Understanding Spinal Muscular Atrophy (SMA) and the related SNM Protein

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Brain Compensation After Motor-Neuron Loss: What Polio Research May Reveal About SMA

The key idea

The brain appears to compensate—or “rewire”—when motor neurons are lost, and researchers may be able to measure this compensation non-invasively. This could provide a useful way to understand neurological adaptation in spinal muscular atrophy (SMA) and potentially help monitor changes in motor-network function over time.

SMA primarily involves degeneration of lower motor neurons, resulting in progressive muscle weakness and atrophy. However, the effects of motor-neuron loss may extend beyond the spinal cord. Research suggests that the brain's motor networks can undergo functional reorganization, potentially recruiting additional or different cortical regions to help maintain movement.

Why researchers studied polio

Researchers investigated adults who survived poliomyelitis (polio) in childhood because polio damages lower motor neurons/anterior horn cells, making it a useful model for studying what happens to the motor system many years after early motor-neuron loss.

This is relevant to SMA because SMA also involves degeneration of lower motor neurons. The comparison does not mean that polio and SMA are the same disease: polio is an infectious disease, whereas SMA is generally a genetic disorder associated with insufficient SMN protein. Rather, the shared loss of lower motor neurons provides an opportunity to investigate whether the nervous system develops similar compensatory responses.

How the researchers measured brain–muscle communication

The researchers recorded two signals simultaneously while participants performed an isometric pincer-grip task:

EEG (electroencephalography) measured electrical activity from the brain, while EMG (electromyography) measured electrical activity from the hand muscles.

They then calculated cortico-muscular coherence (CMC).

CMC can be thought of as a measure of how strongly rhythmic electrical activity in the brain is synchronized with activity in the muscles during movement. In other words, it provides a window into the functional communication between the cortex and the peripheral motor system.

What the polio study found

The study found an abnormal CMC pattern in polio survivors that was not observed in healthy controls. In particular, significant synchronization occurred in low-gamma frequencies (approximately 30–47 Hz) across frontal and parietal brain regions.

Rather than activity being restricted to the motor regions normally expected to dominate control of the task, the results indicated broader engagement of cortical networks.

The researchers interpreted this as evidence of functional reorganization of the cortical motor network following earlier lower-motor-neuron damage.

Why this matters for SMA

The important implication is that losing lower motor neurons may trigger changes upstream in the brain.

This fits with the normal organization of movement: upper motor neurons originate in the brain and communicate with lower motor neurons in the spinal cord and brainstem, which ultimately activate muscles. If part of this downstream motor system is lost, the central nervous system may reorganize how remaining pathways are recruited.

Researchers have also directly examined CMC in adults with SMA. In a study of adults with SMA and healthy controls, simultaneous high-density EEG and surface EMG recordings during precision-grip tasks revealed abnormal CMC involving frontal, central, and parietal regions. The investigators concluded that these patterns were consistent with compensatory changes involving broader cortical regions.

CMC as a possible non-invasive biomarker

This raises an especially interesting possibility:

EEG + EMG → CMC analysis → measurable indicator of motor-network reorganization

Because EEG and surface EMG are non-invasive, CMC could potentially become a neurophysiological biomarker—an objective measurement of how brain–muscle communication changes in SMA.

Such a biomarker might eventually help researchers:

  • quantify compensatory motor-network activity;
  • investigate how the brain responds to progressive motor-neuron loss;
  • follow neurological changes longitudinally;
  • compare patients or stages of disease; and
  • investigate whether disease-modifying treatments alter central–peripheral motor-network function.

The SMA researchers specifically proposed CMC as a potential biomarker for monitoring drug efficacy in future clinical trials.

Bottom line

Motor-neuron loss may change not only the muscles and spinal motor system but also how the brain organizes movement. Studies of polio survivors—and preliminary research directly in SMA—suggest that the brain may recruit broader motor networks as a compensatory response.

By measuring synchronization between brain activity and muscle activity with EEG, EMG, and cortico-muscular coherence, researchers may have a relatively inexpensive, non-invasive way to detect this reorganization.

If validated in larger and longitudinal SMA studies, CMC could potentially complement genetic, clinical, electrophysiological, and imaging measures by providing information about how the functioning brain–muscle network adapts to motor-neuron loss and responds to therapy.

Important limitation

These findings should not be interpreted as showing that CMC is currently an established diagnostic or treatment-monitoring test for SMA.

The polio study demonstrated altered brain–muscle synchronization after longstanding lower-motor-neuron injury and proposed that the approach could be relevant to SMA. The direct SMA work also found pathological CMC patterns, but it involved a small study population.

Therefore, the strongest conclusion at present is that CMC is a promising research biomarker of motor-network reorganization that requires further validation before routine clinical use.


References: 

Altered supraspinal motor networks in survivors of poliomyelitis: A cortico-muscular coherence study
https://pubmed.ncbi.nlm.nih.gov/33271481/

Neuroepithelial Tumor with AAV Integration after Intracisternal Magna Vector Delivery
https://www.nejm.org/doi/10.1056/NEJMoa2601608 

Playing Genetic Roulette | Nigel Laing | TEDxUWA
https://www.youtube.com/watch?v=ZkOLTfEyLXg&t=11

Update: Risdiplam for Prenatal Therapy of Spinal Muscular Atrophy
https://www.nejm.org/doi/full/10.1056/NEJMc2300802

Comparative Clinical Outcomes of Nusinersen and Gene Therapy in Spinal Muscular Atrophy Type 1
https://jamanetwork.com/journals/jamanetworkopen/fullarticle/2839761?guestAccessKey=17ecf7ec-e3eb-441f-ab9c-6a9b73ade32e&utm_medium=email&utm_source=postup_jn&utm_campaign=article_alert-jamanetworkopen&utm_content=new_this_week_&utm_term=100825

FDA approves innovative gene therapy to treat pediatric patients with spinal muscular atrophy, a rare disease and leading genetic cause of infant mortality
https://www.fda.gov/news-events/press-announcements/fda-approves-innovative-gene-therapy-treat-pediatric-patients-spinal-muscular-atrophy-rare-disease

Rare genetic disorder treated in womb for the first time
https://www.nature.com/articles/d41586-025-00534-0?linkId=13041268

First-in-human study of epidural spinal cord stimulation in individuals with spinal muscular atrophy
https://www.nature.com/articles/s41591-024-03484-8

Learn About Spinal Muscular Atrophy

Spinal Muscular Atrophy 2019: Later Onset Spinal Muscular Atrophy
https://www.youtube.com/watch?v=W3MJq9IzSFc

Explore more about SMA and discover how ongoing research continues to pave the way for new treatments and hope for the future.

Improving Outcomes in Spinal Muscular Atrophy:
https://www.youtube.com/watch?v=xNZs7b1_XDk

For more information,  
Spinal Muscular Atrophy (SMA) page on MalaCards. 

My personal experience with SMA:
https://swaresearch.blogspot.com/2024/01/is-me-cfs-spinal-muscular-atrophy.html


© 2000-2030 Sieglinde W. Alexander. All writings by Sieglinde W. Alexander have a fife year copy right.
Library of Congress Card Number: LCN 00-192742 ISBN: 0-9703195-0-9

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