Transcript Slide 1
Spider mechanoreceptors Friedrich Barth (2004) Curr. Opin. Neurobiol. 14: 415-422 Medium-flow sensors Spider: trichobothria Filiform setae 0.1 – 1.4 mm long 10 mm diameter Located on legs (90 per leg) Driven by air flow High sensitivity: threshold work = 2.5 – 15x10-20 J Sensor for medium-flow vs contact Design principles: Resonance in hairs Flow Deflection proportional to velocity Low f Maximum sensitivity Resonant f High f Deflection lags velocity but overshoots due to inertia Inertia so high hair movement is reduced Design principles Table I Hairs detecting medium movement 1. Boundary layer thickness, d dwater = 0.22 dair d= 2.5(n/f)0.5 f = frequency of oscillation A plate of 1 m2 area pushed sideways with a force of 1 N [~100 grams equiv] over a surface coated with a fluid of 1 Pa s viscosity woould move the distance of the fluid depth in 1 second where n is the “kinematic viscosity” of the medium nair= 20 x 10-6 m2 s-1 [20ºC] [12?] nwater= 1x 10-6 m2 s-1 [20ºC] n = m/r n=“kinematic viscosity” m=“dynamic viscosity”; r=denisty mair = 18.3 x 10-6 Pa s [18ºC] mwater= 10-3 Pa s [20ºC] [1 Pa = 1 N m-2] Table I (con’d) Hairs detecting medium movement 2. Drag per unit length, D Dwater = 43 Dair drag = density x area x velocity2 3. Virtual (added) mass, VM Effective inertia, Ieff in water >> Ieff in air [Ieff = f (fluid density, viscosity, oscillation frequency, hair diameter and length)] IVM dominates Ieff in water mainly due to much larger dynamic viscosity m. Resonance frequency in water << resonance frequency in air because fres ~ (S/Ieff)0.5 [S = spring constant ~10-12Nm/rad] Hair length and boundary layers Flow speed Boundary layer Sensor arrays Boundary layer Low Frequency Boundary layer High Frequency Only long hairs move Both hairs move (boundary layers in water are smaller, so hairs can be as well) Behavioral correlates • Typical prey stimuli are highly turbulent (flying insect) (>100 Hz) • Background air velocities low frequency (10Hz) • Prey signals attenuate rapidly with distance (to noise level at 25 cm) • Sensors tuned to 50-120 Hz: prey-specific-range Tactile hairs Bending of the hair shaft • Spring constant 104 x greater than trichobothria • Base deflection <12º owing to proximal shift of force (limits breakage) • Sensory coding range extended • Sensitivity greater for weak stimuli • Structure optimized to keep maximum axial stress fixed Bending of the hair shaft Scaling down the stimulus Overload protection combined with high sensitivity to weak stimuli Movement scale-down 750x Tension on the tip links enhances the probability of an open state for the stretch-gated channel anchored to the link. Threshold = 0.3 nm Model for tip-link-mediated gating Tip-link stretch Opening stretch-gated cation-selective channels Strain detectors: Lyriform organ Membrane potentials • Na+-rich, K+-poor receptor lymph is the spider norm (cf insects: K+-rich) • In lyriform organ, receptor current is Na+ Endolymph (scala media): +85 mV “endocochlear potential” 160 mM K+ 1 mM Na+ 20 mM Ca++ +145 mV outside-positive driving potential Intracellular: -60 mV 140 mM K+ 3 mM Na+ 0.2 mM Ca++ Perilymph 0 mV 4 mM K+ 150 mM Na+ 1 mM Ca++ Site of mechanosensitivity Located at dendrite tips Insensitive to disruption of “tubular body” Initiation of action potentials Initiated at dendrite tips Na+ channel densities high in dendrites & axon Efferent innervation Profuse – why? GABA, glutamate and acetylcholine (peptides?) Conclusions • Spiders rule! • Match between physical characteristics of stimulus environment and receptor structure is noteworthy • Spider studies may be useful in neuromorphic engineering design