Cricket Wind Receptors: Thermal Noise for the Highest Sensitivity Known
The minimum amount of mechanical energy necessary to elicit a neuronal spike in the wind receptor cell of a cricket is determined to be in the order of kBT (4 x 10-21 Joules at 300°K). Insect mechanoreceptors are therefore bordering the range of thermal noise due to Brownian motion when working near to threshold. Evolution, however, has achieved a paradoxical solution for sensory signal transmission in the presence of thermal noise.
The estimation of mechanical energy is based on three measurements: deflection sensitivity of hair to air motion; sensory threshold of receptor cell in terms of air velocity; and mechanical resistance of hair support. The deflection sensitivity to air motion was measured by laser-Doppler velocimetry and Gaussian white noise analysis. Three mechanical parameters, i.e. the moment of inertia of hair shaft, the spring stiffness of hair support, and the torsional resistance within the support were estimated by applying Stokes’ theory for viscous force to the data of deflection sensitivity.
The mechanical energy consumed by the resistance provides a maximum estimate of the energy available to the receptor cell for stimulus transduction. The estimated energy threshold of the mechanoreceptor, is far below that of photoreceptors which can detect a single photon (ca. 3 × 10−19 Joules). The mechanoreceptor is 100 times more sensitive than a photoreceptor.
The spike train of the wind receptor cell fluctuates with time, when responding to weak stimuli near threshold. Simultaneous double recordings from two cells revealed that the fluctuations are uncorrelated between cells. An array of mechanoreceptors exposed to thermal noise is able to detect weaker signals, below the threshold, by a paradoxical use of the thermal noise as the seed of uncorrelated randomness for stochastic sampling.
Biological systems evolved under the inevitable presence of thermal noise teach us a design principle for future information systems that will be faced with the same noise. Future technology needs to be informed by the wide and sound bases of the natural sciences, not only those of physics, chemistry and mathematics, but also of biology.
KeywordsSpike Train Thermal Noise Stochastic Resonance Spring Stiffness Sensory Threshold
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