Spider mechanoreceptors Friedrich Barth (2004) Curr. Opin. Neurobiol. 14:

Spider: trichobothria Filiform setae 0.1 – 1.4 mm long 10  m diameter Located on legs (90 per leg) Driven by air flow High sensitivity: threshold work = 2.5 – 15x J Medium-flow sensors

Sensor for medium-flow vs contact

Design principles: Resonance in hairs Low f Resonant f High f Flow Deflection proportional to velocity Deflection lags velocity but overshoots due to inertia Inertia so high hair movement is reduced Maximum sensitivity

Table I Hairs detecting medium movement 1. Boundary layer thickness,   water  air  = 2.5( /f) 0.5 f = frequency of oscillation where is the “kinematic viscosity” of the medium air = 20 x m 2 s -1 [20ºC] [12?] water = 1x m 2 s -1 [20ºC] =  /  =“kinematic viscosity”  =“dynamic viscosity”;  =denisty  air = 18.3 x Pa s [18ºC] [1 Pa = 1 N m -2 ]  water = Pa s [20ºC] A plate of 1 m 2 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 Design principles

Table I (con’d) Hairs detecting medium movement 2.Drag per unit length, D D water = 43 D air drag = density x area x velocity 2 3.Virtual (added) mass, VM Effective inertia, I eff in water >> I eff in air [I eff = f (fluid density, viscosity, oscillation frequency, hair diameter and length)] I VM dominates I eff in water mainly due to much larger dynamic viscosity . Resonance frequency in water << resonance frequency in air because f res ~ (S/I eff ) 0.5 [S = spring constant ~ Nm/rad]

Hair length and boundary layers Flow speed Boundary layer

High Frequency Sensor arrays Both hairs move Boundary layer Low Frequency Only long 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 Hz: prey-specific-range

Tactile hairs

Bending of the hair shaft Spring constant 10 4 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

Model for tip-link-mediated gating 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 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 + Perilymph 0 mV 4 mM K mM Na + 1 mM Ca ++ Intracellular: -60 mV 140 mM K + 3 mM Na  M Ca mV outside-positive driving potential Endolymph (scala media) : +85 mV “endocochlear potential” 160 mM K + 1 mM Na + 20  M 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