Gordy Clayton, Straka Hans
Department Biology II, Ludwig-Maximilians-University Munich, Munich, Germany.
Graduate School of Systemic Neurosciences, Ludwig-Maximilians-University Munich, Munich, Germany.
Front Syst Neurosci. 2021 Oct 6;15:753207. doi: 10.3389/fnsys.2021.753207. eCollection 2021.
Vestibular endorgans in the vertebrate inner ear form the principal sensors for head orientation and motion in space. Following the evolutionary appearance of these organs in pre-vertebrate ancestors, specific sensory epithelial patches, such as the utricle, which is sensitive to linear acceleration and orientation of the head with respect to earth's gravity, have become particularly important for constant postural stabilization. This influence operates through descending neuronal populations with evolutionarily conserved hindbrain origins that directly and indirectly control spinal motoneurons of axial and limb muscles. During embryogenesis and early post-embryonic periods, bilateral otolith signals contribute to the formation of symmetric skeletal elements through a balanced activation of axial muscles. This role has been validated by removal of otolith signals on one side during a specific developmental period in tadpoles. This intervention causes severe scoliotic deformations that remain permanent and extend into adulthood. Accordingly, the functional influence of weight-bearing otoconia, likely on utricular hair cells and resultant afferent discharge, represents a mechanism to ensure a symmetric muscle tonus essential for establishing a normal body shape. Such an impact is presumably occurring within a critical period that is curtailed by the functional completion of central vestibulo-motor circuits and by the modifiability of skeletal elements before ossification of the bones. Thus, bilateral otolith organs and their associated sensitivity to head orientation and linear accelerations are not only indispensable for real time postural stabilization during motion in space but also serve as a guidance for the ontogenetic establishment of a symmetric body.
脊椎动物内耳中的前庭终器是头部在空间中定向和运动的主要传感器。在这些器官于脊椎动物祖先中进化出现之后，特定的感觉上皮斑块，比如对头部相对于地球重力的线性加速度和定向敏感的椭圆囊，对于持续的姿势稳定变得尤为重要。这种影响通过起源于后脑且在进化上保守的下行神经元群体发挥作用，这些神经元群体直接和间接地控制轴向和肢体肌肉的脊髓运动神经元。在胚胎发生期和胚胎后期早期，双侧耳石信号通过轴向肌肉的平衡激活，有助于对称骨骼元素的形成。这一作用已通过在蝌蚪特定发育时期去除一侧耳石信号得到验证。这种干预会导致严重的脊柱侧弯畸形，这些畸形会一直存在并延续到成年期。因此，负重耳石的功能影响，可能作用于椭圆囊毛细胞及由此产生的传入放电，代表了一种确保对称肌肉张力的机制，而对称肌肉张力对于建立正常体型至关重要。这种影响大概发生在一个关键时期内，这个时期会因中央前庭 - 运动回路的功能完成以及骨骼在骨化之前骨骼元素的可变性而缩短。因此，双侧耳石器官及其对头部定向和线性加速度的相关敏感性不仅对于空间运动期间的实时姿势稳定不可或缺，而且还为对称身体的个体发生建立提供指导。
