Lobbezoo F, van der Glas H W, Buchner R, van der Bilt A, Bosman F
Department of Oral-Maxillofacial Surgery, University of Utrecht, The Netherlands.
Exp Brain Res. 1993;93(1):129-38. doi: 10.1007/BF00227787.
The control of mandibular posture has been related to the activity of the anterior temporal muscles, whereas the masseter muscles have been viewed mainly as force producers. However, these groups of muscles, especially in the deep layers, are highly endowed with muscle spindles, so that a difference in function should imply a difference in the reflex sensitivity. By studying the jaw-jerk reflex by means of bipolar surface electromyogram, the reflex sensitivity was determined from relationships between reflex amplitude and jaw displacement from both groups of muscles in eight subjects. At a constant level of background muscle activity, and hence with a constant excitability of the alpha motoneurons, the reflex sensitivity can be determined from these relationships in terms of gain and threshold. In order to account for differences in thickness of the soft tissues overlying the various muscles studied, the reflex amplitude was normalized with respect to the level of maximal voluntary contraction (MVC). In experiments where the inter-electrode distance was 18 mm over both groups of muscles, the reflex gain of the anterior temporal muscles was larger than that of the masseter muscles (P < 0.05). The threshold value did not differ significantly from zero for either group of muscles. Normalization of the reflex amplitude with respect to MVC can be carried out correctly only if the reflexly activated muscle fibres are distributed uniformly within the muscle. In order to gain an insight into this distribution, control experiments were performed with three subjects in which the inter-electrode distance was varied, thus influencing the depth in the muscle from which active muscle fibres were recorded. The reflex gain of the masseter muscle with an inter-electrode distance of 22 mm, which records from deeper layers of the muscle as well as superficial ones, was larger than with a distance of 11 mm (P < 0.01-0.10 in the various subjects). No such differences were found for the anterior temporal muscle. It was concluded that the afferents of the spindles in the jaw-elevator muscles do not project uniformly upon the motoneurons, but involve mainly fibres in the deeper layers of the muscle. The difference in gain found between the masseter and the anterior temporal muscles with the same inter-electrode distance of 18 mm is likely to be due to a larger distance of the reflexly activated muscle fibres in the masseter muscle with respect to the electrodes rather than to a difference in reflex sensitivity between the muscles.(ABSTRACT TRUNCATED AT 400 WORDS)
下颌姿势的控制与颞前肌的活动有关，而咬肌主要被视为力量产生者。然而，这些肌群，尤其是深层的肌群，富含肌梭，因此功能上的差异应意味着反射敏感性的差异。通过双极表面肌电图研究下颌反射，在8名受试者中根据两组肌肉的反射幅度与下颌位移之间的关系确定反射敏感性。在背景肌肉活动水平恒定，即α运动神经元兴奋性恒定的情况下，可根据这些关系从增益和阈值方面确定反射敏感性。为了考虑所研究的不同肌肉上方软组织厚度的差异，反射幅度相对于最大自主收缩（MVC）水平进行了归一化。在两组肌肉电极间距均为18mm的实验中，颞前肌的反射增益大于咬肌（P<0.05）。两组肌肉的阈值与零均无显著差异。只有当反射激活的肌纤维在肌肉内均匀分布时，才能正确地将反射幅度相对于MVC进行归一化。为了深入了解这种分布情况，对3名受试者进行了对照实验，其中改变电极间距，从而影响记录到活跃肌纤维的肌肉深度。电极间距为22mm（记录肌肉深层和浅层）时咬肌的反射增益大于电极间距为11mm时（不同受试者P<0.01 - 0.10）。颞前肌未发现此类差异。得出的结论是，升颌肌中肌梭的传入纤维并非均匀地投射到运动神经元上，而是主要涉及肌肉深层的纤维。在电极间距均为18mm时，咬肌和颞前肌之间发现的增益差异可能是由于咬肌中反射激活的肌纤维相对于电极的距离更大，而非肌肉之间反射敏感性的差异。（摘要截短于400字）
