Transcranial magnetic-stimulation mapping of the cortical topography of the human masseter muscle

The cortical topography of several limb and trunk muscles has been disclosed using transcranial magnetic stimulation, but the corticomotor representation of the human jaw muscles has not yet been described. An experimental paradigm incorporating transcranial magnetic stimulation of verified sites on the scalp was used (a) to determine the cortical topography of the human masseter muscle and (b) to assess the reproducibility of the motor map, in seven healthy individuals. The results showed that the masseter was discretely represently on the motor and premotor cortex with interindividual variation in map area, volume and height (p<0.05). Coefficients of variation and reliability were low (15–18%) and high (89–96%), respectively, for motor maps obtained at experimental sessions 2 weeks apart, indicating high reproducibility. The findings have clinical relevance in the management of patients with cerebral injury involving corticobulbar projections to the masseter.