Lev-Tov A, Tal M
Department of Anatomy, Hebrew University Medical School, Jerusalem, Israel.
J Neurophysiol. 1987 Sep;58(3):496-509. doi: 10.1152/jn.1987.58.3.496.
The structure and activity patterns of the anterior and posterior heads of the guinea pig digastric muscle (DG) were studied in ketamine-anesthetized guinea pigs. Collagen staining of longitudinal and transverse sections of the muscle revealed that the guinea pig DG is comprised of a unicompartmental anterior head (ADG) and a multicompartmental posterior head (PDG). The two heads are separated by a thin tendinous inscription that, unlike the intermediate tendon of the DG in humans, is not attached to the hyoid bone. The motor nuclei of the guinea pig DG were reconstructed using retrograde labeling with horseradish peroxidase. The motoneurons of the ADG were clustered in a longitudinal column within the trigeminal motor nucleus. The motoneurons of the PDG were segregated into two clusters within the facial motor nucleus. The cross-sectional areas of the ADG and PDG motoneuron somata exhibited unimodal frequency distributions and the average soma area was larger for ADG than PDG motoneurons. Histochemical characterization of ADG and PDG revealed that the two muscle heads contained the three main histochemical types of muscle fibers identified in limb muscles. The frequency distribution of fiber types in ADG and PDG were not significantly different. Both muscle heads were predominantly fast with slow oxidative fibers accounting for only 1.1 and 0.3% of the fibers in narrow dorsal regions of ADG and PDG, respectively, and 13.6 and 12.9% in the more ventral regions of ADG and PDG, respectively. Simultaneous recordings of EMGs from the ADG and PDG were carried out during spontaneously occurring rhythmical jaw movements. These recordings revealed a high degree of synchrony between the activities of the two heads, although differences were observed in the onset and duration of the EMG bursts. Activity in the PDG preceded activity in the ADG in most of the rhythmical cycles and persisted longer. The differences in latencies of time-locked EMGs evoked in the ADG and PDG by four-pulse cortical stimulation were much smaller than those observed between the activity bursts of the two heads during rhythmical jaw movements. It is suggested that the early activity in the PDG is accounted for by shorter central conduction times in the pathways onto it and/or by higher recruitability of its motor units. The early activity in PDG may serve to optimize the location of ADG on its length-tension curve prior to and during the active state.
在氯胺酮麻醉的豚鼠身上研究了豚鼠二腹肌（DG）前后头的结构和活动模式。对该肌肉纵切面和横切面进行胶原染色显示，豚鼠DG由单腔的前头（ADG）和多腔的后头（PDG）组成。两个头被一条细的腱划隔开，这条腱划与人类DG的中间腱不同，不附着于舌骨。使用辣根过氧化物酶逆行标记法重建了豚鼠DG的运动核。ADG的运动神经元聚集在三叉神经运动核内的一个纵向柱中。PDG的运动神经元在面神经运动核内被分隔成两个簇。ADG和PDG运动神经元胞体的横截面积呈现单峰频率分布，且ADG运动神经元的平均胞体面积大于PDG运动神经元。ADG和PDG的组织化学特征表明，这两个肌肉头包含了在肢体肌肉中鉴定出的三种主要组织化学类型的肌纤维。ADG和PDG中纤维类型的频率分布没有显著差异。两个肌肉头主要都是快肌，慢氧化纤维分别仅占ADG和PDG狭窄背侧区域纤维的1.1%和0.3%，在ADG和PDG更腹侧区域分别占13.6%和12.9%。在自发的节律性下颌运动期间，同时记录了ADG和PDG的肌电图。这些记录显示，两个头的活动之间存在高度同步性，尽管在肌电爆发的起始和持续时间上观察到了差异。在大多数节律周期中，PDG的活动先于ADG的活动，且持续时间更长。由四脉冲皮质刺激在ADG和PDG中诱发的锁时肌电图潜伏期差异比在节律性下颌运动期间两个头的活动爆发之间观察到的差异小得多。有人认为，PDG的早期活动是由于其传入通路的中枢传导时间较短和/或其运动单位的更高募集能力所致。PDG的早期活动可能有助于在活动状态之前和期间优化ADG在其长度 - 张力曲线上的位置。
