Revisiting the muscle spindle in ALS: Implications for spinal motor network dysfunction.

Amyotrophic lateral sclerosis (ALS) is typically characterized by progressive degeneration of upper and lower motor neurons. However, the contribution of afferent sensory pathways to spinal motor network disorganization has received little attention. Converging human histological evidence from multiple independent laboratories demonstrates that fusimotor (gamma) motor neuron innervation of muscle spindles degenerates in ALS while sensory endings remain morphologically intact at the end organ, although the afferent neurons and ascending sensory pathways are also affected. This dissociation creates unmodulated proprioceptive input. Spindle-derived sensory signals reach spinal interneuron circuits without fusimotor gain-control adjustment, degrading spindles from forward sensory models encoding predicted kinematic states into crude stretch receptors. We propose that this gain-control failure, compounded by the loss of associated alpha-gamma coactivation and beta skeletofusimotor denervation, constitutes an upstream driver of the spinal inhibitory interneuron circuit dysfunction documented by recent electrophysiological studies in ALS. The resulting early disinhibition, followed by maladaptive excitation, mirrors cortical ALS pathophysiology. However, transgenic mouse ALS models show selective sparing of gamma motor neurons, unlike the human disease, constituting a critical translational limitation. We present a temporal model of human ALS, linking fusimotor denervation to progressive disruption of spinal inhibitory and excitatory circuits.