Corticobulbar Excitability in Parkinson’s Disease: Evidence from TMS-Evoked Potentials of Submental Muscles

Background. The corticobulbar (CB) tract links the motor cortex with orofacial muscles and is closely associated with motivational arousal and reward-related processing. In Parkinson’s disease (PD), where dopaminergic dysfunction is a core feature, alterations in corticobulbar excitability may provide insights into both motor and motivational deficits. Objective. This study investigated whether motor evoked potentials (MEPs) recorded from the submental muscle (SbM) and extensor carpi radialis (ECR) differentiate PD patients from matched healthy controls, and whether SbM excitability is modulated by acute dopaminergic stimulation. Methods. Twenty-two participants (10 PD patients, 12 healthy controls) were tested using transcranial magnetic stimulation (TMS). Patients underwent stimulation in both OFF (≥12 h levodopa withdrawal) and ON (1 h after levodopa) conditions. MEP amplitude, onset latency, and elicitation rate were extracted. Linear and generalized linear mixed models tested between- and within-group effects. Results. At baseline, patients showed a higher proportion of elicited MEPs in ECR compared to controls (p = 0.045), but no differences for SbM. Within patients, levodopa intake was associated with longer SbM onset latency (p = 0.012), reduced SbM elicitation rate (p = 0.034), and a robust increase in ECR amplitude (p < 0.001). Conclusions. These findings suggest a dissociation between control (ECR) and target (SbM) excitability, SbM results differently sensitive to dopaminergic state, supporting its potential linkage to broader dopaminergic network, comprising corticobulbar and motivational function that are disfunctional in PD.Keywords: corticobulbar tract; submental EMG; Parkinson’s disease; transcranial magnetic stimulation; motor evoked potentials; motivational arousal.