Single motor units from the medial pterygoid muscle can be active during isometric horizontal and vertical forces

Abstract

Objectives To determine (a) whether the medial pterygoid muscle is active in an isometric vertical force task and in isometric horizontal force tasks in the contralateral, protrusion and ipsilateral directions; (b) whether the same single motor units (SMUs) could be active across different directions of isometric force generation; and (c) whether different regions of the medial pterygoid muscle exhibit different patterns of SMU activation during the generation of any one direction of isometric force. Methods Intramuscular electromyographic (EMG) recordings were made from the right medial pterygoid muscle in 15 healthy participants during isometric force tasks: vertical and horizontal contralateral, protrusion and ipsilateral. A computed tomography scan divided the EMG recording site into a medial or lateral part in each participant. Single motor units were discriminated in each task. Results Medial pterygoid SMU activity was recorded in 100% of participants for the vertical biting tasks, 86% of participants for the horizontal contralateral and horizontal protrusion tasks and 57% of the horizontal ipsilateral tasks. Of the 72 SMUs that were discriminated, 36% were active in all tasks; 18% were active only in the vertical tasks and 17% were active in the vertical, horizontal contralateral and horizontal protrusion tasks. The proportion of SMUs that was active in at least 1 horizontal task in the lateral part (33/39) was significantly higher than the proportion (21/33) in the medial part (Chi‐Square, P < 0.05). Conclusion The data are consistent with a stabilisation role for the medial pterygoid muscle in isometric jaw forces in the vertical and horizontal planes.

Threshold variations of medial pterygoid single motor units during vertical or horizontal force tasks

“…Evidence from electromyographic (EMG) studies also indicates the medial pterygoid is active during jaw tasks in the closing direction, namely in jaw clenching and chewing, as well as tasks in the horizontal plane including contralateral (ie towards the side opposite to the muscle), ipsilateral (ie towards the same side as the muscle) and protrusive directions. [5][6][7][8][9][10] The possibility of a wide range of force vectors from a complex internal muscle architecture can only be realised if individual or groups of single motor units (SMUs) can be selectively activated within the muscle independently of SMUs in other regions of the muscle. This selective activation within a muscle is termed functional heterogeneity and this property has been described previously in the human spinal motor system 11 and in the human jaw-motor system both for the jaw-closing and jaw-opening muscles including the medial pterygoid muscle.…”

“…This selective activation within a muscle is termed functional heterogeneity and this property has been described previously in the human spinal motor system 11 and in the human jaw-motor system both for the jaw-closing and jaw-opening muscles including the medial pterygoid muscle. 5,[8][9][10][12][13][14][15][16] There is also evidence that the medial pterygoid muscle works in a synergistic role with other jaw muscles to help stabilise mandibular position. For example, whilst the lateral pterygoid muscle is well-established as the prime agonist for generating contralateral and protrusive jaw movements and forces, 12,13,[16][17][18] the presence of medial pterygoid activity throughout these movements and forces with the teeth apart suggests a role for the medial pterygoid in stabilising vertical jaw position in these tasks.…”

“…For example, whilst the lateral pterygoid muscle is well-established as the prime agonist for generating contralateral and protrusive jaw movements and forces, 12,13,[16][17][18] the presence of medial pterygoid activity throughout these movements and forces with the teeth apart suggests a role for the medial pterygoid in stabilising vertical jaw position in these tasks. 8,9 Similarly, whilst the temporalis and masseter muscles appear to be the prime agonists for the generation of jaw-closing movements and forces, the medial pterygoid muscle may play more of a synergistic role in helping to stabilise horizontal jaw positions and movements throughout closing force generation. 8,9 Some of this evidence for a stabilisation role for the medial pterygoid muscle has derived from previous studies demonstrating that the same SMU could be active during different task directions, namely, vertical jaw closing, as well as horizontal movements and forces.…”

“…8,9 Similarly, whilst the temporalis and masseter muscles appear to be the prime agonists for the generation of jaw-closing movements and forces, the medial pterygoid muscle may play more of a synergistic role in helping to stabilise horizontal jaw positions and movements throughout closing force generation. 8,9 Some of this evidence for a stabilisation role for the medial pterygoid muscle has derived from previous studies demonstrating that the same SMU could be active during different task directions, namely, vertical jaw closing, as well as horizontal movements and forces. 8,9 Investigations into the relationship between SMU force thresholds (ie the force at which a motor unit reliably fires) and the rate of force development in isometric force tasks provides another potential line of evidence for a role of the medial pterygoid muscle in stabilising the jaw.…”

“…8,9 Some of this evidence for a stabilisation role for the medial pterygoid muscle has derived from previous studies demonstrating that the same SMU could be active during different task directions, namely, vertical jaw closing, as well as horizontal movements and forces. 8,9 Investigations into the relationship between SMU force thresholds (ie the force at which a motor unit reliably fires) and the rate of force development in isometric force tasks provides another potential line of evidence for a role of the medial pterygoid muscle in stabilising the jaw. Generally, primary agonist muscles demonstrate an inverse relationship between SMU force threshold and the rate of force development.…”