Functional properties of single motor units in inferior head of human lateral pterygoid muscle: Task relations and thresholds

The aim of this study was to clarify the normal function of the inferior he ad of the human lateral pterygoid muscle (IHLP). The hypothesis was that an important function of the IHLP is in the fine control of horizontal jaw mo vements. The activities of 99 single motor units (SMUs) were recorded from IHLP (22 recordings from 16 subjects). Most recording sites were identified by computer tomography (CT). All 99 SMUs were active during contralateral jaw movements with the teeth apart, and protrusive jaw movements with the t eeth apart, and 81% (48 of 59 units studied during all 3 tasks) were active during submaximal jaw-opening movements. None were active on maximal ipsil ateral or retrusive jaw movements with the teeth apart nor on jaw closing/c lenching in intercuspal position; nor were they spontaneously active when t he jaw was at the clinically determined postural jaw position. Thresholds o f SMUs ranged from <0.2 mm of contralateral or protrusive horizontal displa cements to 61-89% of the maximum contralateral or protrusive displacement, respectively. For the 35 units continuously active during the contralateral task, 23 (66%) were recruited within 2 mm of contralateral displacement [2 5 (63% of 40 units) for protrusion]. Recruitment thresholds (mm) of some of the units were rate dependent with thresholds significantly decreasing wit h increasing rate of horizontal jaw movement in protrusion and contralatera l movements. At eight recording sites where up to six SMUs were able to be discriminated, the average thresholds of successively recruited SMUs were w ithin a 1-mm increment of horizontal jaw displacement. After dividing IHLP into four regions, the SMUs recorded in the superior-medial zone exhibited significantly lower mean threshold values than for the SMUs recorded in the other zones (no units were recorded in the inferior-lateral zone). This pr ovides suggestive evidence supporting previously proposed notions of functi onal heterogeneity within IHLP. Taken together, the data suggest that speci fic regions of the IHLP are capable of selective activation in a finely con trolled manner to allow the application of the appropriate force vector (ma gnitude and direction) to effect the required condylar movement needed for the generation and control of horizontal jaw movements.