Whitney Laboratory for Marine Biosciences, University of Florida, St Augustine, FL 32080, USA.
Friday Harbor Laboratories, University of Washington, Friday Harbor, WA 98250, USA.
J Exp Biol. 2020 Dec 23;223(Pt 24):jeb233494. doi: 10.1242/jeb.233494.
Although the bell-shaped nectophores of the siphonophore are clearly specialized for locomotion, their complex neuroanatomy described here testifies to multiple subsidiary functions. These include secretion, by the extensively innervated 'flask cells' located around the bell margin, and protection, by the numerous nematocytes that line the nectophore's exposed ridges. The main nerve complex consists of a nerve ring at the base of the bell, an adjacent column-shaped matrix plus two associated nerve projections. At the top of the nectophore the upper nerve tract appears to have a sensory role; on the lower surface a second nerve tract provides a motor input connecting the nectophore with the rest of the colony via a cluster of nerve cells at the stem. is capable of both forward and backward jet-propelled swimming. During backwards swimming the water jet is redirected by the contraction of the Claus' muscle system, part of the muscular velum that fringes the bell aperture. Contractions can be elicited by electrical stimulation of the nectophore surface, even when both upper and lower nerve tracts have been destroyed. Epithelial impulses elicited there, generate slow potentials and action potentials in the velum musculature. Slow potentials arise at different sites around the bell margin and give rise to action potentials in contracting Claus' muscle fibres. A synaptic rather than an electrotonic model more readily accounts for the time course of the slow potentials. During backward swimming, isometrically contracting muscle fibres in the endoderm provide the Claus' fibres with an immobile base.
尽管水螅水母的钟形浮囊显然是专门用于运动的，但这里描述的其复杂的神经解剖结构证明了它具有多种附属功能。这些功能包括分泌，由位于钟形边缘周围的广泛神经支配的“瓶状细胞”分泌；以及保护，由排列在浮囊暴露脊上的众多刺丝囊提供。主要神经丛由钟形底部的神经环、相邻的柱状基质和两个相关的神经突起组成。在浮囊的顶部，上神经束似乎具有感觉作用；在下部表面，第二个神经束提供了一个运动输入，通过茎上的一簇神经细胞将浮囊与群体的其余部分连接起来。能够进行前向和后向射流推进游泳。在后向游泳过程中，水射流通过 Claus 肌肉系统的收缩重新定向，Claus 肌肉系统是边缘扇形帽的一部分。即使上、下神经束都已被破坏，用电刺激浮囊表面也可以引起收缩。那里引发的上皮冲动会在帽状肌肉组织中产生慢电位和动作电位。慢电位在钟形边缘周围的不同部位产生，并在收缩的 Claus 肌肉纤维中产生动作电位。与电紧张模型相比，突触模型更能解释慢电位的时程。在后向游泳时，内胚层中的等长收缩肌肉纤维为 Claus 纤维提供了一个固定的基础。
