H-reflexes are used to determine the reflex connections of muscle spindle afferents, the excitability of the motoneuron pool and the integrity of the reflex pathways. However, H-reflexes are difficult to elicit in the masseter, limiting their use in the investigation of the masticatory system. H-reflexes may be difficult to elicit in masseter due to the nature of the input from the muscle spindles onto the motoneurons. Thus, this study investigated the recruitment of masseter motoneurons into the H-reflex, to determine the distribution of the effective muscle spindle input.
H-reflexes were recording in simultaneously-active large and small motoneurons. The size of motor units was determined by their representation in the Macro EMG (MacroRep). H-reflexes were found to be more prevalent in large motoneurons than in small ones. This indicates that either Ia input is preferentially distributed to large motoneurons, or that the input to small motoneurons is pre-synaptically inhibited.
A more effective input onto large motoneurons suggests that the stretch reflex in masseter may have a limited role in the maintenance of static posture. The role of the masseter stretch reflex during mastication is likely to be for the development of large, fast forces when an unexpected resistance is encountered. More effective input onto large motoneurons would also allow for the development of large, fast bite forces when the gamma system is activated during attacking or fighting situations.