How did animal bodies, including our own, evolve into such precise movement machines? This study dives deep into important discussions regarding the interaction between the ancient spinal cord and the relatively modern human brain, which together create smooth movements. Additionally, it explores how various neurological disorders can throw off this delicate balance, leading to issues such as slow, imprecise, or jerky movements. It contributes to the understanding of how sensory information is processed and reflexes are controlled during voluntary movement, with significant implications for motor disorders caused by conditions like stroke, cerebral palsy, and Parkinson’s disease.
How did the bodies of animals, including humans, evolve into such precise movement machines? Francisco Valero-Cuevas’ Lab at USC’s Alfred E. Mann Department of Biomedical Engineering is investigating the coordination of the constant struggle between involuntary reflexes and voluntary movements. Their latest computational study, published in the Proceedings of the National Academy of Sciences (PNAS), contributes to understanding how sensory information is processed and reflexes are managed in voluntary movements—showing how disruptions can lead to motor disorders in neurological conditions such as strokes, cerebral palsy, and Parkinson’s disease.
Do you recall a pediatrician tapping your knee to check if you had a strong knee-jerk reflex? This classic test evaluates the stretch reflexes in your spinal cord, which naturally resist muscle stretching to maintain muscle tone, helping to keep your body upright against gravity or to make quick corrections after stumbling. The debate on how these reflexes are adjusted or suppressed for smooth voluntary movement dates back to the foundational research of Charles Scott Sherrington in the 1880s (yes, the 1880s!). This recent study addresses critical discussions about how the ancient spinal cord and the more advanced human brain interact to facilitate smooth movement, along with how certain neurological conditions can disrupt this harmony, leading to slow, inaccurate, or jerky movements.
The research, led by Grace Niyo, a doctoral student in Biomedical Engineering, reveals the existence of a potentially undiscovered system or circuitry within the spinal cord that, when functioning properly, helps “modulate” reflexes during voluntary movements. According to Niyo, this study suggests “a theoretically new mechanism to modulate spinal reflexes at the same level as stretch reflexes within the spinal cord.”
Valero-Cuevas, a Professor at USC specializing in Biomedical Engineering, Aerospace and Mechanical Engineering, Electrical and Computer Engineering, Computer Science, and Biokinesiology and Physical Therapy, is the lead author of the paper titled “A computational study of how α- to γ-motoneurone collateral can mitigate velocity-dependent stretch reflexes during voluntary movement.”
He comments, “Reflexes are intricate and ancient low-level information exchange methods that evolved alongside and adapted to the advancements of the human brain… comprehending their cooperation with the brain is essential for understanding movement, both in health and disease.”
Valero-Cuevas continues, “We continuously benefit from and adjust stretch reflexes, whether we are aware of it or not, as we stand, move, and act.”
His lab is focused on understanding neuromuscular control in animals and robots, which could inform clinical rehabilitation strategies for human mobility.
He elaborates, “Even though our brain is highly sophisticated, we must recognize the significance and strength of the ancient spinal cord — a system that has enabled vertebrates to flourish for millions of years before the evolution of large brains. We seek to comprehend how the spinal cord can facilitate smooth movements, often with minimal brain input, as seen in amphibians and reptiles. This understanding could profoundly influence the treatment and comprehension of movement disorders pertaining to the brain, spinal cord, or both — and could also lead to creating biologically-inspired prosthetics or robots moving smoothly using simulated spinal cords.”
In their simulation experiment, the team, led by Niyo, designed a biomechanical model of a macaque monkey’s arm using a physics simulation software called MuJoCo, producing over a thousand reaching movements. The basic tenet of the stretch reflex is that while muscles experiencing a stretch will resist it, shortening muscles do not exhibit stretch reflexes. They first established that unregulated stretch reflexes indeed cause self-perturbations that hinder voluntary arm movements. Then, they introduced a basic spinal circuit where the same spinal cord neurons controlling muscle force (known as alpha motoneurons) also adjust (or modulate) stretch reflexes in proportion to the muscle excitation levels. This means that highly excited muscles would exhibit strong stretch reflex responses when stretched, and vice versa. Their findings indicated that this straightforward rule — physiologically feasible due to the known pathways from alpha motoneurons (also known as collaterals) to the reflex system — could substantially correct the self-perturbations from stretch reflexes, resulting in smoother and more precise voluntary movements.
From a contemporary engineering standpoint, Valero-Cuevas likens this process to “edge computing,” the concept where information processing occurs at the source (limb sensors and the spinal cord) rather than solely at the central command (the brain) — similar to some smartphone applications that pre-process information before sending it to a cloud server. He draws a mechanical analogy, stating that these low-level connections with the reflex circuitry act like “training wheels on a bicycle, there to help you have fun and support you if you make a mistake while learning to ride.”
These circuits might facilitate novel voluntary movements with minimal disturbances but would still allow the brain and cerebellum to refine and enhance reflex control as the nervous system matures or gains more experience.
Implications: In addition to improving our understanding of movement disorders, Niyo suggests that this discovery could lay the foundation for researchers to seek out and investigate such spinal circuits. “This work could serve as a source of inspiration and direction for new therapies targeting the appropriate levels of the nervous system to treat movement disorders such as strokes and cerebral palsy,” say Niyo and Valero-Cuevas.
The study’s additional co-authors include Lama I. Almofeez, a PhD student in USC’s Alfred E. Mann Department of Biomedical Engineering, and Andrew Erwin, who, at the time of the study, was a post-doctoral scholar in USC’s Division of Biokinesiology and Physical Therapy.
