Brain Matters: Lower Brain Structures (2 of 5)


Uploaded by ltoddrose on 26.07.2011

Transcript:
There are many ways do discuss the organization of the brain. In this chapter we will first
situate the brain as part of the broader nervous system. Secondly, we will discuss one way
of grouping brain structures into four primary areas that will be useful for this chapter
and subsequent chapters. From the beginning it’s important to point
out that the brain does not exist in a vacuum. This is often the way it comes across when
you read articles, or popular press, or even textbooks; they can give the impression that
the brain is doing certain things and leaves out the idea that’s very important which
is that the brain is part of a broader system, in this case the nervous system.
The nervous system can be grouped into two broad categories; the first is the central
nervous system, which consists of the brain and the spinal cord. This central nervous
system acts as a command and control center for the body. In contrast, the second part
of the nervous system is called the peripheral nervous system. The peripheral nervous system
consists of a vast network of cranial and spinal nerves and its function is to link
the central nervous system with tissue and organs and other parts of the body. Our focus
today is exclusively on the central nervous system and in particular on the brain.
As I mentioned earlier there are many ways to discuss the organization of the brain.
One common way that neuroscientists use is to group the brain into four broad areas:
the brain stem, which is largely involved in automatic functions, the cerebellum, which
is involved in among other things, motor control, fine motor movement, balance, posture, the
diencephalon, which is primarily involved in regulatory functions, and the cerebrum,
which is involved in high level function like language, thought, perception. In this chapter
we’ll discuss the first three, the brain stem, the cerebellum, and the diencephalon.
Let’s begin with the brain stem, which is essentially where the spinal cord ends and
the brain formally begins. Now the brain stem technically consists of several structures,
like the medulla oblongata, the ponds, and the midbrain, however, we’ll talk about
it as one unified structure here. The brain stem serves many functions, some of them vital,
like regulating blood pressure, heart rate, and breathing. Now you can think about that
for a moment, of those three, breathing you do have some voluntary control over. So for
example, until I brought it up, your brain stem was controlling your breathing, but now
that I’ve mentioned it, you’ve had to exert some control over it. So you’re mind
is conciously saying ‘breathe in, breathe out, breathe in’. Now, try as you might,
you can’t seem to force yourself to stop thinking about that breathing, it’s kind
of annoying, so you’re welcome. But don’t worry, as you get sucked back in and pay attention
to me instead of your breathing your brain stem will take over. It’s probably a good
thing that you don’t have that kind of voluntary control over the other two, blood pressure
and heart rate, because there’s very little upside to control, sort of slow down your
heart rate, speed it up, but there is a lot of downside to say forgetting to beat your
heart. So your brain stem takes care of that, gives you some control over breathing.
In addition to those vital functions, the brain stem also regulates things like coughing,
sneezing, vomiting and overall arousal level particularly as it relates to the sleep wake
cycle. That last piece, arousal level, you probably have some experience with whether
you realize it or not. Let me paint a picture for you: it’s nighttime, you’re laying
down in bed, you’re about to go to sleep, you feel your body relaxing, and then all
of a sudden, you jerk, and it kind of scares you. And you wonder, ‘is something wrong
with me?’ well the short answer is ‘probably’ but it has nothing to do with this piece.
This is a common occurrence and it’s called the hypnic jerk, and it comes about because
of the brain stem. What it is is this: part of going to sleep involves relaxing your muscles,
and the brain stem deals with that. But it’s not really a gradual sort of relaxing of the
muscles, it’s more like a light switch. And as the brain stem triggers that, as also
it perceives its own response, the muscle’s response to going relaxed, as you’re falling,
and it corrects itself by going rigid. Now that happens every night, it’s just usually
you’re not aware of it because you’re already sort of not conscious at that point,
that is unless you’ve had too much caffeine, or you’re stressed out about something.
Now, if you don’t believe me, you can do an experiment. Tonight, why don’t you watch
a loved one or a neighbor – okay, not a neighbor – a loved one, watch them fall
asleep, and you’ll notice quite early on in that process they’ll show that jerk,
and that’s just a sign that they’re going into sleep. But it’s a common occurrence
and it’s part of brain stem function.
The second structure, the cerebellum, lies directly behind the brain stem. It’s name
literally is latin for ‘little brain’ and it is linked to the rest of the brain
and the spinal cord with massive tracks that all run through the brain stem.
The cerebellum is involved in many functions, including things like attention and language,
but its primary role is in that of skeletal muscle control. Specifically in fine motor
control for things like balance, posture, and muscle tone. It’s important to point
out that although the cerebellum is heavily involved in movement and motor control, the
cerebellum itself cannot generate those movements. Instead, its role is to calibrate and fine
tune ongoing movements.
the third structure, the diencephalon, also consists of several independent parts, however
unlike the brain stem, these parts play such wildly different roles in regulation that
it’s worth briefly touching on them independently. The first part of the diencephalon that we’ll
talk about is the pineal gland, which is a very small gland located right at the center
of the brain. Now I say it’s small because it literally is about the size of a grain
of rice, but it plays a very important role in several functions. Most importantly, it
helps regulate the sleep-wake cycle by producing a substance called melatonin. You may or may
not be familiar with melatonin if you’ve ever taken over-the-counter sleeping aids,
things like that, but melatonin is released in response to darkness, and it creates a
cascading affect that triggers the sleep cycle. In contrast, when there’s light outside,
the production of melatonin in the pineal gland is inhibited. Now the word melatonin
is somewhat misleading here, so it’s worth briefly talking about how it got its name.
It came about because of an early focus in dermatology looked at the role of the pineal
gland and the substance it was producing in terms of its role in skin pigmentation, and
dermatologists thought it may have implications for a wide range of skin problems. Now subsequent
research found that wasn’t the case, but what they did find was that substance, still
called melatonin, was involved in something very important, as we mentioned, the sleep
wake cycle. In addition to the pineal gland, the diencephalon
also includes a small very important structure called the hypothalamus. The hypothalamus
is involved in many regulatory functions; most importantly it’s heavily involved in
the maintenance of homeostasis, which is a technical term for that dynamic balance between
the organism and their environment. Now the hypothalamus is involved in homeostasis through
the regulation of three functions: body temperature, and water balance, and appetite. Let’s begin
with water balance. Now your brain is roughly 70% water, and recall that it’s composed
of neurons. And for those neurons to function, they have to be bathed in a very delicate
solution of not just water but sodium and other sorts of essential chemicals. When that
water balance gets off, either too much sodium for example or too little, bad things happen.
So for those neurons to function, they need that delicate range of chemicals in the water,
and the hypothalamus is the primary regulator of that balance. It does so by regulating
two different types of thirst: osmotic and hypovolemic. Now you’ve probably experienced
both of these but it’s wroth talking about them for a moment so it will help clarify
the times when you should reach for that glass of water, and when you might want to reach
for that diet coke. The first kind of thirst, osmoitic, is probably what you think of when
you think of being thirsty. Here, what’s happened is you’ve taken in too much sodium,
probably from eating like salty foods, and you’ve changed that balance between water
and the chemicals that the neurons are bathed in to the point where there’s too much sodium
outside of the neurons. And again as I’ve said, bad things can happen. In this case,
the hypothalamus will trigger a strong craving for water and water only. In fact when you’ve
been thirsty like that nothing else quite sounds right, you just want water. Now the
problem is, sometimes you can have learned behaviors where you just reflexively kind
of grab for a diet coke, but those kinds of sodas have a lot of sodium and in fact you’re
just exacerbating the problem under those circumstances. So when your brain says “I
want water” listen to it. The second kind of thirst, hypovolemic, is very different.
This happens when you’ve lost a lot of body fluid overall, not just water but also that
whole delicate solution, you’ve just lost it all, the sodium, the electrolytes, and
such. This usually comes about from things like vomiting due to illness, bleeding, or
extreme dehydration. The thing here is that you don’t just want to drink water, because
think about what that would do: say you’ve reduced that overall solution by half so the
glass is literally half full here, if you just fill it back up with water all you’ll
have done is profoundly dilute the solution that the neurons need to function. And in
fact, in extreme cases, they wouldn’t function. What you need at this point is to take back
in not only water but those other essential chemicals like sodium and electrolytes. So
in this case you can imagine that for example if you’ve been sick and you’ve been vomiting,
notice how water doesn’t sound good at all, I mean the thought of drinking water in that
circumstance is just not appealing at all. Instead, your body craves the combination
of salt and water, and often in the form of a sports drink or diet soda, things like that.
In any case, with both types of thirst, your hypothalamus knows what it needs, and it’s
important for you to understand the distinction so you know what to reach for whether it’s
water or Gatorade. In addition to the pineal gland and the hypothalamus,
the diencephalon also includes a large egg-shaped structure called the thalamus. Now the thalamus
is involved in several functions, but its primary role is regulating the flow of information
to the cerebrum. Specifically, all sensory information coming into the brain with the
exception of smell, first is routed to the thalamus, processed, and then sent to the
right areas of the cerebrum for further processing. So in this way, the thalamus serves as a gatekeeper
for information to the rest of the cerebrum.
okay, to summarize, in this chapter we situated the brain as part of the broader nervous system,
emphasizing the fact that the brain does not exist in a vacuum. More specifically we discussed
a way of grouping brain structures that common in neuroscience, separating them into four
broad parts. Specifically, we focused on three: the brain stem, the cerebellum, and the diencephalon.
The next chapter will introduce the fourth part: the cerebrum.