborative and complex than we ever imagined.

Okay.

We've covered the hardware, the software, the modulators, even the construction crew.

We have one last section, section five, and it takes us from the individual to the social.

We've been talking about doing movements.

But as humans, we spend a huge amount of our time watching other people move.

And it turns out when we watch, our motor system isn't just sitting back and observing.

It's participating.

This is the incredible story of mirror neurons.

And like so many great scientific discoveries,

it happened almost by accident.

It did.

The scene is Parma, Italy in the 1990s.

A team led by Giacomo Rizzolotti is studying the premotor cortex of macaque monkeys.

They have these tiny microelectrodes implanted that can record the activity of a single neuron.

And they had found neurons that were involved in specific actions.

Right.

They found a neuron, for instance, that would fire vigorously whenever the monkey reached out and picked up a peanut.

Simple enough.

You'd call it a grasping neuron, the motor command for a pickup peanut.

But then the story goes, it was lunch break.

The legend is a researcher walks into the lab, maybe holding an ice cream cone, or in another version, they just pick up one of the monkey's peanuts to eat it.

The monkey is just sitting there, totally still, just watching the human.

The monkey isn't moving at all.

Not a muscle.

But the recording equipment connected to that grasping neuron goes absolutely crazy.

The exact same neuron that fired when the monkey did the action also fired when the monkey just saw someone else do the action.

Wait.

So from the neuron's point of view, it can't tell the difference between I am grasping and I am watching you grasp.

In a fundamental sense, no.

The neuron appears to be representing the abstract concept of the action itself, regardless of who the actor is.

This was a bombshell.

It suggested that when we watch someone else's actions, we're not just passive observers.

Our brains are actively internally simulating their actions using our own motor circuits.

Which leads to this incredibly profound theory about social connection, about empathy.

If my brain simulates your physical movement, maybe it also simulates your intention or your emotion that goes with that movement.

That's exactly the theory.

And the text connects this specifically to the study of autism.

Figure 11 .20 shows a study that compared brain activation in typically developing children versus children with autism.

And what was the task?

They asked the kids to simply imitate facial expressions that were shown in photographs.

Smile, frown, look surprised, look angry.

And on a behavioral level, could they do it?

Yes.

Both groups of children could physically make the faces.

They could imitate the expressions.

But when the researchers looked at their brain scans, they saw a critical difference.

In the neurotypical kids, when they were imitating, a specific area in the frontal lobe called the pars opercularis lit up brightly.

This is an area known to be rich in these mirror neurons.

But in the kids with autism?

There was significantly less activation in that same area.

The mirror system seemed to be underactive.

So this is the basis for what's called the broken mirror hypothesis of autism.

Exactly.

The hypothesis suggests that some of the core social deficits in autism, the difficulty in understanding other people's intentions or feeling empathy or reading social cues, might stem from an underlying dysfunction in this mirror neuron system.

If my brain doesn't internally simulate your smile, I might not get that automatic resonant feeling of happiness that goes with it.

I can see the shape of your mouth change, but I don't feel the echo of the emotion.

It's a powerful idea.

It suggests that our ability to connect with each other, our very capacity for empathy, might not be some high -level abstract thought.

It might be rooted in the most basic hardware of our motor system.

Which brings us full circle as we wrap up.

We started this whole deep dive with Ian Waterman, the man who lost his internal body sense and had to use his conscious vision to guide his movements.

He lost the input.

But the text ends by mentioning another patient, patient DF, who had the exact opposite problem.

It's such a fascinating counterpoint.

It really is.

Patient DF had brain damage from carbon monoxide poisoning that affected parts of her visual cortex.

She suffered from a condition called visual agnosia.

Which means she couldn't recognize objects.

Consciously, she couldn't.

So for example, if you held up a mail slot in front of her and asked, what is the orientation of this slot?

Is it vertical or horizontal or tilted?

She'd just be guessing.

She couldn't consciously perceive the orientation.

She was blind to the shape.

Consciously, yes.

But, and this is the part that gives you chills, if you then handed her a letter and just said, mail this letter, she would, without a moment's hesitation, reach out, rotate her wrist to the perfect angle and slide it right into the slot.

Without thinking about it.

Without any conscious awareness of the angle at all.

Her conscious brain didn't know the orientation, but her motor system absolutely did.

This was the key evidence for the idea that we have two visual streams in the brain.

One stream for conscious perception, the what pathway, and a separate unconscious stream for guiding action, the how pathway.

So DF had the how pathway, but it lost the what pathway.

Ian Waterman had his what pathway, his vision, but lost the sensory feedback that normally talks to the how pathway.

And between the two of them, they reveal just how much incredible, complex work is happening completely under the hood, completely outside of our awareness.

When you reach out and pick up your coffee cup, you're not consciously thinking about the glia in your cerebellum, or the decussation of your pyramidal tracts, or the acetylcholine at your neuromuscular junctions, or the proprioceptive feedback loops telling your brain your elbow is bent at a 90 degree angle.

Or the mirror neurons that are firing because you see me taking a sip of my coffee too.

But it's all happening.

A silent, magnificent symphony.

And Ian Waterman's story is a powerful reminder that if just one section of that orchestra stops playing, if that sensory feedback cuts out, the music just stops.

It reminds us that what feels effortless in our daily lives is actually the result of the most complex machine in the known universe working perfectly, millisecond by millisecond.

I want to leave you, our listener, with a final thought to chew on.

If this theory is right, if our empathy is at least partly based on these mirror neurons on our motor system simulating the physical actions of others,

what does that mean for us in a world where so much of our interaction happens through screens?

That's a provocative question.

When we send a text message, instead of seeing a person's face crinkle into a smile, or shoulder slump and sadness,

are we are we starving our mirror systems?

Are we exercising our empathy muscles less?

If we don't see the movement, do we lose a crucial part of the feeling?

Something to think about the next time you're on a video call and everyone has their camera turned off.

We want to thank you for joining us on this deep dive.

We hope this journey through the chapter helps you visualize the incredible machinery that's at work inside you right now.

This has been The Last Minute Lecture Team.

Thanks for listening.

See you next time.