Researchers at the University of Pennsylvania and Princeton University have made a breakthrough discovery that explains how the muscle controlling neurons of the brain master skills through practice.
The research was conducted by Javier Medina, assistant professor in the Department of Psychology in Penn's School of Arts and Sciences, and Farzaneh Najafi, a graduate student in the Department of Biology. They collaborated with postdoctoral fellow Andrea Giovannucci and associate professor Samuel S. H. Wang of Princeton University.
The study titled, “Sensory-Driven Enhancement of Calcium Signals in Individual Purkinje Cell Dendrites of Awake Mice” was published in the February 2014 journal Cell Reports.
Purkinje Cells in the Cerebellum Create Muscle Memory
Every fine tuned motor skill—from serving a tennis ball to playing the piano to dancing the tango—requires the brain to orchestrate precise, coordinated control over the body's muscles.
The cerebellum (Latin for “little brain”) is the brain's primary motor control center. It contains thousands of Purkinje cells, each of which collects information from elsewhere in the brain and funnels it down to the muscle-triggering motor neurons. Each Purkinje cell receives messages from a climbing fiber, a type of neuron that extends from the brainstem and sends feedback about the associated muscles.
In order to perfect motor skills, the brain has a feedback system between the senses that monitors when movements go right, and when movements go wrong. Purkinje cells are the neurons that coordinate all the movements of the body. Climbing fibers are the neurons that provide feedback when there is an error or unexpected sensation. Together Purkinje cells and climbing fibers work in harmony to fine-tune motor control.
The Cerebellum Is Mysterious and Powerful
When I published The Athlete’s Way (St. Martins Press) in 2007 neuroscientists were still unsure exactly how the brain created muscle memory. Luckily, my father who was a neuroscientist and neurosurgeon and wrote a book title “The Fabric of Mind” (Viking) had long believed that the Purkinje cells of the cerebellum were the key to mastering a skill through practice. Because of this I put Purkinje cells and the cerebellum in the spotlight as the key to The Athlete’s Way. (Click here to read a detailed description of Purkinje cells from my first book)
The cerebellum is only 10% of brain volume but houses over 50% of the brain’s total neurons. My dad always said, “Whatever the cerebellum is doing, it’s doing a lot of it.” I trusted what he called ‘an educated guess’ that the cerebellum was just as important as the cerebrum when I created the "up brain-down brain" split-brain model for The Athlete's Way.
A decade since I originally wrote the manuscript for The Athlete’s Way—after shaping the neuroscience ideas through daily conversations with my father—it’s exciting to see cutting edge neuroscience confirm my dad's hunches. Although my dad passed away in 2007, I know these new findings about Purkinje cells and climbing fibers would put him over the moon.
For decades, the enigma of how the feedback system between climbing fibers and Purkinje cells work together has perplexed neuroscientists. At the heart of this riddle is the fact that climbing fibers are constantly updating Purkinje cells with random signals. The climbing fibers send urgent signals when there is an error to report—but they also fire about once a second spontaneously with often trivial sensory updates.
"So if you're the Purkinje cell," Javier Medina said, "how are you ever going to tell the difference between signals that are spontaneous, meaning you don't need to change anything, and ones that really need to be paid attention to?"
Medina and his colleagues devised an experiment to test whether there was a measurable difference between legitimate and spontaneous signals from the climbing fibers. In their study, the researchers had mice walk on treadmills while their heads were kept stationary. This allowed the researchers to blow random puffs of air at their faces, causing them to blink, and to use a non-invasive microscopy technique to look at how the relevant Purkinje cells respond.
Because the random puffs of air were unexpected stimuli for the mice, the researchers could directly compare the differences between legitimate and spontaneous signals in the eyelid-related Purkinje cells that made the mice blink.
Before this breakthrough discovery, neuroscientists were unable to identify the mechanism that allowed individual Purkinje cells to detect a legitimate error signal from the barrage of noise coming from the consistent updates coming from climbing fibers every second.
Why is it impossible to tickle yourself?
Using a new microscopy technique the researchers were able to directly visualize the chemical signaling that occurred between the climbing fibers and Purkinje cells of live, active mice. For the first time, the Penn team has shown that there is a measurable difference between "true" and "false" signals. This knowledge will greatly advance future studies of fine motor control, particularly with regards to how movements can be improved with practice.
Javier Medina explains, “Climbing fibers are not just sensory neurons. What makes climbing fibers interesting is that they don't just say, 'Something touched my face'; They say, 'Something touched my face when I wasn't expecting it.' This is something that our brains do all the time, which explains why you can't tickle yourself. There's part of your brain that's already expecting the sensation that will come from moving your fingers. But if someone else does it, the brain can't predict it in the same way and it is that unexpectedness that leads to the tickling sensation."
Not only does the climbing fiber feedback system for unexpected sensations serve as an alert to potential danger—unstable footing, a knife falling from your hands, a creeping predator coming from behind... it helps the brain improve when an intended action doesn't go exactly as planned.
When I was growing up, my dad would coach me at tennis saying, "Think about hammering and forging the muscle memory of your cerebellum with every stroke." These new discoveries help explain exactly how that process works.
"The sensation of muscles that don't move in the way the Purkinje cells direct them to also counts as unexpected, which is why some people call climbing fibers 'error cells,'" Medina said. "When you mess up your tennis swing, they're saying to the Purkinje cells, 'Stop! Change! What you're doing is not right!' This is how Purkinje cells help you learn how to correct your movements and master a skill.
"When the Purkinje cells get these signals from climbing fibers, they change by adding or tweaking the strength of the connections coming in from the rest of the brain to their dendrites. And because the Purkinje cells are so closely connected to the motor neurons, the changes to those synapses are going to result in changes to the movements that Purkinje cell controls."
This discovery gives new and specific evidence to the mechanisms of neuroplasticity. The fact that new neural pathways become hard-wired in response to error signals from the climbing fibers allows the cerebellum to send better instructions to motor neurons the next time the same action is attempted. This is also why you never forget how to ride a bike.
"What we have found is that the Purkinje cell fills with more calcium when its corresponding climbing fiber sends a signal associated with that kind of sensory input, rather than a spontaneous one," Medina said. "This was a bit of a surprise for us because climbing fibers had been thought of as 'all or nothing' for more than 50 years now."
Conclusion: Purkinje Cells and the Cerebellum Remain Mysterious
The exact mechanism that allows individual Purkinje cells to differentiate between the two kinds of climbing fiber signals remains an open question and more research is needed.
"Something that would be very useful for the brain is to have information not just about whether there was an error but how big the error was—whether the Purkinje cell needs to make a minor or major adjustment," Medina concluded. "That sort of information would seem to be necessary for us to get very good at any kind of activity that requires precise control. Perhaps climbing fiber signals are not as 'all-or-nothing' as we all thought and can provide that sort of graded information"
Knowing that Purkinje cells are able to distinguish when their corresponding muscle neurons encounter an error will probably change future studies of fine motor control. Hopefully, this new discovery from Penn scientists will lead to new research into the fundamentals of neuroplasticity and ways to improve the mastery of any skill through practice, practice, practice.
If you'd like to read more on the cerebellum and Purkinje cells, check out my Psychology Today blog posts:
- "No. 1 Reason Practice Makes Perfect"
- "Toward a New Split-Brain Model: Up Brain-Down Brain"
- "How Is the Cerebellum Linked to Autism Spectrum Disorders?"
- “Childhood Family Problems Can Stunt Brain Development”
- "The Neuroscience of Calming a Baby"
- “Why Is Dancing So Good For Your Brain?”
- “The Neuroscience of Madonna’s Enduring Success”
- "Gesturing Engages All Four Brain Hemispheres"
- "The Neuroscience of Superfluidity"
- "One More Reason to Unplug Your Television"
- "Better Motor Skills Linked to Higher Academic Scores"
- "The Neuroscience of Imagination"
- "Too Much Crystallized Knowledge Lowers Fluid Intelligence"
Follow me on Twitter@ckbergland for updates on The Athlete’s Way blog posts.