The neurogenesis in the human adult brain was discovered the early 1980s, surprising the scientific community and the traditional neuroscientists, who thinked were born with all the neurons we were ever going to have.
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.
Neuroscientists reviewed their discover and saw that it worked in several animals and to the human adult brain as well.
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.
In 1962, the scientist Joseph Altman saw evidence of neurogenesis in a region of the adult rat brain called the hippocampus.
He reported that newborn neurons migrated from their birthplace in the hippocampus to other parts of the brain.
In 1979, the scientist Michael Kaplan, confirmed Altman’s findings in the rat brain
In 1983, Kaplan also found neural precursor cells in the forebrain of an adult monkey.
In 1980, the scientist Fernando Nottebohm was trying to understand how birds learn to sing
noted that the numbers of neurons in the forebrains of male canaries dramatically increased during the mating season, exactly at 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.
But most scientists believed these findings could not apply to mammals.
Elizabeth Gould later found evidence of newborn neurons in another area of the brain in monkeys
Fred Gage and Peter Eriksson showed that the adult human brain produced new neurons in a similar area.
For classical 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.
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) and (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.
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
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
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
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
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.
Stem cells differentiate to produce different types of nerve
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
- 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
One method of cell
death results from the release of excess glutamate.
(green) eat dying neurons in order to clear debris.
Hope Through Research
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
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: