The Intricate Role of the "Pause-and-Play" in Mammalian Movement
In the intricate web of mammalian motor function, the concept of “pause-and-play” introduces a refreshing perspective on movement and sensory stimulation. Contrary to what some might think, pausing isn’t merely an absence of movement. It signifies an arrest of movement while activity continues in the spinal cord and nerve cells that control the muscles. In this state, although the body halts, it doesn’t collapse but maintains a form of active inertia.
“We have identified a group of nerve cells in the midbrain which, when stimulated, stop all movement. Not just walking; all forms of motor activity. These cells can cause mice to either stop breathing or breathe more slowly, and even their heart rate decreases,” explains Professor Ole Kiehn, a Danish-Swedish neuroscientist and a co-author of the groundbreaking study. Published on 27 July 2023 in Nature Neuroscience, his team’s research has garnered significant attention. “There are several mechanisms to halt movement. Yet, these particular nerve cells stand out. Once activated, they cause movements to pause or freeze, akin to hitting the pause button on a movie, causing actors to freeze in place,” Kiehn elaborates.
Recent studies on motor control have taken a deep dive into how the brain coordinates movement. Through examination of various circuits in the nervous system, including the spinal cord—the epicenter of brain signals—researchers have zoomed in on the pedunculopontine nucleus (PPN). Earlier studies have pinpointed nerve cells that facilitate locomotion. However, this research has unearthed a unique set of neurons within the PPN. When these are stimulated, they demonstrate a striking “pause and play” pattern, arresting all motion. Such a distinctive response is unmatched and voluntary distinguishing it from other motion-halting mechanisms (e.g., involuntary fear-induced “freeze” responses) previously detected in the brainstem.
Professor Ole Kiehn boasts an illustrious career with dual roles. He is the Professor of Integrative Neuroscience at the Department of Neuroscience, University of Copenhagen, Denmark, and holds the title of Professor of Neurophysiology at the Karolinska Institute in Sweden. His groundbreaking work is a testament to the importance of this discovery.
The pause induced by sensory stimulation from the environment is believed to heighten attention towards that particular stimulus. If deemed non-threatening or irrelevant, movement resumes. This behavior can be observed in the wild, where animals like stalking cats or prowling lions voluntarily exhibit this pause, offering them a heightened, hyper-attentive stance to focus on their prey while preparing to attack or avoid detection.
An intriguing aspect of this mechanism is its influence on critical bodily systems. During this pause, the respiration rate drops, even halting at times, and there’s a noticeable slowing of the heartbeat. Far from being a state of heightened alert, it resembles a focused, attentive posture.
The onset of this pause is remarkably swift, occurring in about 200 milliseconds. Such rapidity suggests the nervous system harbors a memory-like function, recalling prior action and prepping for its continuation after the pause. The nuances of this process, however, are still under extensive investigation.
For routine motions, such as walking, it’s noteworthy that the brain doesn’t relay intricate, step-by-step instructions to muscles. Instead, it sends high level, executive directives to the spinal cord, where neuron circuitry creates rhythm and ensures coordination among various muscles similar to a pre-programmed routine. This rhythmic pattern can be interrupted on executive command and then swiftly resumed, as evidenced in the study.
The potential relation between nerve cells and heightened attention during these pauses could steer the direction of further research. While current findings don’t conclusively associate movement arrest with heightened attention or vice versa, they hint at a compelling interplay worth further exploration.
This study’s implications stretch far and wide, potentially illuminating disorders like Parkinson’s disease, which is characterized by motor symptoms such as slowed movement, tremors, and freezing gait. By exploring the intricacies of this motor arrest, there’s hope to shed light on these challenging symptoms, potentially leading to novel treatments.
In the upcoming decade, research is poised to extend into deeper realms of motor control, focusing on the brain’s regions and discerning how advanced brain functions influence motor actions. Through these studies, there’s potential to bridge the chasm between motor output and cognitive processes, potentially illuminating facets of consciousness and decision-making.
While therapeutic treatments based on these discoveries remain on the horizon, this foundational research sets the stage for potential future innovations. There is an optimistic view that these findings might eventually elevate the quality of life for individuals battling movement disorders.