Division of Human Anatomy and Cell Biology, School of Biomedical Sciences, University of Liverpool, Sherrington Buildings, Liverpool, UK.
J Anat. 2010 Jan;216(1):37-47. doi: 10.1111/j.1469-7580.2009.01172.x. Epub 2009 Nov 25.
The semicircular canals of the inner ear sense angular accelerations and decelerations of the head and enable co-ordination of posture and body movement, as well as visual stability. Differences of agility and spatial sensitivity among species have been linked to interspecific differences in the relative size of the canals, particularly the radius of curvature (R) and the ratio of the canal plane area to streamline length (P/L). Here we investigate the scaling relationships of these two size variables and also out-of-plane torsion in the three semicircular canals (anterior, posterior and lateral), in order to assess which is more closely correlated with body size and locomotor agility. Measurements were computed from 3D landmarks taken from magnetic resonance images of a diverse sample of placental mammals encompassing 16 eutherian orders. Body masses were collected from the literature and an agility score was assigned to each species. The R and P/L of all three semicircular canals were found to have highly significant positive correlations with each other and no statistical difference was found between the slope of 2P/L against R and 1. This indicated that, contrary to initial hypotheses, there is little difference between 2P/L and R as measures of semicircular canal size. A measure of the in-plane circularity of the canal was obtained by dividing 2P/L by R and out-of-plane torsion was measured as angular deviation from a plane of best fit. It was predicted that deviations from in-plane and out-of-plane circularity would increase at small body size due to the constraints of fitting a proportionately larger canal into a smaller petrous bone. However, neither measurement was found to have a significant correlation with body mass, indicating that deviations from circularity (both in-plane and out-of-plane) are not sufficient to alter P/L to an extent that would impact the sensitivity of the canals. 2P/L and R were both shown to be significantly correlated with locomotor agility. The posterior canal was the least correlated with agility, suggesting that it may be generally less closely aligned to the direction of movement than the anterior canal. Of the three canals, the lateral canal was the most highly correlated with agility. In particular, it could be used to distinguish between species that move in a largely 2D environment and those that locomote in 3D space (aerial, arboreal and aquatic species). This complements previous work suggesting that the lateral canal primarily commands navigation, whereas the vertical canals control reflex adjustments. It was also found that 2P/L is substantially better correlated with agility than is R in the lateral canal. This result is intriguing given the above finding that there is no statistical difference between 2P/L and R, and requires further investigation.
内耳的半规管感知头部的角加速度和减速度，并使姿势和身体运动协调，以及视觉稳定。物种间的敏捷性和空间敏感性的差异与管腔的相对大小有关，特别是曲率半径 (R) 和管腔平面面积与流线长度的比值 (P/L)。在这里，我们研究了这两个大小变量以及三个半规管（前、后和外侧）的平面外扭转的缩放关系，以评估哪个与体型和运动敏捷性更密切相关。测量值是从涵盖 16 个真兽类目的胎盘哺乳动物的磁共振图像中的 3D 标记计算得出的。体质量是从文献中收集的，并且为每个物种分配了一个敏捷度评分。发现三个半规管的 R 和 P/L 之间具有高度显著的正相关关系，并且 2P/L 对 R 的斜率与 1 之间没有统计学差异。这表明，与最初的假设相反，2P/L 和 R 作为半规管大小的测量值之间几乎没有差异。通过将 2P/L 除以 R 来获得管腔的平面内圆度的度量，并且将平面外扭转测量为与最佳拟合平面的角度偏差。预测由于比例较大的管腔要适应较小的岩骨，因此在小体型时，平面内和平面外的圆度偏差会增加。然而，没有发现这两个测量值与体质量有显著相关性，这表明圆度偏差（平面内和平面外）不足以改变 P/L，以至于会影响管腔的灵敏度。2P/L 和 R 均与运动敏捷性显著相关。后管腔与敏捷性的相关性最小，这表明它可能与运动方向的一般对齐程度不如前管腔。在三个管腔中，外侧管腔与敏捷性的相关性最高。特别是，它可以用于区分在二维环境中运动的物种和在三维空间（空中、树栖和水生）中运动的物种。这补充了先前的工作，表明外侧管腔主要负责导航，而垂直管腔控制反射调整。还发现，在外侧管腔中，2P/L 与敏捷性的相关性明显优于 R。鉴于上述发现，2P/L 和 R 之间没有统计学差异，这一结果令人好奇，需要进一步研究。
