One of the brain’s most important properties is its flexibility. Our cerebral circuitry changes constantly—every day, new links are made amongst the 86 billion individual neurons in our heads, and old connections are allowed to fall away.
The result is a dizzyingly complicated network that is in a constant state of flux, rewiring itself on the fly in response to its environment and the life experience of its owner. The brain’s ability to do this is called neuroplasticity, and it’s what gives us the capacity to learn, grow, develop new skills and ideas, and adapt to the environment in which we live. We understand some aspects of neuroplasticity fairly well but others, including the reason that certain connections get made instead of others, remain deeply mysterious.
Now, a new study published April 17 in Science promises to shed light on at least one aspect of that mystery. The research examines how neuroplasticity functions in mice learning to tackle a new task, and its results hint at the possibility that neurons’ shape may influence whether their connections are included in the changes wrought by this new skill.
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Neuroplasticity: the how
The last century has seen a huge amount of research into how the brain works. As a result of all this work, we have a good grasp on the physical processes that enable neuroplasticity. When a neuron “fires,” it sends an electrochemical signal called an action potential. These signals then travel down long strands of neural tissue called axons.The signals are then received by intricate, branched filaments called dendrites and the junctions at which these structures connect are called synapses.
We also know that not all neural connections are equal. Some are stronger than others, with a more robust connection making it more likely that the neuron on the receiving end will fire in response to a signal from the neuron at the other end of the link. The strength of these connections can also change over time.
Neuroplasticity allows for the formation of new connections, the purging of obsolete ones, and changes in the strength of existing connections—all processes that make subtle changes to the structure of our neural networks.
Neuroplasticity: the why
While we may well understand how neuroplasticity works, it can be more challenging for scientists to explain why this is the case. Specifically, it remains unclear how neurons are “chosen” for connection or modification. Why are certain connections created or modified and not others? Does the process work to some sort of cerebral blueprint? Or are the vast swathes of neurons in our brain more like chunks of RAM in our computers, sitting and waiting to be used as they become available?
William Wright—a postdoctoral scholar at the University of California San Diego’s neurobiology faculty and one of the Science paper’s co-authors—explains that there must be some degree of determinism to the way neuroplasticity plays out.
“Synaptic plasticity can’t just be happening randomly,” Wright tells Popular Science. “For us to learn … the right synapses must undergo the right types of changes (i.e., get stronger or weaker).”
But what determines which are the right neurons? According to Wright, this is the problem. “We [don’t] really understand how this process [gets] initiated at specific synapses and not others.”
Of mice and dendrites
To look for patterns in learning-related neuroplasticity, Wright and his team designed an experiment where each member of a population of mice in a lab was taught to activate a lever to receive a reward. They used a technique called longitudinal in vivo two-photon fluorescence imaging, which allows for the study of individual synapses in living creatures, and enabled the team to map how the acquisition of this skill altered each mouse’s brain.
Wright explains that every mouse brain shares a basic layout, “a sort of general blueprint [that] sets the general connectivity patterns of the brain (i.e., which brain areas connect with which).” However, once an individual is born, their brain is on its own. The brain will develop and change in a way that reflects that individual’s life experience and circumstances. As a result, every brain is different–a statement as true of mice as it is of humans.
These individual differences in the brain mean that a similar piece of learning might manifest quite differently in the brain of one mouse to how it does in another’s. In turn, this means that studying the effects of neuroplasticity has less to do with trying to identify whether the exact same neurons are being connected in different individuals, and more to do with looking for patterns that hint at some sort of underlying rules.
Wright and his team found that their results did indeed hint at such patterns, and therefore at some sort of accompanying rules. The changes observed after the skill acquisition were localized to the animals’ primary motor cortices, and seemed to be particularly pronounced in a type of neuron called a pyramidal neuron. As their name suggests, these cells are characterised by their shape. In addition to making them reasonably easy to identify, that triangular shape affects the nature of the cells’ dendrites.
Wright compares the structure of a pyramidal neuron and its connections to a tree. The cell has two types of dendrites–one trunk-like structure extending from the apex of the pyramid and a series of “roots” emerging from the bottom. These are called apical and basal dendrites, respectively, and the study’s results suggest that they have distinctly different functions.
Connections made via apical synapses seemed to be strengthened by movement information more than those made via basal synapses. The paper notes that “these results suggest apical synapses in … [these] pyramidal neurons are organized into task-related functional clusters, while this tendency is much weaker for basal synapses.”
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This hints that dendritic structure may be one factor in determining why a certain neural connection might end up being modified as a result of neuroplasticity. More generally, Wright emphasises the importance of the simple fact that different dendritic connections appear to perform different functions.
“We still don’t fully understand why neurons have these different types of dendrites,” Wright says, “[or] what different functions they may be [performing].” The study provides clues to the nature of at least one of these functions, and in doing so, points at directions for future research. This, Wright says, is “one of [our] most exciting [results].”
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