“Neurons that fire together wire together” is a typical way of summarizing the now very well-supported Hebbian theory that (in the words of the current version of the Wikipedia article “Hebbian theory”) can be described more precisely as

… an increase in synaptic efficacy arises from a presynaptic cell's repeated and persistent stimulation of a postsynaptic cell.

The more often the firing of one neuron stimulates another to fire, the stronger and more efficient that causal connection becomes. Conversely, if a neuronal pathway isn’t used very often, that pathway becomes weaker. The upshot is that our brains are changing all the time. Otherwise we would never learn anything.

In Keep Sharp: Build a Better Brain at any Age, Sanjay Gupta lays out brain plasticity in some detail. Each bullet below is a quotation from that useful book (the bullet is added).

Remarkably, sometimes the changes in our brains from learning are big enough to be seen by the crude instruments we now have to look at neurons from outside those neurons:

• … in William James’s 1890 book The Principles of Psychology, in which the Harvard University psychologist writes: “Organic matter, especially nervous tissue, seems endowed with a very extraordinary degree of plasticity,” but only in my lifetime have we begun to measure and visualize this phenomenon with technology. And with tools like fMRI, we can see the brain changes in response to certain stimulation. We can also see parts of the brain that are not in use being pruned away. The brain constantly and dynamically shapes and reshapes itself in response to experiences, learning, or even an injury. What’s more, what you choose to focus your attention on rewires the brain from a structural and functional perspective.

After a certain point, the key changes in the brain are not about size but about connections and function:

• A newborn’s brain triples in size in the first year of life; after that, the rate of physical growth slows as we learn and pack more into our roughly 3.3-pound brains. What continues to develop, allowing this tremendous ability to process more and more information, is the complexity of the networks of neurons as they go through a process of pruning, whereby certain synapses that are not being used are trimmed to make room for new ones.

• You know that as an adult, adding more information to your brain doesn’t increase the size of it (and imagine what people would look like if brain size increased with learning new information). But what does grow larger is the number of neurons—nerve cells—and the complexity of their network through ongoing and active pruning and “growth.”

The rewiring of the brain after an injury shows just how much potential there is for rewiring. The dramatic improvements in skills that are possible through practice are also testament to the potential for rewiring:

• The brain remains plastic throughout life and can rewire itself in response to your experiences. It can also generate new brain cells under the right circumstances. Take, for example, what blind people experience, as parts of their brain that normally process sight may instead be devoted to exceptional hearing. Someone practicing a new skill, like learning to play the violin, “rewires” parts of the brain that are responsible for fine motor control. People who’ve suffered brain injuries can recruit other parts of their brain to compensate for the lost or damaged tissue. Intelligence is not fixed either.

• If people who have suffered a devastating stroke can learn to speak again and those born with partial brains or who lose significant brain tissue to disease or surgical removal can propel their brains’ rewiring to work as a whole, think of the possibilities for those of us who just hope to preserve our mental faculties as we age. Even people who’ve had an entire hemisphere removed in childhood to treat rare neurological conditions such as intractable epilepsy or brain cancer can go on to function in adulthood. Their brains reorganize and various networks pick up the slack.

However, rewiring is not always an improvement. Both “Use it or lose it” and “You can get good at being dysfunctional” are genuine concerns:

• It’s important to note that brain plasticity is a two-way street. In other words, it’s almost as easy to drive changes that impair memory and physical and mental abilities as it is to improve these things. I love how Dr. Michael Merzenich, a leading pioneer in brain plasticity research and professor emeritus at the University of California at San Francisco, puts it: “Older people are absolute masters at encouraging plastic brain change in the wrong direction.” You can change your brain for the better or worse through behaviors and even ways of thinking. Bad habits have neural maps that reinforce those bad habits. Negative plasticity, for example, causes changes in neural connections that can be harmful. Negative thoughts and constant worrying can promote changes in the brain that are associated with depression and anxiety. Repeated mental states, where you focus your attention, what you experience, and how you respond to situations indeed become neural traits. One of Dr. Merzenich’s often-cited quotes is the following: “The patterns of activity of neurons in sensory areas can be altered by patterns of attention. Experience coupled with attention leads to physical changes in the structure and future functioning of the nervous system.

On this point, also see

Mental Retirement: Use It or Lose It—Susann Rohwedder and Robert Willis

In many ways, the growing accumulation of different kinds of evidence for brain plasticity is simply a reminder of what we knew all along (and culturally almost caused ourselves to forget): that the capacity for human learning is powerful and doesn’t suddenly disappear when one becomes an adult.

On the dramatic potential for learnings, see:

There's One Key Difference Between Kids Who Excel at Math and Those Who Don't

How to Turn Every Child into a 'Math Person'

The Most Effective Memory Methods are Difficult—and That's Why They Work

For annotated links to other posts on diet and health, see:

• Miles Kimball on Diet and Health: A Reader's Guide