Chapter 12: Physiology of Neurons

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Have you ever just stopped and thought about how fast your brain works?

I mean, the speed from seeing something to reacting or even just thinking a thought.

It's incredible, how does it happen?

Well, today we're diving deep into that.

We're tackling Chapter 12 from Boron and Bullpapes, Medical Physiology.

The focus, neurons, the very cells doing all that work.

Our goal here is to break this down for you.

Make these really complex ideas about neurons feel understandable, accessible.

Whether you're meeting the stuff for the first time in college or need that solid foundation for med school, we want to give you that aha moment.

Think of this as your confidence booster for understanding the brain's wiring.

Exactly.

Neurons, they really are something else.

Probably the most complex cells in the body, honestly.

Each one is like a tiny, super sophisticated computer.

A computer?

How so?

Well, imagine one single neuron getting chemical messages from maybe tens of thousands of other neurons.

It takes all that in, processes it, transforms it, and then sends out a completely new message.

It's just amazing how they combine information, change it, store it sometimes for years like memories, and then pass it on.

Okay, let's zoom in then.

One neuron.

How does the information actually flow?

It's not just random, is it?

Not at all.

It's very directional.

Think of a neuron like a tree.

You've got the dendrites, these extensive branches reaching out to collect signals.

They're the main input sites.

Like roots gathering water.

Kind of, yeah.

Good analogy.

All that information flows towards the center, the soma, or the cell body.

Then, from the soma, one main trunk emerges.

That's the axon.

That axon branches out again, multiple times, ending at these little presynaptic terminals.

That's where the neuron talks to the next cell.

Dendrites in, soma processes, axon out.

That's the typical flow, yeah.

Although, you can get synapses directly on the soma, or even the axon sometimes.

For sensory neurons, the dendrites themselves can act as transducers, turning things like light or touch directly into electrical signals.

But generally, yes.

The signal is a change in voltage across the cell membrane, and it moves.

Dendrites, soma, axon, synapse.

Okay, voltage changes.

So, when these chemical messages arrive at the dendrites, how do they become electrical?

What's happening there?

Right.

That's the crucial first step.

These initial changes are called postsynaptic potentials, or PSPs.

If the input is excitatory, it causes positive ions like sodium to flow into the cell.

Since the inside of the neuron is normally negative, this positive influx makes the inside less negative, more positive.

We call that depolarization.

And the potential itself is an excitatory postsynaptic potential, an EPSP.

EPSP.

Makes sense.

Excites it towards firing, maybe.

Exactly.

Now, if the input is inhibitory, it usually does the opposite.

It might let positive ions flow out or negative ions flow in.

Either way, it makes the inside more negative.

That's hyperpolarization.

And that would be an IPSP.

You got it.

An inhibitory postsynaptic potential.

Just quickly, for sensory neurons, things like light or chemicals directly cause some more voltage changes called receptor potentials.

Now, here's a really key point about all these PSPs and receptor potentials.

They are graded responses.

Graded, meaning?

Meaning their size, how much the voltage changes and how long they last depends directly on how the input was.

More neurotransmitter or a stronger stimulus gives you a bigger, longer potential.

Less input, smaller potential.

Okay.

So it's not just on or off.

It's like a volume control.

Precisely.

It's a fundamental type of neural coding.

The neuron encodes the strength and duration of the input signal right there in the size and shape of that initial voltage change in the dendrite.

Like a whisper whose volume reflects the importance of the message.

So these whispers start out in the dendrites, but you said the axon can be really long.

How do these whispers travel all the way to the soma without just fading out completely?

That is a huge challenge for the neuron.

And you're right, they do fade.

These synaptic potentials travel towards the soma, but they undergo substantial attenuation.

They get weaker.

Attenuation, like they diminish.

Exactly.

Think of a dendrite, like a leaky garden hose.

You put pressure on one end, but water leaks out all along its length, so the pressure drops the further you go.

Dendrites leak electrical current through ion channels in their membrane.

So the signal gets weaker with distance.

Yep.

How much weaker depends on things like how leaky the membrane is, the resistance inside the dendrite, and even the dendrite's diameter.

Thicker dendrites carry signals farther, generally.

And there's another interesting thing.

Dendrites act like low -pass filters.

Rapidly changing signals, high frequency inputs get attenuated more than slow, steady ones.

Huh.

Okay, so distance and speed are both working against the signal.

How does Neuron solve this?

Two main ways.

First, just passive properties.

Sometimes dendrites are short enough that the signal doesn't have that cardigo before it reaches the soma, minimizing the decay.

But the more dynamic solution involves their active properties.

Dendrites aren't just passive cables.

They actually have voltage -gated ion channels embedded in their membranes.

Voltage -gated channels, like the ones involved in action potentials?

Sort of, yeah.

But usually at a much lower density in most dendrites.

We'll get to the big action potentials in a second.

Okay, because that's what I'm wondering.

If these signals start small and fade, how on earth do you get a signal from your brain down to your foot?

That's a meter.

A passive whisper won't cut it.

Absolutely not.

And that's where the Neuron makes a critical switch.

If enough of those attenuated EPSPs arrive at the soma around the same time, and they sum up, if that combined voltage change is large enough to reach a specific threshold voltage, then what?

Then it triggers an action potential, or maybe a whole train of them.

Action potentials, or APs, are completely different.

They're large, rapid swings in voltage.

And crucially, they're all or nothing.

All or nothing.

Meaning they have a fixed size and shape.

They don't get weaker with distance.

Once triggered, an AP travels down the axon like a wave, regenerating itself along the way.

It's the Neuron's way of shouting the message reliably over long distances.

Wow.

So it converts the whisper into a shout.

Beautifully put.

The Neuron transforms the graded voltage code of the dendrites,

the varying sizes of PSPs, into a temporal code of action potentials in the axon.

Temporal code?

You mean timing?

Exactly.

The information isn't in the size of the AP, because they're all the same size.

It's in how many APs are fired, how fast they're fired, or the specific pattern of firing over time.

More input generally means faster firing.

Okay, that makes sense for long distance, but what about those voltage gated channels you mentioned in the dendrites?

What do they do if they don't usually fire full APs?

Ah, good question.

Those dendritic voltage gated channels, usually sodium or calcium channels, act like little boosters.

They don't typically initiate a full AP themselves because their density is too low, but as that graded synaptic potential travels past them, they open and add a little extra inward current.

So they help counteract the leakiness?

Precisely.

They amplify or boost the signal, making it fall off less steeply as it travels towards the soma.

They help the whisper carry a bit further.

Now some neurons, like the amazing Purkinje cells in the cerebellum, actually do have enough voltage gated calcium channels in their dendrites to fire slower, large dendritic calcium spikes.

These can be big enough to then trigger the regular fast sodium -based APs down to the soma and axon.

So some dendrites can shout, in a way.

In a way, yes, a slower calcium shout.

But interestingly, the fast sodium APs from the soma don't travel very far back into the dendrites.

Remember that low -pass filter effect.

The rapid frequency of sodium APs gets attenuated quickly, going backward into the dendrites.

Right.

It seems like there's an advantage to the dendrites not being too easily excitable, maybe, so they can actually sum up all those inputs.

That's a key insight.

If dendrites fired full APs easily, one strong input might max out the neuron's response right away.

By being only weakly excitable, they can integrate thousands of inputs, maintaining a wider dynamic range before deciding whether to fire the axon.

Okay, so we have this process.

Grated inputs summed up, potentially boosted, then maybe triggering all -or -nothing action potentials at the axon if threshold is reached.

But if all APs are the same size, how do different neurons create such different outputs, different behaviors?

That's where the soma and the beginning of the axon, the axon hillock, really show their personality.

The specific mix of different ion channels located there determines the neuron's characteristic firing pattern.

Firing pattern.

Yeah, how it responds to a steady ongoing input.

Some neurons, you give them a stimulus, and they just fire a rapid, regular train of APs for as long as the stimulus lasts.

We call them non -adapting.

Okay, steady output.

Others fire quickly at first, but then they slow down.

Even if the input stays strong, that's adapting.

Gets tired, sort of.

Sort of, yeah.

And then there are bursting neurons.

These might fire a quick burst of several APs, then stop for a bit, then maybe burst again, rhythmically.

Wow, so the same input can lead to very different output rhythms.

Absolutely.

And these patterns are intrinsic properties.

It's not just the input dictating the pattern, it's the neuron's own built -in machinery, its unique collection of ion channels, especially channels with slower kinetics than the super -fast ones making the AP spike itself.

Each type of neuron has its own signature tune.

That sounds incredibly important for function.

It is.

Think about it.

Rhythmically bursting cells are perfect for jobs like controlling breathing or walking.

You need that regular pattern.

Or certain hormone -releasing neurons in the hypothalamus.

They often burst because bursts cause a bigger buildup of internal calcium, which is needed to trigger hormone release more effectively than single spaced out spikes.

It really is.

And bursting helps synchronize brain activity during things like sleep.

You even see it in hearing different neurons in the auditory pathway respond to the same sound input with totally different firing patterns, allowing the brain to extract different features like timing versus loudness.

Okay, so the neuron computes the input, decides whether to fire, shapes the firing pattern.

Now we need to send that pattern down the axon.

You said it's like a high -speed highway.

Exactly.

The axon's job is rapid, reliable, efficient transmission.

It might seem less glamorous than the computation, but it's critical,

especially with myelin.

Ah, yes, the insulation.

Right.

Myelin allows for saltatory conduction where the AP effectively jumps between gaps in the myelin called nodes of Ranvier.

This makes conduction vastly faster.

How much faster?

Orders of magnitude faster.

Think about that signal from your foot to your spinal cord.

Maybe takes 20, 30 milliseconds.

Without myelin, it would take vastly longer, maybe hundreds of milliseconds or more.

Our large brains little depend on myelin for speed and also for saving space and energy.

Where does the AP actually start?

Not in the soma.

Usually it kicks off at the axon initial segment, right where the axon leaves the soma.

This tiny region is special.

It has an incredibly high density of voltage -gated sodium channels, much higher than anywhere else.

So it's like the trigger zone, easiest place to start the AP.

Precisely.

It has the lowest threshold.

Once it fires there, the AP travels orthodromically forward down the axon to the terminals.

Orthodromically.

Okay.

But interestingly, it also propagates backward into the soma and even a little way into the dendrites.

This backward signal is weaker, attenuated, but it might play roles in things like learning and memory by modifying synaptic strength.

Fascinating.

And you mentioned axon diameter matters too.

It does.

Larger diameter means faster conduction, both for myelinated and unmyelinated axons.

But myelin is the big winner for speed and efficiency.

A thin myelinated axon can be as fast as a giant unmyelinated axon, like the squid giant axon famous in research, but takes up way less space.

So we use myelinated axons for most things.

For anything requiring speed over distance, yes.

The really thick fast ones are used for crucial things like muscle stretch reflexes or fine motor control.

Thinner axons, especially unmyelinated ones called C fibers, are used for slower signals like chronic pain or temperature, where millimeter per second differences aren't as critical.

Got it.

So the axon is this optimized cable, but what happens if that optimization breaks down if the myelin, the insulation gets damaged?

Yeah, that leads to serious problems.

We're talking about demyelinating diseases, conditions where the myelin sheath is attacked and destroyed, leaving the axon underneath exposed.

Like stripping the insulation off a wire.

Exactly.

And the consequences for signal conduction are severe.

Remember how the AP jumps between nodes?

In a healthy axon, the current generated at one node is easily strong enough to trigger the next node.

But when the axon is demyelinated between nodes, that same current now has to spread out over a much longer section of membrane that's leaky and it has high capacitance.

It just dissipates.

So the signal struggles.

It struggles badly.

You get slow conduction velocity.

Signals take longer.

You can get frequency related block, meaning the axon can't keep up with rapid firing.

The signal might just drop out.

Oh.

In worst case, you get total conduction block.

The signal just stops.

Dead end.

This block is often the direct cause of the symptoms people experience.

Like weakness or numbness.

Exactly.

Plus demyelinated bits of axon can become irritable, firing spontaneously ectopic impulses or becoming sensitive to touch or pressure.

Sometimes signals can even jump inappropriately between adjacent demyelinated axons, causing crosstalk.

That sounds like chaos.

What's a major example of this?

The most well known in the central nervous system is multiple sclerosis, or MS.

It's an autoimmune disease where the body's own immune system attacks myelin, or the cells that make myelin in the brain and spinal cord.

Autoimmune.

So the body attacks itself.

Yes.

We don't know the exact trigger, maybe a virus in susceptible people.

It leads to patches of demyelination, causing symptoms like optic neuritis, vision problems, double vision, pins and needles, weakness, fatigue, and that strange lermot sign and electric shock feeling down the spine when bending the neck.

And it comes and goes, relapses and remissions.

Often, yes.

The relapses are periods of active inflammation and demyelination causing symptoms.

Remissions happen when the inflammation calms down, and sometimes the axons can recover some function, even without full remyelination, maybe by inserting different types of sodium channels.

You mentioned temperature sensitivity earlier.

Ah, yes.

The temperature sensitivity in MS is fascinating.

Conduction through those partially demyelinated segments is precarious.

It turns out that lower temperatures actually help.

Lower temps help.

How?

Lowering the temperature slows down the opening and closing of the sodium channels.

This makes the action potential slightly longer, wider.

That wider pulse of current has a better chance of successfully triggering the next bit of axon, overcoming the leakiness and capacitance of the demyelinated part.

So cooling down can actually improve symptoms temporarily.

It can.

Conversely, getting hotter, like with a fever, exercise, or even a hot bath, speeds up the channels, narrows the AP, and makes conduction failure more likely.

That's why heat can worsen MS symptoms transiently.

It's called Uthoff's phenomenon.

Wow.

That's a direct link between physiology and experience.

What about the peripheral nerves?

The main demyelinating disease there is Guillain -Barre syndrome, GBS.

It often causes ascending paralysis, starting in the legs.

But the key difference from MS is that the peripheral nervous system can remyelinate much better than the central nervous system.

So people with GBS often recover.

Thankfully, yes.

Most people with GBS eventually make a good, often complete, recovery because their peripheral nerves can repair the myelin damage over time.

That's a much better outlook than for CNS, demyelination, and MS.

So what a journey.

From these tiny graded potentials and dendrites, through amplification and threshold decisions, the shaping of firing patterns, the high -speed chase down the axon, and then what happens when that vital myelin insulation fails?

It really covers the neuron's whole electrical life story, doesn't it?

The key things to take away are that incredible integration of inputs, the conversion from graded signals to timed action potentials, the specialized conduction, and the intrinsic ability of each neuron to shape its output.

Understanding these fundamentals really is the bedrock for figuring out how neural circuits work and what's going wrong in diseases like MS or GBS.

Absolutely.

Your brain, your nervous system, it's this amazing electrical network.

And getting a handle on how these individual cells operate, well, it empowers you to understand health, disease, and just the incredible complexity of yourself.

You've just taken another deep dive, absorbed a lot of complex information, and you should feel really good about that.

Keep digging.

Keep asking questions.

Remember, you're part of the deep dive family, and you absolutely have what it takes to master this stuff.