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Spider mechanoreceptors
Friedrich Barth (2004) Curr. Opin. Neurobiol. 14: 415-422
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Spider: trichobothria
Filiform setae0.1 – 1.4 mm long10 m diameter
Located on legs (90 per leg)
Driven by air flow
High sensitivity: threshold work = 2.5 – 15x10-20 J
Medium-flow sensors
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Sensor for medium-flow vs contact
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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
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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 10-6 m2 s-1 [20ºC] [12?]
water= 1x 10-6 m2 s-1 [20ºC]
= / =“kinematic viscosity” =“dynamic viscosity”; =denisty
air = 18.3 x 10-6 Pa s [18ºC] [1 Pa = 1 N m-2]
water= 10-3 Pa s [20ºC]
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
Design principles
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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 .
Resonance frequency in water << resonance frequency in air because fres ~ (S/Ieff)0.5 [S = spring constant ~10-12Nm/rad]
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Hair length and boundary layers
Flow speed
Boundary layer
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Boundary layerHigh Frequency
Sensor arrays
Both hairs move
Boundary layerLow Frequency
Only long hairs move
(boundary layers in water are smaller, so hairs can be as well)
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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
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Tactile hairs
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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
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Bending of the hair shaft
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Scaling down the stimulus
Overload protection combined with high sensitivity to weak stimuli
Movement scale-down 750x
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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
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Strain detectors: Lyriform organ
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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+
150 mM Na+
1 mM Ca++
Intracellular: -60 mV140 mM K+
3 mM Na+
0.2 M Ca++
+145 mV outside-positive driving potential
Endolymph (scala media): +85 mV “endocochlear potential”160 mM K+
1 mM Na+
20 M Ca++
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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?)
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Conclusions
• Spiders rule!• Match between physical characteristics of
stimulus environment and receptor structure is noteworthy
• Spider studies may be useful in neuromorphic engineering design