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Internal seminar.
By Léo Régnier and Elie Wandersman
November 5, 2024 at 11:00AM - 4th floor seminar room - LJP - Tower 22-23

Abstract

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Random Walks Exploration and Starving, Léo Régnier
 
How long does it take for a tracer to reach a position it has never visited before [1]? When will a forager starve to death, depending on the medium it moves through or its walking behavior [2]?  In this presentation, I will address these two questions by quantifying the exploration dynamics of a memoryless (Markovian) random walk using the time between visits to new sites. We will observe that this observable displays universal behaviors. Next, we will see how these findings apply to the foraging random walk problem, where the tracer has a finite metabolic time, serving as a minimal model for a depletion-controlled system. 
 
References:
[1] Régnier, L., Dolgushev, M., Redner, S. & Bénichou, O., Universal exploration dynamics of random walks. Nat Commun 14, 618 (2023). https://doi.org/10.1038/s41467-023-36233-5
[2] Régnier, L., Dolgushev, M. & Bénichou, O., From Maximum of Inter-Visit Times to Starving Random Walks, Phys. Rev. Lett. 132, 127101, https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.132.127101, Arxiv (2023) https://arxiv.org/abs/2310.09082.
 
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Mechanical noise integration in tactile perception, Elie Wandersman

 

Most animals and micro-organisms can detect mechanical features of their environment with an exquisite sensitivity and rapidity. Humans, for instance can detect and discriminate micrometric variations of a surface roughness (its texture) by rubbing their fingers across it. Aquatic animals and micro-organisms often use myriads of high aspect ratio ciliated structures as tactile organs. The tactile detection is here mediated by hydrodynamic interactions and allows, for instance, for food/predators/flow detection and taxis.

In both cases (aquatic/terrestrial), the very first step of the tactile processes is purely mechanical: the interaction between the tactile organ and its mechanical environment elicits mechanical stresses which are propagated through the organism. The second step is biological: mechanosensitive cells (mechanoreceptors) embedded in the skin encode the mechanical signal in series of action potentials. The triggering of this neural response is due to the presence of mechanosensitive protein nanopores in the plasma membrane of mechanoreceptors. During these two tactile steps, the input mechanical signals are filtered, both by the mechanical properties of the tactile organ and by the biophysical properties of the mechanoreceptor membrane itself.

 

In this talk we will discuss how a noisy mechanical input (a texture) is integrated using biomimetic approaches, first at the scale of a tactile organ. Using artificial cilia, we will show how the texture of a heterogeneous liquid can be finely encoded by cilia’s bending fluctuations. We will then discuss our first attempts to develop artificial touch mechanoreceptors, made of lipid vesicles embedded in a gel, the membrane of which is decorated with mechanosensitive proteins. We will describe how these mechanoreceptors are deformed under static and dynamic perturbations, and model them as cavities embedded in an elastic medium. We will show that membrane stresses are anisotropic and that their time fluctuations can encode the texture of a solid.

 

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