The role of the human lateral pterygoid muscle in the control of horizontal jaw movements

There is a limited understanding of the normal function of the lateral pter ygoid muscle (LP) and the role that this muscle plays in temporomandibular disorders. This article addresses the hypothesis that a major function of t he LP is in the control of horizontal jaw movements. The range of fiber ali gnments suited to generating a major horizontal force vector (magnitude and direction), together with the likelihood of independent activation of subc ompartments (that is, functionally heterogeneous zones) within each head, p rovide the possibility of a finely graded range of force vectors on the con dyle to effect the fine control of horizontal jaw movements. This level of control does not appear to extend to the control of resting jaw posture, as recent single motor unit (SMU) data indicate that the LP is inactive with the jaw in the postural jaw position. Available electromyographic data demo nstrate graded changes in multiunit and SMU activity with small horizontal jaw displacements at low force levels, a single preferred direction of the SMU firing rate during horizontal isometric jaw tasks, and graded changes i n the SMU firing rate with horizontal force magnitude and direction. The ev idence suggests that a major function of the LP is in the generation and fi ne control of the horizontal component of jaw movement by the graded activa tion of a subset of SMUs within the LP. The data also suggest that the LP i s involved in the generation of horizontal force vectors, as required in pa rafunctional activities and heavy mastication.