W2 Gating Failure in Motor Neurone Disease: TRN Gate Coherence Loss as a Proposed Upstream Cause of Motor Cortical Hyperexcitability — A Pre-Symptomatic Intervention Hypothesis

Motor neurone disease (MND/ALS) is characterised by progressive degeneration of upper and lower motor neurons. The dominant mechanistic account — the dying forward hypothesis — correctly identifies motor cortical hyperexcitability as an early and specific feature of ALS that drives lower motor neurone death via glutamate excitotoxicity, but does not identify why motor cortex becomes hyperexcitable. We present this paper as the ninth application of the Return Constraint Model (RCM) W2 gating framework (Conway, 2026a, submitted to Neuroscience of Consciousness), which has established the thalamic reticular nucleus (TRN) alpha gate as the common circuit mechanism underlying a family of neurological conditions, with the gate mechanism rated CCT-3.5 Harvey-Level Established across eight independent prior clinical domains and two independent external confirmations (Wehmeyer et al., 2025; Dai et al., 2024). The MND application rests on two independently confirmed anatomical facts: (1) Layer 5 Betz cells are the primary drivers of TRN motor sector activity via monosynaptic corticothalamic projections (Hadinger et al., 2023; Carroll et al., 2022); and (2) Betz cells in layer Vb of the primary motor cortex are among the first and most severely affected neurons in sporadic ALS (Braak et al., 2013). The mechanistic connection between these two facts — that progressive Betz cell loss removes the primary drive source sustaining TRN motor sector gate coherence, and that gate coherence failure is upstream of motor cortical hyperexcitability — is an original hypothesis of this programme that has not previously been stated in the ALS literature. We further propose the Betz Cell Protection Hypothesis: that motor cortical hyperexcitability is not only a consequence of gate failure but an excitotoxic accelerant of further Betz cell degeneration, creating a self-amplifying loop. Evidence that cortical hyperexcitability alone is sufficient to drive upper motor neurone degeneration in wild-type mice (Haidar et al., 2022) provides biological support for this extension. Four falsifiable predictions follow. The most immediately testable — TRN alpha coherence in the motor sector declines before cortical hyperexcitability reaches clinical threshold — requires no new device and can be assessed in existing presymptomatic genetic carrier cohorts using TMS-EEG.