Download Unit 2: What are the building blocks of our brains?

Unit 2: What are the building
Overview
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 the 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.
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?
For a complete list of defined
terms, see the Glossary.
Wo r k b o o k
Lesson 2.1
Neurons are the most important functional cells in our nervous system. The adult human brain contains
roughly 86 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 regrow long distances to repair its connections. Why? No one really knows. (Interestingly, CNS neurons can grow in lower vertebrates like fish). One potential reason why our CNS neurons
aren’t able to regrow lays in the hypothesis that all of our complex behaviors demand a neuronal network
with a very precise architecture. Meaning that, CNS neurons have had to trade off the ability to regrow, so
that the network remains stable. Even so, some nervous system damage can be repaired if we can induce
neurons to rewire over short distances.
Can neurons in the PNS repair themselves?
What would this mean in regards to recovery after an injury to the PNS?
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Can neurons in the CNS repair themselves? What would this mean in regards
to recovery after an injury to the CNS?
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LESSON READING
What are the three functional regions of the
neuron?
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:
DEFINITIONS OF TERMS
Cell body – part of the neuron
containing the nucleus, but not
including the axon and dendrites.
Also called the soma.
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.
Nucleus – the DNA containing
structures of cells.
For a complete list of defined
terms, see the Glossary.
Wo r k b o o k
Lesson 2.1
•
•
•
The cell body
The dendrites
The axon
Dendrites 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 parts of
the neuron supplied with protein is a major
task carried out by the cell body.
Figure 2: Cell body. The cell body is the metabolic center of the cell and contains all the cellular
organelles required to support cell life: the nucleus,
mitochondria, ribosomes, rough and smooth endoplasmic reticulum.
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Name two important functions carried out
by the neuron’s cell body.
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39
LESSON READING
We can identify two types of outgrowths sprouting off from the cell body, the dendrites and the axon.
The dendrites
DEFINITIONS OF TERMS
Dendrites — branched
projection(s) of a neuron that
functions as the receptive area
of a neuron.
Dendritic spines — tiny spikes
of various shapes that are
located on the surfaces of many
dendrites and are the sites of
synapses.
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
Figure 3: Dendrites. The dendritic arbor of two
dendrites is to convert these signals, which may
neurons (a Purkinje neuron on the left, and a senbe in the form of physical signals if they are from
sory neuron on the righ) illustrating the extensive
the environment (such as light, sound or touch) or
branching of dendrites..
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.
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 because they look like spikes (Figure 4). Each
Figure 4: Dendritic spines. Dendrites
have small protuberances called spines.
dendritic spine usually contains one synapse, which is an
Each spine can contain a synapse.
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 also 86 billion neurons — makes you 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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LESSON READING
What is the function of the axon?
The axon
DEFINITIONS OF TERMS
Action potential – the electrical
signal of the axon.
Axon – projection of a neuron that functions to conduct
electrical impulses away from a
neuron’s cell body.
Presynaptic cell – neuron
located before the synapse, and
thus sending the signal
Postsynaptic cell – neuron
located after the synapse, and
thus receiving the signal
For a complete list of defined
terms, see the Glossary.
Wo r k b o o k
Lesson 2.1
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 its target cell, for example a muscle, than it could if it
didn’t have ‘roots’.
Just as dendrites have specific points of contact called synapses, where they receive information from
the environment or other cells, so too do axons. Axons form synapses with muscles, glands, or when
located deep within a network of the CNS, with other neurons (Figure 5). In fact the synapse actually
contains both the transmitting point of contact (axon) and the receiving point of contact (dendrite).
The cell transmitting the signal is called the presynaptic cell for before the synapse, whereas the cell
receiving the signal is the postsynaptic cell for after the synapse.
Our neurons are classified into two main
groups depending on what other cells they
make connections with and what type of
information they convey. Neurons that
receive input from the environment, and
transmit that input into the CNS are called
sensory neurons. Whereas neurons that
carry information out of the CNS and make
connections with muscles and glands are
called motor neurons. 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.
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At a synapse, the cell sending a signal is
called what? (Hint: It’s the cell before the
synapse.)
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At a synapse, the cell receiving the signal
is called what? (Hint: It’s the cell after the
synapse.)
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LESSON READING
DEFINITIONS OF TERMS
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.
Axon terminals/Presynaptic
terminals – swellings at the end
of the axon’s branches that serve
as the transmitting site of the
presynaptic cell.
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.
Synaptic cleft – small gap in
the synapse that separates the
presynaptic cell from postsynaptic
cell.
So now we can classify neurons into three groups on the basis of their function:
For a complete list of defined
terms, see the Glossary.
Wo r k b o o k
Lesson 2.1
Neuronal function
•
Sensory neurons carry information into the central nervous system for perception.
•
Motor neurons carry commands out of the central nervous system to muscles and glands.
•
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 axon that
connects to the dendrite?
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What is the name of the site on the dendrite
that connects to the axon?
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Are the axons and dendrites in physical
contact with each other?
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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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