Study topic · about 8 minutes

Neuroplasticity: how the brain rewires itself

Neuroplasticity is the brain's ability to change its own wiring. Connections between neurons grow stronger or weaker with use (synaptic plasticity), new connections form, and unused ones are pruned away. After an injury, surviving areas can take over lost jobs, which is called functional recovery. The brain is most plastic in childhood, but it keeps changing throughout life.

Explore the hippocampus in 3D

What is neuroplasticity?

For much of the 20th century, many scientists thought the adult brain's wiring was fixed. We now know it changes all the time. Every time you learn a fact, practice a skill or recover from an injury, connections between neurons change. This ability is called neuroplasticity, or brain plasticity, and it works at several levels.

Synaptic plasticity: cells that fire together wire together

In 1949, the Canadian psychologist Donald Hebb proposed that when one neuron repeatedly helps fire another, the connection between them grows stronger. His idea is often summed up as 'cells that fire together wire together'.

The first strong evidence came in 1973, from the hippocampus of rabbits. After a burst of rapid, repeated signals, a pathway passed on signals more strongly than before, and it stayed that way for hours. This is long-term potentiation (LTP). The reverse, long-term depression (LTD), weakens synapses that are less active or rarely help the next cell fire. Together they turn each connection up or down, which is thought to be how memories are stored (see how memory works).

How does a synapse know it is being used together with its target? At many synapses that use glutamate, the key is the NMDA receptor. It lets calcium in only when two things happen at once. Glutamate must arrive, and the receiving cell must already be excited, which pushes a blocking magnesium ion out of the channel. The calcium tells the cell to add more AMPA receptors, so the next signal has a bigger effect. That makes the NMDA receptor a coincidence detector.

With repeated use, the change becomes physical. Dendritic spines, the tiny knobs on dendrites where most excitatory synapses sit, grow larger. New spines and synapses form, and unused ones shrink and disappear.

The growing brain: pruning and critical periods

A baby's brain makes new connections at an astonishing rate, and by the toddler years the cortex has far more synapses than it will keep. Through childhood and the teen years, the brain prunes the ones that are rarely used and strengthens the rest. The rule is often summed up as 'use it or lose it'.

At the same time, oligodendrocytes wrap more and more axons in myelin, which speeds up signals. Myelination goes on well into the 20s. The prefrontal cortex, which plans, weighs choices and holds back impulses, is one of the last areas to mature. The brain as a whole keeps maturing until the mid-to-late 20s.

Some kinds of learning depend on a critical period, a window when experience wires a system for life. In the 1960s, David Hubel and Torsten Wiesel sewed one eye of young kittens shut for their first few months. When the eye was reopened, it was healthy, yet the kitten was nearly blind in it. Its primary visual cortex had rewired to respond to the open eye instead. Closing an adult cat's eye for months had little effect. Hubel and Wiesel shared the 1981 Nobel Prize for their work on vision.

The same is true in people. A child born with a cataract, or with badly misaligned eyes, can develop amblyopia (lazy eye) unless it is treated early. Treatment often means patching the stronger eye. Language has a sensitive period too. Babies can hear the sound differences of every language, but by about their first birthday they tune in to the sounds of their own. People who learn a second language after childhood rarely lose their accent. Scientists say 'sensitive' rather than 'critical' because learning later is harder, not impossible.

Can adults grow new neurons?

For decades, textbooks said you are born with all the neurons you will ever have. For many animals, that is wrong. Rats, mice and many other mammals keep making neurons all their lives, mainly in the dentate gyrus of the hippocampus. There they become new granule cells. In mice, running roughly doubles the number of new neurons, while long-term stress lowers it.

In adult humans, the question is still debated. A 2013 study dated brain cells using carbon-14 left in the air by nuclear bomb tests. It estimated that each hippocampus adds about 700 new neurons a day. A 2018 study found almost none after childhood, while others have found signs of young neurons even in people in their 70s and older.

Either way, neurons in the cerebral cortex are not replaced: they are about as old as you are. So most plasticity in the adult brain comes from changing connections, not from new cells.

Learning changes the brain

To get a license, London taxi drivers must learn 'the Knowledge': the layout of about 25,000 streets, which takes years. In 2000, brain scans showed that the back part of their hippocampus was larger than in other people, and larger the longer they had driven. Bus drivers, who follow set routes, did not show this pattern. A later study followed trainees: those who passed the test gained gray matter there, while those who failed did not.

Skills leave traces too. In string players, the fingers of the left hand, which press the strings, take up more room in the brain's touch map than in non-musicians. The effect is biggest in those who started young. Even three months of learning to juggle enlarged an area that tracks movement in what you see, and it shrank again once people stopped practicing.

Phantom limbs and remapping

After losing an arm or leg, most people still feel the missing limb, and many feel pain in it. This is called a phantom limb. In the 1990s, the neuroscientist V. S. Ramachandran found that touching the face of a man who had lost his arm made him feel touches on his missing hand.

The reason lies in the body map of the primary somatosensory cortex, where the face area sits right next to the hand area. With no signals coming from the hand, inputs from the face had spread into its patch of cortex. This is cortical remapping. Remapping happens in other ways too: in people blind from birth, the visual cortex becomes active when they read Braille with their fingertips.

See the body map in 3D

Recovery after brain injury

After a stroke or head injury, the brain can regain lost skills, which is called functional recovery. Some early gains come as swelling goes down and stunned neurons near the damage start working again. Most recovery happens in the first weeks to months, but it can continue for years. Lasting recovery depends on plasticity: surviving areas, mostly near the damage, learn to do the lost jobs.

UK A-level psychology (AQA) uses these terms for the ways the brain repairs its wiring:

Rehabilitation guides this rewiring. Constraint-induced movement therapy is one example. For about two weeks, a person with one weak arm wears a mitt on the stronger hand for most of the day. They practice tasks with the weaker arm for hours each day. This breaks the habit of relying on the good arm, called learned non-use. Brain maps show the weak hand's area in the primary motor cortex growing.

Children often recover better than adults, because their brains are more plastic. The most striking example is hemispherectomy, an operation that removes or disconnects one hemisphere to stop severe seizures. Many of these children go on to walk, talk and go to school. The hand on the opposite side usually stays weak, and half of the visual field is lost. If the left hemisphere is removed early, the right can take over much of language (see language and the brain).

Education seems to help too: people with more years of schooling tend to recover better from head injuries, an idea called cognitive reserve.

In these study guides

• MCATSleep, drugs, memory, language and reward
• AP Psychology1.4 The brain
• Intro psychologyThe brain and spinal cord (section 3.4)
• A-level PsychologyPlasticity and functional recovery after trauma

Take it further in 3D

• MemoryThe hippocampus and its circuit. A short guided lesson in the 3D app.
• Moving and feelingThe motor and sensory strips. A short guided lesson in the 3D app.

Sources

1. Psychology 2e, 3.4 The Brain and Spinal Cord, OpenStax
2. Long-term synaptic plasticity, Queensland Brain Institute, University of Queensland
3. The Teen Brain: 7 Things to Know, National Institute of Mental Health (NIMH)

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