Circular continuum of alpha motoneuron types

Alpha-motoneurons (-MNs) are traditionally classified into slow (S), fast fatigue-resistant (FR), and fast-fatigable (FF), which exist along a continuum of properties between slow and fast, enabling the generation of graded force and seamless movement. Using combinations of markers, we developed novel immunohistochemistry protocols that enabled co-labeling of six major and transitional -MN types throughout the mouse lumbar spinal cord with unprecedented detail. Intriguingly, our protocols labeled for the first time: -MNs of the fast fatigue intermediate (FI) type; a previously undescribed transitional -MN subtype (FR/FI); and a novel subtype of -MNs exhibiting hybrid characteristics of both S and FF types - termed S/FF - which resist ALS degeneration. Electrophysiological recordings confirmed FR/FI and S/FF subtypes, both exhibiting mixed traits. The discovery of S/FF subtype reveals that -MNs exist along a circular continuum between slow and fast types, challenging the traditional linear model and reshaping our understanding of their role in motor control. New & NoteworthyThis study introduces a novel immunohistochemistry protocol to co-label six distinct types of adult spinal cord motor neurons. This technical innovation led to a conceptual paradigm shift, revealing a circular continuum of motor neuron subtypes rather than a linear one. This new framework provides unprecedented precision for studying their varying susceptibility to degeneration in diseases like ALS, with broad implications for understanding motor control in health and disease.