Revisiting the gamma loop in ALS.

Amyotrophic Lateral Sclerosis (ALS) is a fast progressive neurodegenerative disease characterized by muscle denervation, weakening and atrophy, which eventually culminates into death, mainly due to respiratory failure. The traditional view of ALS as a disorder affecting selectively motor neurons throughout the central nervous system has been progressively dispelled by innumerous lines of evidence indicating that other cells but motor neurons may be affected as well. Remarkably, this disorder is not limited to the motor system but rather configures as a systemic disease yielding a plethora of clinical signs. Among this broad clinical spectrum, sensory neuropathy occurring parallel to motor dysfunction is a quite frequent feature within ALS patients, which has spurred the interest of many investigators during the years. In line with this, morphological studies have confirmed that sensory neurons and axons' degeneration may occur in both ALS- experimental models and -patients. Noteworthy, this may have a nonetheless negligible role in ALS -related motor decline, as highlighted by recent studies showing that, degeneration of type I/II proprioceptive fibers is a primary source of alpha-motor neurons' death. These latter in fact, differently from gamma motor neurons, are a direct monosynaptic target of proprioceptive fibers. The present findings contribute to define a novel scenario of sensorimotor ALS pathophysiology where the gamma loop's fine connectivity may play a key role. In support to this view, in the present manuscript we provide a reappraisal on the role of single gamma loop's components in ALS.