Touch-induced mechanical strain in somatosensory neurons is independent of extracellular matrix mutations in Caenorhabditis elegans.

Cutaneous mechanosensory neurons are activated by mechanical loads applied to the skin, and these stimuli are proposed to generate mechanical strain within sensory neurons. Using a microfluidic device to deliver controlled stimuli to intact animals and large, immobile, and fluorescent protein-tagged mitochondria as fiducial markers in the touch receptor neurons (TRNs), we visualized and measured touch-induced mechanical strain in Caenorhabditis elegans worms. At steady state, touch stimuli sufficient to activate TRNs induce an average strain of 3.1% at the center of the actuator and this strain decays to near zero at the edges of the actuator. We also measured strain in animals carrying mutations affecting links between the extracellular matrix (ECM) and the TRNs but could not detect any differences in touch-induced mechanical strain between wild-type and mutant animals. Collectively, these results demonstrate that touching the skin induces local mechanical strain in intact animals and suggest that a fully intact ECM is not essential for transmitting mechanical strain from the skin to cutaneous mechanosensory neurons

Forces applied during classical touch assays for Caenorhabditis elegans.

For decades, Caenorhabditis elegans roundworms have been used to study the sense of touch, and this work has been facilitated by a simple behavioral assay for touch sensation. To perform this classical assay, an experimenter uses an eyebrow hair to gently touch a moving worm and observes whether or not the worm reverses direction. We used two experimental approaches to determine the manner and moment of contact between the eyebrow hair tool and freely moving animals and the forces delivered by the classical assay. Using high-speed video (2500 frames/second), we found that typical stimulus delivery events include a brief moment when the hair is contact with the worm's body and not the agar substrate. To measure the applied forces, we measured forces generated by volunteers mimicking the classical touch assay by touching a calibrated microcantilever. The mean (61 μN) and median forces (26 μN) were more than ten times higher than the 2-μN force known to saturate the probability of evoking a reversal in adult C. elegans. We also considered the eyebrow hairs as an additional source of variation. The stiffness of the sampled eyebrow hairs varied between 0.07 and 0.41 N/m and was correlated with the free length of hair. Collectively, this work establishes that the classical touch assay applies enough force to saturate the probability of evoking reversals in adult C. elegans in spite of its variability among trials and experimenters and that increasing the free length of the hair can decrease the applied force

The mechanical properties of eyebrow hair tools.

<p>(A) Schematic of the method for measuring the stiffness of an eyebrow hair. (B) Representative trace of the deflection of the hair as a function of time. (C) Representative force-displacement curve. At least four curves were collected for each eyebrow hair. The slope of this curve is the stiffness. (D) Stiffness vs. length of the hair. Points are the measurements from five different hairs, and the smooth line was fit to the data according to <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0178080#pone.0178080.e001" target="_blank">Eq 1</a>.</p

Representative examples of high-speed videos that indicate maximum force is applied to the worm.

<p>(A) Schematic representing the hair moving across the worm. (B) Cross-section showing when the hair first comes into contact with the worm. At this instant, it is also in contact with the agar and bacterial lawn. (C) As the hair moves across the worm, it lifts off the agar and is only in contact with the worm. (D-E) Images from representative videos of the hair sliding over the worm, temporarily lifting off the agar. (F) In this case, the worm is pushed across the agar laterally before the hair slides over the top of the worm. The dashed line reference shows lateral movement. Times are in reference to the time when the hair is only in contact with the worm as in panel C.</p

Forces applied by volunteers all exceeded the force required to produce the maximum probability of response from a worm.

<p>(A) Volunteers used an eyebrow hair to touch a force-sensing cantilever 30 times. A sensing circuit records the z-force applied to the cantilever as a function of time. The traces plotted here are the minimum, median, and maximum for touch events for the median volunteer (volunteer G). (B) Box plots of forces delivered by human volunteers. The central mark is the median, the edges of the boxes are 25th and 75th percentiles, and the whiskers indicate the minimum and maximum forces. The red line indicates the force at which the probability of a worm reversing is saturated (about 2 μN, F_sat).</p