Fetcho J R
Department of Neurobiology and Behavior, State University of New York, Stony Brook 11794.
Brain Behav Evol. 1992;40(2-3):82-97. doi: 10.1159/000113905.
Recent studies of the spinal motor systems of vertebrates allow us to begin to infer the organization of the motor apparatus of primitive vertebrates. This paper attempts to define some of the features of the motor system of early vertebrates based on studies of the motor systems in anamniotes and in Branchiostoma. It also deals with some changes in the primitive motor system during evolution. The primitive motor system consisted of myomeric axial muscles, with a functional subdivision of the musculature into non-spiking slow muscle fibers segregated in the myomeres from spiking fast ones. These fibers were innervated by two major classes of motoneurons in the cord-large motoneurons innervating faster fibers and small motoneurons innervating slow fibers. There was not a simple isomorphic mapping of the position of motoneurons in the motor column onto the location of the muscle fibers they innervated in the myomeres. Early vertebrates used these axial muscles to bend the body, and the different types of muscle fibers and motoneurons reflect the ability to produce slow swimming movements as well as very rapid bending associated with fast swimming or escapes. The premotor network producing bending was most likely a circuit composed of a class of descending interneurons (DIs) that provided excitation of ipsilateral motoneurons and other interneurons, and inhibitory commissural interneurons (CIs) that blocked contralateral activity and played an important role in generating the rhythmic alternating bending during swimming. This DI/CI network was retained in living anamniotes. At least two major descending systems linked the sensory systems in the head to these premotor networks in the spinal cord. The ability to turn on swimming by activation of DI/CI premotor networks in the cord resided at least in part in a midbrain locomotor region (MLR) that influenced spinal networks via projections to the reticular formation. Reticulospinal neurons were important not only for initiation of rhythmic swimming but also in the production of turning movements. The reticulospinal cells involved in turns produced their effects in part via monosynaptic connections with motor neurons and premotor interneurons, including some involved in rhythmic swimming. A prominent and powerful Mauthner cell was most likely present and important for rapid escape or startle movements. Some features of this primitive motor apparatus were conserved during the evolution of vertebrate motor systems, and others changed substantially. Many features of the early motor system were retained in living anamniotes; major changes occur among amniotes.(ABSTRACT TRUNCATED AT 400 WORDS)
最近对脊椎动物脊髓运动系统的研究使我们能够开始推断原始脊椎动物运动器官的组织方式。本文试图根据对无羊膜动物和文昌鱼运动系统的研究来界定早期脊椎动物运动系统的一些特征。它还探讨了原始运动系统在进化过程中的一些变化。原始运动系统由肌节轴肌组成，肌肉组织在功能上分为两类：不产生动作电位的慢肌纤维，它们在肌节中与产生动作电位的快肌纤维分开。这些纤维由脊髓中的两类主要运动神经元支配：支配较快纤维的大型运动神经元和支配慢纤维的小型运动神经元。运动柱中运动神经元的位置与它们在肌节中所支配的肌肉纤维的位置之间不存在简单的同构映射关系。早期脊椎动物利用这些轴肌来弯曲身体，不同类型的肌肉纤维和运动神经元反映了产生缓慢游泳运动以及与快速游泳或逃避相关的非常快速弯曲的能力。产生弯曲的运动前网络很可能是一个由一类下行中间神经元（DIs）组成的回路，这些下行中间神经元为同侧运动神经元和其他中间神经元提供兴奋，以及抑制性连合中间神经元（CIs），它们阻断对侧活动，并在游泳过程中产生有节奏的交替弯曲中起重要作用。这种DI/CI网络在现存的无羊膜动物中得以保留。至少有两个主要的下行系统将头部的感觉系统与脊髓中的这些运动前网络连接起来。通过激活脊髓中的DI/CI运动前网络来启动游泳的能力至少部分存在于中脑运动区（MLR），该区域通过向网状结构的投射来影响脊髓网络。网状脊髓神经元不仅对有节奏游泳的启动很重要，而且在转弯运动的产生中也很重要。参与转弯的网状脊髓细胞部分通过与运动神经元和运动前中间神经元的单突触连接产生作用，包括一些参与有节奏游泳的神经元。一个突出且强大的毛特纳细胞很可能存在，并且对快速逃避或惊吓动作很重要。这种原始运动器官的一些特征在脊椎动物运动系统的进化过程中得以保留，而其他特征则发生了很大变化。早期运动系统的许多特征在现存的无羊膜动物中得以保留；在羊膜动物中发生了重大变化。（摘要截选至400字）
