Workbooks
Unit 2:
What are the building blocks of
our brains?
In the last unit we discovered that complex brain functions occur as individual structures in the brain work together like an orchestra.
We also discussed one of the limitations of our new visualization techniques – that they only sample populations of hundreds or thousands of neurons, so they don’t give us any information about how they individual cells of the nervous system work together. So now
we’re going to take another step back and dial down our focus to the primary building blocks of our brains, the neurons and the glial
cells. In this unit we will explore how these basic cells are built and how they work, and importantly what can go wrong when these
building blocks are diseased and their functions are compromised.
Remember our graphic from the beginning of this workbook? This unit focuses on the neuron, which is the building block of our
brains.
LESSON 2.1 WORKBOOK
What is the structure of a neuron?
DEFINITIONS OF TERMS
Neuron – cells of the nervous
system that are specialized for the
reception, conduction and transmission of electrochemical signals
Cell body – part of the neuron
containing the nucleus, but not
including the axon and dendrites.
Also called the soma.
Nucleus – the DNA containing
structures of cells
Endoplasmic reticulum – organelle in the cell that forms a network
of tubules and vesicles. It functions
to synthesize proteins and lipids as
well as metabolize carbohydrates.
For a complete list of defined
terms, see the Glossary.
Wo r k b o o k
Lesson 2.1
This unit introduces you to the building blocks of our
brains: neurons and glia cells. In this lesson, we will
begin our exploration of how the brain is put together
by investigating why neurons have such complex
structures and how these structures allow the neurons
to perform highly specialized functions
What are neurons?
Neurons are the most important functional cells in our nervous system. The brain itself contains more
than 100 billion individual neurons. Each neuron is interconnected forming a precise network. Within that
network neurons are assembled into many different kinds of functionally distinct regions (like Broca’s area
for example). As we saw in the last lesson these regions interact with each other to produce our perception
of the external world, to fix our attention on the responses that need to be made, and to control our bodily
functions. Our first step in understanding the brain, therefore, has to be to understand the neuron – how it
is put together and how it works.
Neurons are cells with highly complex structures, much more complex than any other cell in the body.
Wiggle your big toe. The neuron that controls that wiggle starts off in the spinal cord somewhere in your
upper chest and ends up at your big toe, a distance that would be tens of meters if you were a giraffe
(which don’t have toes, but whatever, you get the point). Neurons are different from other cells in a number
of ways especially because, unlike most cells, neurons don’t divide - the number of neurons you had when
you are born is the maximum you will ever have. This means that when a neuron is damaged the only
possibility you have to restore its function is to fix it, you can’t simply make another one to take its place,
like you could in the liver. In the peripheral nervous system you can fix damaged neurons so that they’ll
grow slowly back to make their original connections. The central nervous system is different. When a CNS
neuron is damaged it cannot gros back long distances to repair its connections. Why? No one really knows
(CNS neurons can grow in lower vertebrates like fish). Scientists have hypothesized that all the complex
behaviors we can do demand that the neuronal network has a very precise architecture, and since this
could be completely destroyed if random neurons were growing every which way, they have had to trade
off the ability to grow so the network stays stable. Even so, some nervous system damage can be repaired
if we can induce neurons to rewire over short distances.
Can the peripheral nervous system repai irself if it is damaged?
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Can the central nervous system repai irself if
it is damaged?
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LESSON MATERIALS
Neurons have three distinct functional regions
The typical neuron contains three different regions, each of which looks different and each of which has
its own specialized function (Figure 1). These regions are:
Dendrites DEFINITIONS OF TERMS
Dendrites – branched
projection(s) of a neuron that
functions as the receptive area of
a neuron.
Axon – projection of a neuron that functions to conduct
electrical impulses away from a
neuron’s cell body
Dendritic spines – tiny spikes of
various shapes that are located
on the surfaces of many dendrites and are the sites synapses
Synapse – junction between two
neurons, consisting of a small
gap across which impulses pass
from one neuron to another
Axon hillock – specialized part
of a neuron’s cell body that connects to the axon. As a result, the
initial segment or axon hillock is
the site where action potentials
originate.
Wo r k b o o k
Lesson 2.1
•
•
•
The cell body
The dendrites
The axon
Axon Cell Body Ini2al Segment Synapse Presynap2c cell Postsynap2c cell Figure 1: Neuron structure. Neurons have three distinct regions: the
dendrites, the cell body, and the axon.
The cell body
The cell body (also sometimes called the soma) is the metabolic center of the neuron (Figure 2). It
contains the nucleus, which stores the genes of the cell in chromosomes, and the smooth and rough
endoplasmic reticulum, which are the sites where proteins are synthesized. It also contains the lysosomes that degrade proteins that have become old or damaged.
Because the ribosomes are mostly concentrated in
the cell body, protein synthesis primarily occurs there
and in the dendrites that are closest to it. Because
of this, a major role of the cell body is to package
the proteins it has made so they can be transported
over long distances down the leg and into the foot to
our big toe (or our little finger etc.). Similarly, because
the cell body is also the site were lysosomes are
concentrated, any big toe protein that has reached
its sell-by date needs to be transported back up the
leg to the cell body for destruction. Keeping all the Figure 2: Cell body. The cell body is the metaparts of the neuron supplied with protein is a major bolic center of the cell and contains all the cellular
organelles required to support cell life: the nucleus,
task carried out by the cell body.
mitochondria, ribosomes, rough and smooth endoplasmic reticulum.
What are the three functional regions of the
neuron?
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Name two important functions carried out
by the neuron’s cell body.
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3
LESSON MATERIALS
We can identify two types of outgrowths sprouting off from the cell body, the dendrites and the axon.
The dendrites
DEFINITIONS OF TERMS
Action potentials – the electrical
signal of the axon
Presynaptic cell – neuron
located before the synapse, and
thus sending the signal
Postsynaptic cell – neuron
located after the synapse, and
thus receiving the signal
Axon terminals/Presynaptic
terminals – swellings at the end
of the axon’s branches that serve
as the transmitting site of the
presynaptic cell
Synaptic cleft – small gap in
the synapse that separates the
presynaptic cell from postsynaptic
cell
For a complete list of defined
terms, see the Glossary.
Wo r k b o o k
Lesson 2.1
Most neurons have several dendrites (Figure 3).
These dendrites branch out from the cell body in a
shape that makes them look like a tree. In fact the
dendrites are often called ‘the dendritic tree’. The
dendritic tree is the main region of the neuron that receives signals. These signals can come in the form of
sensations from the environment. Alternatively, in the
depths of the neuronal network they may come from
other neurons. The role of the dendrites is to convert Figure 3: Dendrites. The dendritic arbor of two
these signals, which may be in the form of physical neurons (a Purkinje neuron on the left, and a sensory neuron on the righ) illustrating the extensive
signals if they are from the environment (such as light, branching of dendrites..
sound or touch) or chemicals if they are from other
neurons, into an electrical signal. Dendrites do this by
changing the electrical properties of their membranes via depolarization or hyperpolarization. We will talk
more about the important processes of depolarization and hyperpolarization later on in this unit.
As we saw, each of our sensory systems contains unique
neurons that are specialized to detect specific types of sensory stimuli in the environment. The dendrites from these
neurons are able to convert these stimuli into a neural response that the brain can understand. For example, different
types of sensory dendrites in our skin are uniquely tuned to
detect changes in pressure. They then convert the physical
sensation of pressure into a neural response by depolarizing
or hyperpolarizing their membranes.
The branches of the dendritic tree often have many hundreds of thousands of little twigs that we call dendritic spines
Figure 4: Dendritic spines. Dendrites have
because they look like spikes (Figure 4). Each dendritic spine
small protuberances called spines. Each
spine can contain a synapse.
usually contains one synapse, which is an exact area where
the dendrite can receive a signal, whether from the environment or from another neuron. You can appreciate that if a
single dendritic tree has hundreds of thousands of spines, then it can have hundreds of thousands of different inputs. Remember that there are are also 100 billion neurons and you can appreciate that trying to
understand how everything is connected is a massive task. No wonder neuroscientists were excited by the
development of supercomputers!
What is the function of the dendritic tree?
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Which kind of neuron has more inputs? a
neuron without dendritic spines, or a neuron
with dendiritc spines? Why?
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4
LESSON MATERIALS
DEFINITIONS OF TERMS
Action potentials – the electrical
signal of the axon
Presynaptic cell – neuron
located before the synapse, and
thus sending the signal
Postsynaptic cell – neuron
located after the synapse, and
thus receiving the signal
Axon terminals/Presynaptic
terminals – swellings at the end
of the axon’s branches that serve
as the transmitting site of the
presynaptic cell
Synaptic cleft – small gap in
the synapse that separates the
presynaptic cell from postsynaptic
cell
For a complete list of defined
terms, see the Glossary.
Wo r k b o o k
Lesson 2.1
The axon
The other type of sprout we can detect coming off the cell body is the axon. Unlike the branches of
the dendritic tree, which are tapered just like real branches, the axon can be identified because it
looks just like a cylindrical tube. There is usually only one axon per neuron. The axon grows out from
a specialized region of the cell body called the axon hillock or initial segment. This structure is important because the axon is the main transmitting or conducting unit of the neuron, conveying electrical
signals from the dendritic tree down to its very tip. In our big toe analogy, the axon would convey the
signal from dendrites in the spinal cord along your leg to tell your muscles to wiggle your toe. The axon
hillock gathers together all the signals the neuron has received from the dendritic tree, converts them
into the single output response and sends them down the axon. This output response is an electrical
signal called the action potential. We will focus on how the action potential is made and transported
in another lesson in this unit. Many axons split into several branches at their tips (like the roots of the
tree). This means that the action potential can affect a larger area of muscle than it could if it didn’t
have ‘roots’.
Just like each dendrite had specific points of contact called synapses where it received information
from the environment or other cells, so too do axons. Axons also may connect with the environment
(such as the muscles in your toe or glands) or when located deep within the network of the central
nervous system, with other neurons (Figure 5). In fact the synapse actually contains both the transmitting point of contact (environment or axon) and the receiving point of contact (dendrite or environment.
The cell transmitting the signal is called the presynaptic cell, and the point where it forms the synapse is the presynaptic site whereas the cell receiving the signal is the postsynaptic cell and the
point of where it forms the synapse is the
postsynaptic site.
In the peripheral nervous system, whether
the presynaptic cell is an axon or an environmental cell (like the retina of the eye) will
depend on whether the neuron is sensory
or motor. Similarly, whether the postsynaptic cell is an environmental cell (like a
muscle) or a dendrite will also depend on
whether the neuron is motor or sensory. If
we are going to be able to understand how
neurons make functional networks it is going to be very important to understand exactly how the neurons connect together.
Presynap)c cell Axon terminal Synap)c cle4 Postsynap)c cell Figure 5: Synapse. The end of the axon divides into
fine branches that swell to form axon terminals. These
axon terminals are separated from the postsynaptic
cell by the synaptic cleft.
What is the function of the axon?
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What are the names of the site on the axon
that contacts the dendrite? You are standing
on the beach and feel the sand. Draw the
pathway that feels the sand and tells you to
wiggle your big toe.
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5
LESSON MATERIALS
DEFINITIONS OF TERMS
Action potentials – the electrical
signal of the axon
Presynaptic cell – neuron
located before the synapse, and
thus sending the signal
Postsynaptic cell – neuron
located after the synapse, and
thus receiving the signal
Axon terminals/Presynaptic
terminals – swellings at the end
of the axon’s branches that serve
as the transmitting site of the
presynaptic cell
Synaptic cleft – small gap in
the synapse that separates the
presynaptic cell from postsynaptic
cell
For a complete list of defined
terms, see the Glossary.
Wo r k b o o k
Lesson 2.1
Within the depths of the network in the central nervous system neurons connect to other neurons,
so the presynaptic site is usually on an axon and the postsynaptic site is usually a dendrite (we may
run into exceptions later, but for now don’t worry about them). The points of contact on the axon are
specialized swellings on the axon’s branches called axon terminals or presynaptic terminals, while the
points of contact on the dendrite are called, not surprisingly, postsynaptic terminals. It is an important
characteristic of synapses that the pre- and postsynaptic terminals do not physically touch each other.
Instead, they are separated by a space called the synaptic cleft. In order to get the signal across the
synaptic cleft, and depolarize or hyperpolarize the dendritic membrane the presynaptic terminal turns
the action potential into a chemical signal that can cross the physical space. We will talk about this
process of transmitting a signal across the synapse called synaptic transmission in another lesson.
As you might imagine, the function of the neuron critically depends on how long its axon is. Neurons
with long axons are able to convey information over long distances to your big toe and so are called
projection or relay neurons. Neurons with short axons are only able to convey information into a limited
region and integrate information within a specific local area.
Neuronal function
So now we can classify neurons into three groups on the basis of their function:
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Sensory neurons carry information into the central nervous system for perception.
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Motor neurons carry commands out of the central nervous system to muscles and glands.
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Interneurons carry information from area to area within the nervous system. They are by far the
largest class, consisting of all the neurons that are not specifically sensory or motor.
In summary, although all neurons contain the same three functional components, they do not all look
or behave the same (Figure 6).
Figure 6: Examples of neurons.
Neurons that perform different functions have different shapes. Sensory
neurons receive input from a sensory
organ, like the ear. Motor neurons
control muscle information. Local
interneurons integrate activity within
a small area. Projection neurons
convey information for long distances. Neuroendocrine cells release
hormones into blood vessels. Model
neurons show that each of the different types have the same functional
components.
What is the name of the site on the dendrite
that contacts the axon?
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Are the axons and dendrites in physical
contact with each other?
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6
STUDENT RESPONSES
How are neurons specialized to complete their functions?
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Remember to identify your
sources
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Given what you know about the different types of neurons, what types of neurons do you predict to be involved in your ability to
smell warm chocolate chip cookies? And then taste one after you eat it? What has to happen after you’ve smelled the cookie, but
before you make the first bite? Be as specific as you can.
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Wo r k b o o k
Lesson 2.1
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