Brain Matters: The Neuron (1 of 5)


Uploaded by ltoddrose on 31.07.2011

Transcript:
Brain Matters: Chapter 1 The Neuron
Transcript
The human brain is an incredibly complex dynamic system made up of vast overlapping neural
networks. These neural networks, together, give rise to everything from blood pressure
control to monitoring heart rate, to thought, and emotion. In order to understand something
as complex as that it’s important to start small. In this chapter we start small by considering
the cells that make up those neural networks. From the outset it’s worth pointing out
that the brain consists of two broad classes of cells: neurons and glia. In this chapter
we’ll focus exclusively on neurons, but it’s worth pointing out that glia actually
outnumber neurons 50 to 1 in the brain and they do play an important but less understood
role in information processing. That said, we’ll focus on neurons. Specifically, we’ll
focus on their structure and on their function, that is, how they communicate.
To begin, let’s start with a definition. A neuron is a specialized cell that’s capable
of sending and receiving information. This last part is important. Neurons handle information
to, within, and from the brain. That’s a lot of information, so it’s not surprising
that you have a lot of neurons, upwards of two hundred billion. In addition to the sheer
number of neurons your brain has a lot of different types of neurons, upwards of ten
thousand different types depending on how you classify them. This isn’t surprising
either given the wide range of different kinds of information that you have to process, everything
from sensations, emotions, and thought for example. Now, we’re going to talk about
the structure of a neuron, but it’s worth mentioning that as late as the nineteenth
century most scientists didn’t believe that the brain was made of up discrete cells. In
fact, the prevailing opinion of the time was that the brain was conceptualized as one gigantic
interconnected network, a web if you will. This view is perhaps not that surprising given
the technology of the time, when you looked at that brain it did kind of look like one
jumbled mess, a web. It was the pioneering work of Santiago Ramon y Kahal that showed
us that in fact the brain did consist of discrete cells, which he called Neurons. Now, the way
he discovered this is kind of an interesting story. He actually used a staining method
developed by his arch rival, Golgi. The funny thing about this staining method is that it
randomly stains only a very very small percentage of neurons. So for Golgi, who didn’t believe
in the discrete cell theory of the brain this was kind of a failure, he was trying to stain
the whole network. But on the other hand for Ramon y Kahal it was basically a Godsend because
the brain was such a jumbled mess that if it wasn’t’ for the fact that in only stained
such a small number of them he wouldn’t have been able to separate those cells at
all. But lucky for us he was able to. Now not only was Ramon y Kahal able to show that
neurons did exist as discrete cells, he went further. He showed that neurons had several
identifiable parts in common. So let’s talk about what those parts are.
Like all other cells, neurons have a cell body. The cell body is in fact the largest
part of a neuron and it contains the nucleus, the DNA, and it’s involved in really essential
things like protein synthesis. But as important as a cell body is, what we care about in this
chapter are the parts that allow a neuron to send and receive information. The first
part are called dendrites. These are branch-like structures that come out of the cell body
and collect incoming signals. The second part is called the axon. This is a fiber bundle
that carries signals away from the cell body. Now, it’s important to point out that while
a neuron may have many dendrites, it only has one axon. The axon may branch at the end,
but it’s still only one signal. Most axons are covered in a fatty insulation called myelin.
The myelin allows the axons to send information at a much faster rate than it could otherwise.
The final part that’s important to consider are terminals. These exist at the very end
of those axon branches, and they house neurotransmitters which are the chemical signal used to communicate
between neurons.
Okay, so now we have a basic sense for the structure of neurons; but how do they function,
that is, how do they communicate? It turns out that neurons have evolved unique capabilities
for communication both within a neuron and between neurons. Within a neuron communication
is entirely electrical. Between neurons, communication can be electrical or chemical, but it’s
mainly chemical. Let’s start by considering communication within a neuron.
A simplified version of this process goes something like this: the dendrites capture
information coming from other neurons and pass that to the cell body. The cell body
integrates those signals into one unified signal. Now, if that signal reaches a particular
threshold, then the cell body will initiate its own electrical signal to pass down the
axon. This signal is called an action potential. If the signal in the cell body does not reach
the threshold, then no action potential will be initiated. In this way communication within
a neuron is all or nothing. Either the action potential is triggered or it is not, there
is no in between. Lets assume that an action potential was initiated and the signal is
passed down the axon until it arrives at the axon terminals. This is essentially where
we go from communication within a neuron to communication between neurons.
In reality a neuron is going to be connected to thousands of other neurons, but for our
purposes let’s keep it simple. Let’s ask this question: how does neuron A communicate
with neuron B? it turns out to answer that question we have to appreciate that neurons
do not indiscriminately communicate with other neurons. In fact, they mainly communicate
at very specialized locations called synapses. In this case we’re talking about a synapse
between the axon terminal of neuron A and the dendrite of neuron B. It turns out in
the human brain there are two types of synapses: electrical and chemical. Now, as late as the
19th century most scientists believed the brain only had electrical synapses. This is
not that surprising given that the brain is incredibly fast in the way that it handles
information. So it was thought that anything other than electrical synapses would simply
be too slow for the kinds of information that the brain has to process at the speed with
which it has to process it. The idea that something other than electrical synapses was
at play in neuronal communication was given a boost with the discovery of what is called
the synaptic cleft. This is a very small gap between the terminal of neuron A and the dendrite
of neuron B. Now, I say it’s small and I mean it’s very small, maybe a few billionths
of a meter wide. But it’s just big enough to act as a short to the electrical circuit.
That is, that electrical signal coming down the axon of neuron A to its terminal cannot
jump to the dendrite of neuron B. So, with the discovery of the synaptic cleft, it became
clear that something else had to be going on. And in this case that something else is
chemical. Specifically, the brain has very specialized chemicals called neurotransmitters
that it uses to bridge that gap between the terminal of neuron A and the dendrite of neuron
B. And in the human brain we have over a hundred different neurotransmitters and that allows
us all kinds of complex signaling. But just having neurotransmitters doesn’t tell us
how it is that we can go from electrical signaling in neuron A to chemical signaling between
neuron A and neuron B, to electrical signaling again in neuron B. So, let’s consider that
process again for a moment. Okay, we have electrical communication within
a neuron, that is, between dendrites and axon terminals. And we have electrical and chemical
communication between neurons. But it’s worth asking, why even have chemical communication
at all? Why not just rely on the efficiency and speed that comes with electrical to electrical
signaling? And in fact remember that the brain does have electrical synapses. But these synapses
tend to be relegated to areas where speed matters more than anything else. So, areas
like reflexes. It turns out that chemical communication is absolutely critical for all
kinds of complex behaviors that we consider human. That is, any kind of flexible, adaptive
behavior, including learning, depends fundamentally on chemical signaling and on neurotransmitters.
In this way, the brain has really struck an optimal balance between the electrical synapses
and their speed and efficiency and chemical synapses that bring with it the flexibility
and adaptability to respond to your environment.
Okay, to summarize. In this chapter we introduced the idea of a neuron, which is the fundamental
building block of the brain. Specifically we talked about the structure of a neuron
and introduced several key parts, including dendrites, the cell body, the axon, the myelinated
sheath around an axon, and the axon terminals. And we also discussed how neurons function,
that is, how they communicate, and we made a distinction there between communication
within a neuron which is through electrical signaling, and communication between neurons
which can either be electrical or chemical in nature. And in the case of chemical signaling,
we discussed how the brain makes use of specific chemicals called neurotransmitters.
In future chapters we’ll build on this understanding to consider the structure and function of
the brain as a whole.