Table of Contents
Until recently, most neuroscientists thought we were born with all the neurons
we were ever going to have. As children we might produce some new neurons
to help build the pathways - called neural circuits - that act as information
highways between different areas of the brain. But scientists believed that
once a neural circuit was in place, adding any new neurons would disrupt
the flow of information and disable the brain’s communication system.
In 1962, scientist Joseph Altman challenged this belief when he saw
evidence of neurogenesis (the birth of neurons) in a region of the adult rat brain called the hippocampus. He
later reported that newborn neurons migrated from their birthplace in the hippocampus to other parts of the
brain. In 1979, another scientist, Michael Kaplan, confirmed Altman’s findings in the rat brain, and in 1983 he
found neural precursor cells in the forebrain of an adult monkey.
These discoveries about neurogenesis in the adult brain were surprising to other researchers who didn’t think
they could be true in humans. But in the early 1980s, a scientist trying to understand how birds learn to sing
suggested that neuroscientists look again at neurogenesis in the adult brain and begin to see how it might
make sense. In a series of experiments, Fernando Nottebohm and his research team showed that the numbers
of neurons in the forebrains of male canaries dramatically increased during the mating season. This was the
same time in which the birds had to learn new songs to attract females.
Why did these bird brains add neurons at such a critical time in learning? Nottebohm believed it was because
fresh neurons helped store new song patterns within the neural circuits of the forebrain, the area of the brain
that controls complex behaviors. These new neurons made learning possible. If birds made new neurons to
help them remember and learn, Nottebohm thought the brains of mammals might too.
Other scientists believed these findings could not apply to mammals, but Elizabeth Gould later found evidence
of newborn neurons in a distinct area of the brain in monkeys, and Fred Gage and Peter Eriksson showed that
the adult human brain produced new neurons in a similar area.
For some neuroscientists, neurogenesis in the adult brain is still an unproven theory. But others think the
evidence offers intriguing possibilities about the role of adult-generated neurons in learning and memory.
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Neuron | |
The central nervous system (which includes the brain
and spinal cord) is made up of two basic types of
cells: neurons (1) and glia (4) & (6). Glia
outnumber neurons by a substantial amount -- some
scientists have estimated it to be as large as nine to
one -- but in spite of their smaller numbers, neurons
are the key players in the brain.
Neurons are information messengers. They use
electrical impulses and chemical signals to transmit
information between different areas of the brain, and
between the brain and the rest of the nervous system.
Everything we think and feel and do would be
impossible without the work of neurons and their
support cells, the glial cells called astrocytes (4)
and oligodendrocytes (6).
Neurons have three basic parts: a cell body
and two extensions called an axon (5) and a
dendrite (3). Within the cell body is a nucleus (2),
which controls the cell’s activities and contains the
cell’s genetic material. The axon looks like a long
tail and transmits messages from the cell. Dendrites
look like the branches of a tree and receive messages
for the cell. Neurons communicate with each other
by sending chemicals, called neurotransmitters,
across a tiny space, called a synapse, between the
axons and dendrites of adjacent neurons.
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The architecture of the neuron. | |
There are three classes of neurons:
- Sensory neurons carry information from the
sense organs (such as the eyes and ears) to the brain.
- Motor neurons have long axons and carry
information from the central nervous system to the
muscles and glands of the body.
- Interneurons have short axons and
communicate only within their immediate region.
Scientists think that neurons are the most diverse kind
of cell in the body. Within these three classes of
neurons are hundreds of different types, each with
specific message-carrying abilities.
How these neurons communicate with each other by
making connections is what makes each of us unique
in how we think, and feel, and act.
The extent to which new neurons are generated in the
brain is a controversial subject among neuroscientists.
Although the majority of neurons are already present in
our brains by the time we are born, there is evidence to
support that neurogenesis (the scientific word for the birth
of neurons) is a lifelong process.
Neurons are born in areas of the brain that are rich
in concentrations of neural precursor cells (also called
neural stem cells). These cells have the potential to
generate most, if not all, of the different types of
neurons and glia found in the brain.
Neuroscientists have observed how neural precursor
cells behave in the laboratory. Although this may not
be exactly how these cells behave when they are in the
brain, it gives us information about how they could be
behaving when they are in the brain’s environment.
The science of stem cells is still very new, and could
change with additional discoveries, but researchers have
learned enough to be able to describe how neural stem
cells generate the other cells of the brain. They call it
a stem cell’s lineage and it is similar in principle to a
family tree.
Neural stem cells increase by dividing in two and
producing either two new stem cells, or two early
progenitor cells, or one of each.
When a stem cell divides to produce another stem cell,
it is said to self-renew. This new cell has the potential to
make more stem cells.
When a stem cell divides to produce an early progenitor
cell, it is said to differentiate. Differentiation means that
the new cell is more specialized in form and function.
An early progenitor cell does not have the potential of
a stem cell to make many different types of cells. It can
only make cells in its particular lineage.
Early progenitor cells can self-renew or go in either of
two ways. One type will give rise to astrocytes. The
other type will ultimately produce neurons or
oligodendrocytes.
Once a neuron is born it has to travel to the place in the
brain where it will do its work.
How does a neuron know where to go?
What helps it get there?
Scientists have seen that neurons use at least two
different methods to travel:
- Some neurons migrate by following the long fibers of
cells called radial glia. These fibers extend from the
inner layers to the outer layers of the brain. Neurons
glide along the fibers until they reach their destination.
- Neurons also travel by using chemical signals.
Scientists have found special molecules on the surface of
neurons -- adhesion molecules -- that bind with similar
molecules on nearby glial cells or nerve axons. These
chemical signals guide the neuron to its final location.
Not all neurons are successful in their journey. Scientists
think that only a third reach their destination. The rest
either never differentiate, or die and disappear at some
point during the two to three week phase of migration.
Some neurons survive the trip, but end up where they
shouldn’t be. Mutations in the genes that control
migration create areas of misplaced or oddly formed
neurons that can cause disorders such as childhood
epilepsy or mental retardation. Some researchers
suspect that schizophrenia and the learning disorder
dyslexia are partly the result of misguided neurons.
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Some neurons migrate by riding along extensions (radial glia) until they reach their final destinations. | |
Once a neuron reaches its destination, it has to settle
in to work. This final step of differentiation is the least
well-understood part of neurogenesis.
Neurons are responsible for the transport and uptake of
neurotransmitters - chemicals that relay information
between brain cells.
Depending on its location, a neuron can perform the job
of a sensory neuron, a motor neuron, or an interneuron,
sending and receiving specific neurotransmitters.
In the developing brain, a neuron depends on molecular
signals from other cells, such as astrocytes, to determine
its shape and location, the kind of transmitter it produces,
and to which other neurons it will connect. These freshly
born cells establish neural circuits - or information
pathways connecting neuron to neuron - that will be in
place throughout adulthood.
But in the adult brain, neural circuits are already
developed and neurons must find a way to fit in.
Researchers suspect that astrocytes play a similar role
in the adult brain, actively regulating the function and
synapse formation of new neurons.
As a new neuron settles in, it starts to look like
surrounding cells. It develops an axon and dendrites
and begins to communicate with its neighbors.
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Stem cells differentiate to produce different types of nerve cells. | |
Although neurons are the longest living cells in the body,
large numbers of them die during migration and
differentiation.
The lives of some neurons can take abnormal turns.
Some diseases of the brain are the result of the unnatural
deaths of neurons.
- In Parkinson’s disease, neurons that produce the
neurotransmitter dopamine die off in the basal ganglia,
an area of the brain that controls body movements. The
brain can no longer control the body and people shake
and jerk in spasms.
- In Huntington’s disease, a genetic mutation causes
over-production of a neurotransmitter called glutamate,
which kills neurons in the basal ganglia. As a result,
people twist and writhe uncontrollably.
- In Alzheimer’s disease, unusual proteins build
up in and around neurons in the neocortex and
hippocampus, parts of the brain that control memory.
When these neurons die, people lose their capacity to
remember and their ability to do everyday tasks.
Physical damage to the brain and other parts of the
central nervous system can also kill or disable neurons.
- Blows to the brain, or the damage caused by a
stroke, can kill neurons outright or slowly starve them of
the oxygen and nutrients they need to survive.
- Spinal cord injury can disrupt communication
between the brain and muscles when neurons lose their
connection to axons located below the site of injury.
These neurons may still live, but they lose their ability
to communicate.
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One method of cell death results from the release of excess glutamate. | |
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Macrophages (green) eat dying neurons in order to clear debris. | |
Scientists hope that by understanding more about the life
and death of neurons they can develop new treatments,
and possibly even cures, for brain diseases and disorders
that affect the lives of millions of Americans.
The most current research suggests that neural stem cells
can generate many, if not all, of the different types of
neurons found in the brain and the nervous system.
Learning how to manipulate these stem cells in the
laboratory into specific types of neurons could produce a
fresh supply of brain cells to replace those that have died
or been damaged.
Therapies could also be created to take advantage of
growth factors and other signaling mechanisms inside the
brain that tell precursor cells to make new neurons. This
would make it possible to repair, reshape, and renew the
brain from within.
For information on other neurological disorders or research programs funded by the National Institute of Neurological Disorders and Stroke, contact the Institute's Brain Resources and Information Network (BRAIN) at:
BRAIN
P.O. Box 5801
Bethesda, MD 20824
(800) 352-9424
www.ninds.nih.gov