Chapter 6: Neuronal Signaling and the Structure of the Nervous System

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Welcome back curious learners to the deep dive.

Today we're embarking on a journey really into the nuts and bolts of how our nervous system sends signals.

Yeah, it's a fascinating topic.

We're diving deep into chapter six of Vander's human physiology, getting our insights right from the source.

Exactly,

and our mission, well it's pretty straightforward,

break down these complex ideas, we'll navigate the chapter, simplify the mechanisms, the definitions,

make it all understandable.

So it guides through the jungle of neurophysiology?

Kind of.

We want to pull out the really key pieces of information, explain the jargon simply.

Maybe use an analogy if it helps clarify something, you know, like the book uses diagrams for.

The goal being a clear picture of how this command center works without feeling totally swamped.

Precisely, no overwhelm, just understanding.

All right, let's unpack this.

Where do we start?

The absolute basics, I guess.

Yeah.

The cells.

Good place to start.

So the nervous system, big picture, has two main divisions.

You've got the central nervous system, the CNS, that's your brain and spinal cord, the control hub, and then everything else branching out.

That's the peripheral nervous system, the PNS,

nerves connecting the CNS to, well, everywhere else.

Muscles, organs, skin.

And the key players in this communication network are the neurons.

The neurons, yeah.

They're the functional units.

They generate electrical signals,

release chemical messengers, neurotransmitters to talk to other cells.

They're also amazing integrators handling tons of input.

Okay, so what does a typical neuron actually look like?

What are its parts?

Well, they vary, but generally you have the cell body, or soma, that's command central for the cell, contains the nucleus, makes proteins.

Then branching out from the cell body, you have dendrites.

Think of them like antennas, highly branched, designed to receive information.

And often these dendrites have tiny little bumps called dendritic spines.

They just massively increase the surface area, letting the neuron pick up even more signals.

It's incredible.

So dendrites receive.

How does the neuron send its own message?

That's the axon's job.

It's usually a single long extension that carries signals away from the cell body.

Can be really long too.

Like really long.

Oh yeah.

Think feet, not inches.

Sometimes.

The signal usually gets generated at the base of the axon, a spot called the axon helic.

Then it travels down, the axon might branch out into collaterals, and it ends at axon terminals, or sometimes swellings called

And that's where the neurotransmitters get released.

Exactly.

That's the output zone.

These axons sound like critical communication lines.

How does the system make sure signals travel fast, especially over those long distances?

Ah, great question.

That's where myelin comes in.

It's a fatty sheath, like insulation on a wire, wrapped around many axons.

Insulation, so it speeds things up.

Massively speeds things up, and saves energy too.

In the CNS, cells called oligodendrocytes make myelin, often wrapping multiple axons.

In the PNS, it's Schwann cells, each wrapping a segment of just one axon.

Are there gaps in the insulation?

Yes.

Crucial gaps called the nodes of Ranvier.

The axon membrane is exposed there, and these nodes are absolutely key for that rapid signal transmission we'll talk about later.

Okay, so the axon is this long pathway.

It must need supplies delivered all the way to the end, right?

How does that work?

It's got its own internal delivery service called axonal transport.

It's like a microscopic railway system.

Seriously?

A railway?

Well, kinda.

It uses microtubules as rails, and motor proteins, like kinesins and dyneins, as the engines.

Kinesins move stuff like organelles and vesicles away from the cell body to the terminals, that's intergrade transport.

And dyneins?

Dyneins move stuff back towards the cell body retrograde transport, essential for recycling materials, getting feedback signals.

Could that retrograde transport be a problem?

Like a pathway for bad stuff?

Uh, yeah.

Unfortunately it is.

It's a vulnerability.

Things like tetanus, toxin, rabies, virus, herpes.

They can hijack this transport system to travel right into the central nervous system.

Wow, okay.

So we have this structure.

How about function?

Different types of neurons doing different jobs.

Absolutely.

Three main functional classes.

First, afferent neurons.

Think A for arriving.

They carry information from your body's tissues and organs towards the CNS.

They have sensory receptors at their ends.

So like feeling heat on your skin.

Exactly.

Then you have effort neurons.

Think E for exiting.

They carry commands away from the CNS to affect your cells, muscles, or glands, telling them what to do.

Contract a muscle, secrete a hormone.

Right.

And the vast majority, like over 99%, are interneurons.

They live entirely within the CNS.

They connect other neurons, forming complex circuits.

They're the processors, the integrators.

Think simple reflex versus complex memory.

The difference is often the number of interneurons involved.

Connections seem key.

Let's talk about those junction synapses.

Synapses, yes.

The specialized points where one neuron influences another.

The sender is the presynaptic neuron.

The receiver is the postsynaptic.

Neurons can get input from many others.

Oh, definitely.

That's convergence.

Many neurons talking to one allows integration of lots of information.

And the opposite is divergence, one neuron talking to many others, spreading its signal widely.

So it's a really complex web of communication.

Incredibly complex.

Now, neurons get the spotlight, but you mentioned other cells are important too.

The support crew.

Can't forget the glial cells.

They actually outnumber neurons in the CNS.

They provide physical support, metabolic help, just crucial allies.

I mentioned oligodendrocytes and Schwann cells making myelin.

But there are others too.

Astrocytes are like the Swiss army knives of glial cells in the CNS.

They regulate the chemical environment, help form that protective blood -brain barrier, provide nutrients, guide neuron growth, maybe even signal themselves.

Wow.

What else?

Microglia are the immune defenders, like macrophages cleaning up debris.

And ependymal cells line the fluid -filled spaces in the brain, helping manage the cerebrospinal fluid.

That's quite a team supporting the neurons.

What about how this system develops and changes?

It's not static, right?

Not at all.

It starts with stem cells differentiating, migrating, sending out processes.

The tip of a growing axon, the growth cone, is guided by chemical cues.

But it's a vulnerable time.

Things like alcohol or viruses can really disrupt development.

And I read something about programmed cell death, apoptosis.

Yeah, it's fascinating.

A huge percentage, maybe 50 -70 percent, of developing neurons actually self -destruct through apoptosis.

The thinking is it's a way to prune connections, refine the wiring for efficiency,

maybe even why we don't remember early childhood clearly.

Pruning the network.

And this ability to change continues throughout life.

Plasticity.

Exactly.

Plasticity is the brain's lifelong ability to rewire itself based on experience or even injury.

Remodeling synapses, sometimes even generating new neurons in certain areas.

It's amazing.

But what about major injuries, like severing nerves?

Can they repair?

It depends.

In the PNS, peripheral nerves can often regenerate if the cell body is intact.

Axons regrow, slowly, about a millimeter a day.

But in the CNS, brain and spinal cord regeneration is much, much more limited.

Why the difference?

It's complex.

Oligodendrocytes often die after injury, and CNS allele release factors that actually inhibit axon regrowth.

It might be a protective mechanism to prevent faulty connections in the incredibly complex CNS circuitry, but it makes recovery from spinal cord injury, for example, very difficult.

Lots of research trying to overcome that.

Okay, that gives us the players and the structure.

Now let's dive into the language they speak electricity.

How do neurons use electrical signals?

It boils down to basic physics.

Opposite charges attract, like charges repel.

If you separate charges across a barrier, like a cell membrane, you create an electrical potential of voltage, measured in millivolts, usually.

And the movement of charge is current.

Right.

Current.

And resistance is anything hindering that movement.

In cells, the fluids inside and out are good conductors, because they're full of ions charged particles.

But the cell membrane itself, the lipid bilayer, is a great insulator, high resistance.

So neurons exploit this separation of charge.

Precisely.

Even when resting, a neuron has a resting membrane potential.

The inside is negative compared to the outside, usually around, and it gets 40 to 90 millivolts, let's say, negative 70 milliliter for a typical value.

Why negative inside?

It's due to an unequal distribution of ions.

Lots of sodium, necklace, and chloride, Cl, outside.

Lots of potassium, K +, and large negatively charged anions, like proteins, inside.

And something maintains this imbalance.

Yes.

The sodium -potassium ATPase pump.

It's constantly working, pumping 3 Na plus ions out for every 2K plus ions it pumps in.

It actively maintains those concentration gradients across the membrane.

Absolutely vital.

So the pump sets up the gradients.

What else determines that resting negative 70 millivy value?

The other key factor is membrane permeability to those ions.

The membrane isn't equally leaky to everything.

At rest, it's much, much more permeable to potassium than to sodium.

Maybe 50 to 100 times more.

Why?

Because there are more open leak channels for potassium.

So potassium tends to leak out down its concentration gradient, taking positive charge with it, leaving the inside more negative.

Sodium leaks in a bit, but much less.

So the resting potential is close to potassium's ideal potential.

Exactly.

It's close to the equilibrium potential for potassium, EK, which is around managed 90 millivy.

Sodium's equilibrium potential, ENA, is way up at plus 60 millivy.

The resting potential sits closer to EK because of that higher K plus permeability.

The pump's direct contribution, 3 Na plus out for 2K plus in, adds a little bit of negativity, too.

But its main job is maintaining the gradients that the ion leaks rely on.

So it's a dynamic balance.

Leaks are balanced by the pump.

A steady state, yeah.

Gradient stable, potential stable, until something changes.

Right, the signals.

How do neurons change this potential?

By changing ion permeability.

They open or close specific gated ion channels.

These can be opened by chemicals, ligand gated, physical force, mechanically gated, or crucially by changes in voltage itself, voltage gated.

These changes create signals,

graded potentials, and action potentials.

Those are the two main types.

We talk about changes like depolarization becoming less negative, moving towards zero, or even positive.

Repolarization is returning to rest, and hyperpolarization is becoming more negative than rest.

Let's tackle graded potentials first.

Okay.

Graded potentials are small, localized changes, like ripples.

Their size or amplitude is graded.

It depends on the stimulus strength.

Bigger stimulus, bigger potential change.

But they don't travel far.

Nope, they're decremental.

They fizzle out over short distances, just a few millimeters, because charge leaks out across the membrane.

Think of them as short range signals.

But, importantly, they can add up.

They summate.

Add up?

How?

If multiple graded potentials happen close together in time or space, their effects combine.

This summation is really important for triggering the other type of signal.

Which is the action potential, the long distance runner.

Exactly.

Action potentials are totally different.

They're large, like a 100 millivolt swing, very rapid, and crucially, all or none.

All or none?

Meaning once you trigger one, it fires completely.

Always the same size.

It doesn't get weaker as it travels.

It's non -decremental.

It's the signal for long distance communication down the axon.

What makes them happen?

Which channels are involved?

The stars here are the voltage gated sodium channels and the voltage gated potassium channels.

The sodium channels open really fast when the membrane depolarizes to a certain point, and they also have an inactivation gate that closes them soon after.

The potassium channels are slower to open and close.

Walk me through the steps of an action potential firing.

Sure.

Starts at resting potential, say negative 70 millivie.

A stimulus, maybe some graded potentials, causes depolarization.

If it reaches the threshold potential, maybe 55 millivie.

That's the trigger point.

Yes.

That threshold, boom.

Voltage gated sodium channels fly open.

Sodium ions rush in down their electrochemical gradient.

This influx of positive charge causes rapid depolarization.

Positive feedback.

Totally.

More depolarization opens more sodium channels, which causes more depolarization.

The membrane potential shoots up, even becomes positive inside, that's the overshoot, approaching sodium's equilibrium potential.

What stops it?

Two things happen near the peak.

The fast inactivation gates on the sodium channels slam shut, stopping sodium influx.

And the slower voltage gated potassium channels finally open fully.

So potassium flows out.

Right.

Potassium ions rush out down their gradient, carrying positive charge out of the cell.

This rapidly brings the membrane potential back down repolarization.

Does it overshoot on the way down, too?

Often, yeah.

Those potassium channels close slowly, so for a brief period, potassium permeability is even higher than at rest.

This can cause an after hyperpolarization, where the membrane dips more negative than the resting potential, closer to potassium's equilibrium potential.

And then everything resets.

Then the voltage gated potassium channels close, and the membrane returns to its resting potential, ready for the next signal.

The NA plus K plus pump cleans up the tiny ion shifts over the long run, maintaining the gradients.

So the all or none means stimulus intensity isn't coded by the size of the action potential?

Correct.

It's coded by the frequency how many action potentials fire per second, and the pattern of firing, like Morse code.

A stronger stimulus causes more frequent firing.

And this mechanism is what local anesthetics target.

Exactly.

Drugs like a lidocaine block those voltage gated sodium channels.

No sodium influx, no action potential, no pain signal transmission.

You mentioned the neuron needs time to reset.

What are refractory periods?

Crucial concept.

Right after an action potential, there's the absolute refractory period.

During this time, the sodium channels are either already open or inactivated, so it's impossible to fire another action potential, no matter how strong the stimulus.

What's the point of that?

It ensures the action potential travels in one direction, down the axon, away from the cell body, and limits the maximum firing rate.

Is there another period?

Yes.

Following that is the relative refractory period.

Here, some sodium channels have reset, but the voltage gated potassium channels might still be open, causing hyperpolarization.

So you can fire another action potential, but you need a stronger than usual stimulus to reach threshold.

This also helps regulate firing frequency.

Okay, so how does this signal actually travel down the axon without weakening propagation?

Right, action potential propagation.

When one patch of membrane fires an action potential, the influx of positive charge creates local currents that spread sideways and depolarize the adjacent patch of membrane to its threshold.

So each action potential triggers the next one down the line.

Exactly, like a wave or that burning trail of gunpowder analogy.

It regenerates itself at each point, so it doesn't decrease in amplitude.

It's non -decremental.

In speed,

does axon size matter?

Myelin.

Both matter a lot for velocity of propagation.

Wider axons have less internal resistance, so current flows more easily, making conduction faster, and myelination makes a huge difference.

How does myelin speed it up?

The nodes.

Yes, the nodes of Ranvier.

In myelinated axons, the action potential doesn't happen along the whole membrane.

It essentially jumps from one node to the next.

The myelin insulates the segments between nodes, forcing the current to flow quickly down the axon interior to the next node, where it triggers another action potential.

This is saltatory conduction.

Saltatory, like leaping.

From the Latin saltare to leap.

It's much faster and more energy efficient than continuous conduction in unmyelinated axons.

That's why our motor commands and sharp pain signals travel so quickly.

Okay, fantastic.

We've got the signal traveling.

Now, how does it pass from one neuron to the next?

Back to synapses.

Back to synapses.

The communication hubs.

Remember, they can be excitatory, making the next cell more likely to fire, or inhibitory, making it less likely.

And you said there are two main types, electrical and chemical.

Right.

Electrical synapses use gap junctions.

They physically connect the cytoplasm of two cells, allowing ions to flow directly.

Very fast, good for synchronizing cells.

But chemical synapses are the vast majority.

Remind me how those work again.

Presynaptic terminal has synaptic vesicles filled with neurotransmitter.

There's a gap, the synaptic cleft.

Post -synaptic membrane has receptors.

No direct connection.

This allows for much more complex signal processing.

So how does the neurotransmitter get released?

What's the trigger?

Calcium.

When the action potential invades the presynaptic terminal, it opens voltage -gated calcium channels.

Calcium ions, say 2 +, rush into the terminal.

And calcium does what?

Calcium influx is the key signal.

It triggers a cascade involving specific proteins like snares that causes the synaptic vesicles docked at the membrane to fuse with it and release their neurotransmitter content into the synaptic cleft via exocytosis.

And once it's in the cleft?

The neurotransmitter diffuses across that tiny gap and binds briefly and reversibly to receptors on the post -synaptic membrane.

What kinds of receptors?

Two main families.

Ionotropic receptors are themselves ion channels.

Binding opens the channel directly fast effect.

Metapetropic receptors are usually linked to G proteins and intracellular signaling pathways slower, more modulatory effects.

And the signal needs to stop, right?

How is the neurotransmitter cleared?

Crucial step.

It has to be removed quickly.

Either by enzymes in the cleft breaking it down like acetylcholinesterase for AC, or by being actively pumped back into the presynaptic terminal reuptake or into nearby glial cells or simply diffusing away.

Okay, let's link this to excitation and inhibition.

What happens post -synaptically at an excitatory synapse?

At an excitatory chemical synapse, the neurotransmitter typically binds to receptors that open channels permeable to both sodium and potassium.

But because of the electrochemical gradients, more sodium flows in than potassium flows out.

Net effect.

Depolarization.

A small depolarization.

Usually, yes.

A small graded depolarization called an excitatory post -synaptic potential, or EPSP.

Its job is to bring the post -synaptic neuron closer to its threshold for firing an action potential.

And at an inhibitory synapse.

There, the neurotransmitter opens channels usually for either chloride, Cl, or potassium K+.

If chloride channels open, Cl usually flows in, making it more negative or stabilizes the potential near rest.

If potassium channels open, K -plus flows out, also making it more negative.

So the effect is?

The effect is either hyperpolarization or stabilization, making the inside more negative or harder to depolarize.

This graded potential is called an inhibitory post -synaptic potential, or IPSP.

It moves the neuron further away from threshold.

One EPSP probably isn't enough to make a neuron fire, though.

Usually not.

An EPSP might only be half a millivolt, and you might need 15 or 20 millivade of depolarization to reach threshold.

That's where synaptic integration and summation come in.

Ah, the adding up of signals.

Exactly.

Temporal summation is when a single pre -synaptic neuron fires repeatedly in quick succession.

The EPSPs arrive close together and add up over time.

And spatial summation.

Spatial summation is when multiple different pre -synaptic neurons fire at roughly the same time.

Their EPSPs arrive at different spots on the post -synaptic neuron, but add up spatially.

Of course, IPSPs can submit too, and they can cancel out EPSPs.

Where does the neuron decide whether to fire?

The axon hillock, that initial segment of the axon, is usually the decision point.

It has the highest density of voltage -gated sodium channels, and thus the lowest threshold.

So the net sum of all the EPSPs and IPSPs arriving there determines if threshold is reached and an action potential is generated.

Location matters too.

A synapse closer to the hillock has more influence.

Is the strength of a synapse always the same?

Oh, definitely not.

Synaptic strength is highly variable, highly plastic.

It can change based on activity.

How?

What changes it?

Lots of ways.

Presynaptically, the amount of neurotransmitter released can change.

For instance, if a terminal fires rapidly, residual calcium might build up, boosting subsequent release.

Are there other inputs that affect release?

Yes, through axonic synapses.

One axon terminal can synapse directly onto another axon terminal.

This can either inhibit neurotransmitter release, presynaptic inhibition, or enhance it, presynaptic facilitation.

It's a way to selectively fine -tune specific inputs without affecting the whole neuron.

Also, autoreceptors on the presynaptic terminal can bind the neuron's own neurotransmitter, usually acting as a negative feedback break.

What about changes on the postsynaptic side?

The postsynaptic cell can change its sensitivity.

It might make more or fewer receptors, up or down regulation.

Receptors can also become temporarily unresponsive, even if neurotransmitter is still present.

Desensitization.

And this variability is where drugs and diseases often act?

Very often.

They might mimic a neurotransmitter, agonists, block a receptor, antagonists, block reuptake, or interfere with release.

Like tetanus toxin, it destroys snare proteins needed for release at inhibitory synapses in the CNS.

Result, uncontrolled muscle contractions, spastic paralysis.

And Botox.

Botulinum toxin does something similar, destroying snares, but at excitatory synapses, controlling skeletal muscles.

Result, flaccid paralysis, muscles can't contract.

That's why it's used therapeutically to relax specific muscles.

Incredible how these tiny molecular mechanisms have such dramatic effects.

Let's talk about the messengers themselves.

What are the main types?

Huge diversity.

We distinguish between neurotransmitters, which usually cause fast, direct effects like EPSPs or IPSPs, and neuromodulators, which often act more slowly, maybe through G proteins, having longer lasting effects, tuning the neuron's overall responsiveness.

But the line can be blurry.

Many substances do both.

Let's run through some key examples.

Acetylcholine.

Classic one.

Crucial in the PNS at the neuromuscular junction, also important in the brain for things like attention and memory.

Broken down by acetylcholinesterous, ACE,

binds to nicotinic receptors, fast, excitatory, targeted by nicotine, and muscarinic receptors, slower, metabotropic.

Degeneration of AC neurons is linked to Alzheimer's.

What about the biogenic amines?

Dopamine, norepinephrine, serotonin.

Yes, catecholamines, dopamine, norepinephrine, and serotonin.

Synthesized from amino acids.

Huge roles as neuromodulators in mood, motivation, alertness, sleep, reward, you name it.

Removed by reuptake and broken down by MAO.

Many antidepressants, like SSRIs, selective serotonin reuptake inhibitors, target these systems.

And the most common neurotransmitters in the CNS are amino acids.

By far.

Glutamate is the primary excitatory neurotransmitter.

Axon receptors like AMP and NMDA.

Their interaction is vital for long -term potentiation.

LTP.

A key mechanism thought to underlie learning and memory.

But too much glutamate can be toxic exciter toxicity.

And the main inhibitory ones.

GABA, gamma -aminobutyric acid, is the major inhibitory player in the brain.

Glycine is key in the spinal cord and brainstem.

Both typically open chloride channels, causing inhibition.

IPSPs.

Drugs like Valium or Xanax and alcohol enhance GABA's effects.

Strychnine poison blocks glycine receptors causing convulsions.

Are there other classes beyond these?

Oh yes.

Neuropeptides like endorphins, our endogenous opioids, are larger molecules, often acting as neuromodulators with longer -lasting effects, involved in pain relief.

Even gases like nitric oxide act as signals, diffusing directly.

Purions like ATP.

And lipids like endocannabinoids, which our own body makes and are targeted by THC from marijuana.

It's a really diverse chemical language.

And neurons talk to muscles and glands too, right?

Neuro -effector communication.

Yeah, similar principles.

A neuron releases a neurotransmitter, like HE or norepinephrine, near an effector cell, muscle or gland, which has receptors for it, causing a response.

OK, amazing detail at the cellular level.

Let's zoom out now for the final segment, the overall structure of the nervous system.

Right.

The big picture anatomy.

Need some terms first.

In the PNS, a bundle of axons is a nerve.

In the CNS, it's a pathway or tract.

Cell bodies clustered together are ganglia in the PNS, nuclei in the CNS.

And the brain itself, major divisions.

Develops from three embryonic parts, forebrain, midbrain, hindbrain.

The forebrain becomes the cerebrum and deencephalon.

The hindbrain becomes the pons, medulla oblongata, and cerebellum.

The midbrain stays the midbrain.

Pons, medulla, and midbrain together make up the brainstem.

Let's start with the cerebrum, the biggest part.

Dominated by the two cerebral hemispheres.

The outer layer is the cerebral cortex that folded gray matter.

This is where higher functions happen.

Perception, voluntary movement, reasoning, learning, memory.

The folding vastly increases the surface area.

Deep inside are clusters of gray matter.

The subcortical nuclei, including the basal nuclei, crucial for movement control.

And the limbic system, a functional network involved in emotion, learning, and motivation.

What about the deencephalon also in the forebrain?

Contains the thalamus.

Think of it as a major relay station and filter for sensory info going to the cortex.

Helps you focus.

Below it is the hypothalamus, the absolute master command center for homeostasis.

Regulates temperature, hunger, thirst, controls the pituitary gland.

Incredibly important.

Moving back, the cerebellum.

The cerebellum tucked under the back of the cerebrum.

Its main job is coordinating movement, ensuring posture and balance are smooth and precise.

Also involved in some types of learning.

And the brainstem.

Right.

Midbrain, pons, medulla.

The vital conduit.

All nerve tracks between the forebrain, cerebellum, and spinal cord pass through here.

It contains nuclei for most cranial nerves and centers, controlling essential functions like breathing, heart rate, swallowing.

Plus, the reticular formation runs through it, critical for arousal, wakefulness, and attention.

Damage here can be catastrophic.

Finally, the spinal cord.

The spinal cord, protected by the vertebrae.

Central H, or butterfly shape, is gray matter interneurons, motor neuron cell bodies, initial synapses for sensory input.

Surrounding it is white matter myelinated axons, forming ascending sensory tracks going up to the brain,

and descending motor tracks coming down from the brain.

The information highway.

So you see we've gone from the tiny details of ion channels and neurotransmitters.

Right, the absolute basics.

All the way up to the major divisions of the brain and spinal cord.

We saw how neurons generate electrical signals, graded potentials locally,

action potentials for distance, and how they communicate chemically at synapses.

With all the possibilities for modulation and integration,

it's this intricate interplay, from the molecular to the macroscopic, that allows the nervous system to control and coordinate everything the body does, maintaining that essential ballast, homeostasis.

It truly is incredible.

And that brings us to a final thought for you, our curious learner.

We've explored this stunningly precise machinery of electrical and chemical signaling.

How might these fundamental processes give rise to something as complex and frankly mysterious as consciousness?

Or creativity?

Or dreams?

What other emergent properties are waiting to be understood once we fully grasp these building blocks?

Something to ponder.

Thank you for joining us on this deep dive.

Yes, thank you for listening.

From the whole Last Minute Lecture team, we appreciate you tuning in.