Spider mechanoreceptors

Friedrich Barth (2004) Curr. Opin. Neurobiol. 14: 415-422

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

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 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

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]

Hair length and boundary layers

Flow speed

Boundary layer

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)

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

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+

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++

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