What Was Observed? (Introduction)

• The study explored how the single‐celled slime mold Physarum polycephalum – an organism without a nervous system – makes decisions based on physical cues.
• Researchers discovered that Physarum can “feel” the weight of inert objects from a distance by sensing tiny deformations in its growth surface.
• This process, called mechanosensation, enables the organism to grow preferentially toward heavier masses even when no chemical or light signals are present.

Key Concepts and Terms

• Physarum polycephalum: A giant, single-celled organism (slime mold) that shows surprisingly complex behavior despite lacking a brain.
• Mechanosensation: The ability to detect mechanical forces (like pressure or strain) in the environment. Think of it as “feeling” the push or pull on a surface.
• Shuttle streaming: A rhythmic, back-and-forth flow of the cell’s internal fluid. It acts like a natural “pulse” or heartbeat that helps the organism probe its surroundings.
• Strain: A measure of how much a surface is deformed by a force. In this context, heavier objects cause small “dents” or deformations in the gel substrate.
• TRP channels: Specialized proteins in the cell membrane that help convert mechanical stimuli into signals. Blocking these channels disrupts the organism’s ability to sense weight.

Experimental Methods (Step-by-Step)

• Setting Up the Assay:
  • A small piece of Physarum was placed in the center of a petri dish containing a soft agar gel.
  • Inert glass fiber discs were positioned at opposite edges of the dish. In some tests, one side had a single disc while the other had three discs.
• Observing Growth:
  • Time-lapse imaging was used to record the organism’s growth over many hours.
  • Early on, Physarum spread equally in all directions, but after a few hours it began to extend a branch toward the heavier mass.
• Additional Manipulations:
  • A machine learning model was applied to predict which side Physarum would choose – revealing that decisions were made within about 4 hours.
  • Researchers introduced mechanical disruptions (like tilting the dish) to test how external motion affects mass sensing.
  • The stiffness of the agar substrate was varied to see how surface firmness influenced the strain signals.
  • A chemical inhibitor (GsMTx-4) was used to block TRP channels and verify the role of these channels in mechanosensation.

Results and Observations

• Physarum consistently grew toward regions with heavier masses without first physically exploring the area.
• Its decision-making occurred in two distinct phases:
  • Sensing Phase: Within the first few hours, the organism detected differences in the strain (deformation) caused by different masses.
  • Execution Phase: After sensing, Physarum rapidly directed its growth toward the heavier mass.
• When the dish was gently rocked (tilted), the organism’s ability to choose the heavier side was disrupted – it often grew in both directions instead.
• A softer substrate (low-concentration agar) enhanced the ability to detect small differences in mass, whereas a stiffer substrate reduced this discrimination.
• Blocking TRP channels with a specific inhibitor prevented Physarum from distinguishing between heavy and light masses, confirming the importance of these channels in its decision-making process.

Mechanism and Theoretical Model

• The researchers propose that Physarum uses its rhythmic shuttle streaming to “pull” on the substrate, creating strain (small deformations) that it can detect.
• Rather than measuring the absolute force, the organism seems to sense the fraction of its outer edge that is experiencing strain above a certain threshold – much like checking how much of a balloon’s surface is stretched.
• Finite element simulations (computer models of physical stress) confirmed that heavier objects create broader and stronger strain fields on the gel.
• This leads to a “fluidically-coupled clutch model” where the periodic pulling (like a repetitive grip) helps the slime mold align its growth toward the area with the most favorable (heavier) mechanical signal.

Key Conclusions (Discussion)

• Physarum polycephalum, despite lacking a nervous system, can process physical information to make long-range decisions about where to grow.
• This study shows that mechanosensation – the ability to “feel” differences in weight via physical deformations – is a crucial mechanism for spatial decision-making in even the simplest organisms.
• The findings offer insight into an ancient biological process that may have inspired advanced systems in robotics and synthetic biology.
• Understanding this process might also help explain how multicellular organisms use physical forces to guide development, regeneration, and even healing.

观察到什么？ (引言)

• 本研究探讨了单细胞黏菌 Physarum polycephalum 如何利用物理信号做出决策，这种生物没有神经系统。
• 研究人员发现，Physarum 能够“感觉”到距离较远的无生命物体所施加的重量，通过检测其生长表面上的微小变形来做出判断。
• 这种过程称为机械感受（mechanosensation），使得该生物即使在没有化学或光信号的情况下，也会优先向较重的区域生长。

关键概念和术语

• Physarum polycephalum：一种巨大的单细胞生物（黏菌），尽管没有大脑，但表现出复杂行为。
• 机械感受（Mechanosensation）：检测环境中机械力（如压力或拉力）的能力。可以把它想象成“感觉”表面受到的推或拉。
• 往返流动（Shuttle streaming）：细胞内部液体有节奏地来回流动，就像自然的“脉搏”或心跳，有助于生物探测周围环境。
• 应变（Strain）：衡量表面在受力时发生形变的程度。在此背景下，较重的物体会使琼脂凝胶产生微小的“凹陷”。
• TRP 通道：细胞膜中的特殊蛋白，帮助将机械刺激转化为细胞信号。阻断这些通道会破坏生物的重量感知能力。

实验方法（分步说明）

• 实验装置的搭建：
  • 将一小块 Physarum 放置在含有柔软琼脂凝胶的培养皿中央。
  • 在培养皿两侧分别放置无营养的玻璃纤维圆盘。例如，一侧放置一个圆盘，另一侧放置三个圆盘。
• 观察生长过程：
  • 利用延时摄影记录生物在数小时内的生长情况。
  • 早期，Physarum 在各个方向均匀扩散，但数小时后，它开始向较重的一侧延伸出分枝。
• 进一步实验操作：
  • 使用机器学习模型提前预测生长方向，显示出约4小时内就能做出决策。
  • 引入机械干扰（例如轻微倾斜培养皿）来测试外部运动如何影响质量感知。
  • 改变琼脂基质的硬度（使用不同浓度的琼脂）以观察表面硬度对应变信号的影响。
  • 使用 TRP 通道抑制剂（GsMTx-4）阻断相关通道，验证其在机械感受中的作用。

结果和观察

• Physarum 总是优先向较重区域生长，即使在没有先探索该区域的情况下也能做到这一点。
• 其决策过程可分为两个阶段：
  • 感知阶段：在最初几小时内，生物检测到由不同质量引起的基质应变差异。
  • 执行阶段：在感知到信号后，Physarum 快速将生长方向导向较重的区域。
• 当培养皿受到轻微摇晃（倾斜）时，生物辨别不同质量的能力受到干扰，往往在两个方向上同时生长。
• 较软的基质（低浓度琼脂）有助于更好地检测微小的质量差异，而较硬的基质则会削弱这种辨别能力。
• 使用 TRP 通道抑制剂后，Physarum 无法区分重与轻的区域，这证明了这些通道在其决策过程中的重要性。

机制和理论模型

• 研究人员提出，Physarum 通过其节律性的往返流动来“拉”动基质，从而在表面上产生可被检测到的应变。
• 生物并不是直接测量施加的绝对力，而是感知其外围有多少部分经历了超过某个阈值的形变，就像检测气球表面被拉伸的面积一样。
• 有限元模拟（利用计算机模型模拟物理应力）证明，较重的物体会在凝胶上产生更宽、更强的应变场。
• 这种观察支持了“流体耦合离合模型”，即周期性的拉动（类似重复的抓握动作）帮助黏菌调整生长方向，朝向机械信号更强的区域。

关键结论（讨论）

• 即使没有神经系统，Physarum polycephalum 也能处理物理信息，从而在长距离上做出生长决策。
• 研究表明，利用机械感受——即通过检测基质形变来“感觉”质量差异——是这种简单生物实现空间决策的重要机制。
• 这些发现为理解一种古老的生物物理过程提供了新视角，该过程可能为仿生机器人和合成活体系统的发展提供灵感。
• 深入了解这种机制还能帮助解释多细胞生物如何利用物理力来指导发育、再生乃至愈合过程。