Intrinsic properties of spinal motoneurons degrade ankle torque control in humans
Bibliographic record
Abstract
Motoneurons are the final common pathway for all motor commands and possess intrinsic electrical properties that must be tuned to control muscle across the full range of motor behaviours. Neuromodulatory input from the brainstem is probably essential for adapting motoneuron properties to match this diversity of motor tasks. A primary mechanism of this adaptation, control of dendritic persistent inward currents (PICs) in motoneurons by brainstem monoaminergic systems, generates both amplification and prolongation of synaptic inputs. While essential, there is an inherent tension between this amplification and prolongation. Although amplification by PICs allows for quick recruitment and acceleration of motoneuron discharge, PICs must be deactivated to derecruit motoneurons upon movement cessation. In contrast, during stabilizing or postural tasks, PIC-induced prolongation of synaptic inputs is critical for sustained motoneuron discharge. Here, we designed two motor tasks that challenged the inhibitory control of PICs, generating unduly PIC prolongation that increases variability in human torque control. This included a paradigm combining a discrete motor task with a stabilizing task and another involving muscle length-induced changes to the balance of excitatory and inhibitory inputs available for controlling PICs. We show that prolongation from PICs introduces difficulties in ankle torque control and that these difficulties are further degraded at shorter muscle lengths when PIC prolongation is greatest. These results highlight the necessity for inhibitory control of PICs and showcase issues introduced when inhibitory control is constrained. Our findings suggest that, like sensory systems, errors are inherent in motor systems. These errors are not due to problems in the perception of movement-related sensory input but are embedded in the final stage of motor output. This has many implications relevant to clinical conditions (e.g. chronic stroke) where pathological shifts in monoamines may further amplify these errors. KEY POINTS: All motor commands are processed via spinal motoneurons, whose intrinsic electrical properties are adapted by brainstem neuromodulatory input. The effects of these neuromodulatory inputs (i.e. persistent inward currents; PICs) must be tightly regulated by inhibitory inputs to allow for the large repertoire of human motor behaviours. We designed two motor tasks to restrict the ability of inhibitory synaptic inputs to control PICs and show that this generates substantial errors that reduce the precision of motor output in humans. Our findings suggest that errors are inherent in motor systems and embedded in the final stage of motor output. This has many implications relevant to clinical conditions (e.g. chronic stroke) and may, speculatively, shed light on contributing factors to muscle cramps.
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How this classification was reachedexpand
Full frame machine prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. The Gemma side is a direct model label for every work in the frame, read from the title-only record. The Codex side is a classifier learned from the 10,348 direct Codex labels and calibrated to design-weighted sample rates; fields without enough sample support carry no Codex call. Candidate is the union of the two sides; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels.
Distilled classifier scores by category (both heads)
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.000 | 0.002 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.000 | 0.000 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.000 | 0.000 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.001 | 0.000 |
Machine scores (provisional)
The two teacher heads of the student model, read on this work. A score orders the frame for review; it never asserts a category, and the validation status ships verbatim with every row.
Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
score_only:v0-immature-baseline · verbatim from the scoring run: score_only means the number may rank works, and no category label ships from itClassification
machine, unvalidatedMachine predicted; a candidate call from one source (direct Gemma or distilled Codex), not a consensus.
How this classification was reached, model by model and score by score, is at the end of the page under "How this classification was reached".