Structural correlates of mechanosensory transduction and adaptation in identified neurons of spider slit sensilla

Force transformation in spider strain sensors: white light interferometry

Scanning white light interferometry and micro-force measurements were applied to analyse stimulus transformation in strain sensors in the spider exoskeleton. Two compound or 'lyriform' organs consisting of arrays of closely neighbouring, roughly parallel sensory slits of different lengths were examined. Forces applied to the exoskeleton entail strains in the cuticle, which compress and thereby stimulate the individual slits of the lyriform organs. (i) For the proprioreceptive lyriform organ HS-8 close to the distal joint of the tibia, the compression of the slits at the sensory threshold was as small as 1.4 nm and hardly more than 30 nm, depending on the slit in the array. The corresponding stimulus forces were as small as 0.01 mN. The linearity of the loading curve seems reasonable considering the sensor's relatively narrow biological intensity range of operation. The slits' mechanical sensitivity (slit compression/force) ranged from 106 down to 13 nm mN(-1), and gradually decreased with decreasing slit length. (ii) Remarkably, in the vibration-sensitive lyriform organ HS-10 on the metatarsus, the loading curve was exponential. The organ is thus adapted to the detection of a wide range of vibration amplitudes, as they are found under natural conditions. The mechanical sensitivities of the two slits examined in this organ in detail differed roughly threefold (522 and 195 nm mN(-1)) in the biologically most relevant range, again reflecting stimulus range fractionation among the slits composing the array.

In search of differences between the two types of sensory cells innervating spider slit sensilla (Cupiennius salei Keys.)

The metatarsal lyriform organ of the spider Cupiennius salei is a vibration detector consisting of 21 cuticular slits supplied by two sensory cells each, one ending in the outer and the other at the inner slit membrane. In search of functional differences between the two cell types due to differences in stimulus transmission, we analyzed (1) the adaptation of responses to electrical stimulation, (2) the thresholds for mechanical stimulation and (3) the representation of male courtship vibrations using intracellular recording and staining techniques. Single- and multi-spiking receptor neurons were found among both cell types, which showed high-pass filter characteristics. Below 100-Hz threshold, tarsal deflections were between 1 degrees and 10 degrees. At higher frequencies, they decreased down to values as small as 0.05 degrees, corresponding to 4.5-nm tarsal deflection in the most sensitive cases. Different slits in the organ and receptor cells with slow or fast adaptation did not differ in this regard. When stimulated with male courtship vibrations, both types of receptor cells again did not differ significantly regarding number of action potentials, latency and synchronization coefficients. Surprisingly, the differences in dendrite coupling were not reflected by the physiological responses of the two cell types innervating the slits.

Lung ischemia: a model for endothelial mechanotransduction

Endothelial cells in vivo are constantly exposed to shear associated with blood flow and altered shear stress elicits cellular responses (mechanotransduction). This review describes the role of shear sensors and signal transducers in these events. The major focus is the response to removal of shear as occurs when blood flow is compromised (i.e., ischemia). Pulmonary ischemia studied with the isolated murine lung or flow adapted pulmonary microvascular endothelial cells in vitro results in endothelial generation of reactive oxygen species (ROS) and NO. The response requires caveolae and is initiated by endothelial cell depolarization via K(ATP) channel closure followed by activation of NADPH oxidase (NOX2) and NO synthase (eNOS), signaling through MAP kinases, and endothelial cell proliferation. These physiological mediators can promote vasodilation and angiogenesis as compensation for decreased tissue perfusion.

Evolutionary changes in sensory precursor formation in arthropods: embryonic development of leg sensilla in the spider Cupiennius salei

We describe here for the first time the development of mechanosensory organs in a chelicerate, the spider Cupiennius salei. It has been shown previously that the number of external sense organs increases with each moult. While stage 1 larvae do not have any external sensory structures, stage 2 larvae show a stereotyped pattern of touch sensitive 'tactile hairs' on their legs. We show that these mechanosensory organs develop during embryogenesis. In contrast to insects, groups of sensory precursors are recruited from the leg epithelium, rather than single sensory organ progenitors. The groups increase by proliferation, and neural cells delaminate from the cluster, which migrate away to occupy a position proximal to the accessory cells of the sense organ. In addition, we describe the development of putative internal sense organs, which do not differentiate until larval stage 2. We show by RNA interference that, similar to Drosophila, proneural genes are responsible for the formation and subtype identity of sensory organs. Furthermore, we demonstrate an additional function for proneural genes in the coordinated invagination and migration of neural cells during sensory organ formation in the spider.

Mechanosensitive ion channels of spiders: mechanical coupling, electrophysiology, and synaptic modulation

Arthropods have provided several important mechanoreceptor models because of the relatively large size and accessibility of their primary sensory neurons. Three types of spider receptors- tactile hairs, trichobothria, and slit sensilla-have given important information about the coupling of external mechanical stimuli to the neuronal membrane, transduction of mechanical force into receptor current, encoding of afferent action potentials, and efferent modulation of peripheral sensory receptors. Slit sensilla, found only in spiders, are particularly important because they allow intracellular recording from sensory neurons during mechanical stimulation. Experiments on slit sensilla have shown that their mechanosensitive ion channels are sodium selective, blocked by amiloride, and open more at low pH. This evidence suggests that the channels are members of the same molecular family as degenerins, acid-sensitive ion channels, and epithelial sodium channels. Slit sensilla have also yielded evidence about the location, density, single-channel conductance, and dynamic properties of the mechanosensitive channels. Spider mechanoreceptors are modulated in the periphery by efferent neurons and possibly by circulating chemicals. Mechanisms of modulation, intracellular signaling, and role of intracellular calcium are areas of active investigation.

Intracellular recording from a spider vibration receptor

The present study introduces a new preparation of a spider vibration receptor that allows intracellular recording of responses to natural mechanical or electrical stimulation of the associated mechanoreceptor cells. The spider vibration receptor is a lyriform slit sense organ made up of 21 cuticular slits located on the distal end of the metatarsus of each walking leg. The organ is stimulated when the tarsus receives substrate vibrations, which it transmits to the organ's cuticular structures, reducing the displacement to about one tenth due to geometrical reasons. Current clamp recording was used to record action potentials generated by electrical or mechanical stimuli. Square pulse stimulation identified two groups of sensory cells, the first being single-spike cells which generated only one or two action potentials and the second being multi-spike cells which produced bursts of action potentials. When the more natural mechanical sinusoidal stimulation was applied, differences in adaptation rate between the two cell types remained. In agreement with prior extracellular recordings, both cell types showed a decrease in the threshold tarsus deflection with increasing stimulus frequency. Off-responses to mechanical stimuli have also been seen in the metatarsal organ for the first time.

Mechanotransduction in spider slit sensilla

Mechanoreception is a vital constituent of several sensory modalities and a wide range of internal regulatory processes, but fundamental mechanisms for neural detection of mechanical stimuli have been difficult to characterize because of the morphological properties of most mechanoreceptors and the nature of the stimulus itself. An invertebrate preparation, the VS-3 lyriform slit sense organ of the spider, Cupiennius salei, has proved useful because it possesses large mechanosensory neurons, whose cell bodies are close to the sites of sensory transduction, and accessible to intracellular recording during mechanotransduction. This has made it possible to observe and experiment with all the major stages of mechanosensation. Here, we describe several important findings from this preparation, including the estimated number, conductance and ionic selectivity of the ion channels responsible for mechanotransduction, the major voltage-activated ion channels responsible for action potential encoding and control of the dynamic properties of the neurons, the location of action potential initiation following mechanical stimulation, and the efferent control of mechanoreception. While many details of mechanosensation remain to be discovered, the VS-3 system continues to offer important opportunities to advance our understanding of this crucial physiological process.

Post-embryonic development of cuticular caps of campaniform sensilla of the cockroach leg: potential implications in scaling force detection

All animals generate progressively larger forces as they increase in size and mass. Their abilities to detect these forces must be similarly adjusted. In insects, campaniform sensilla monitor strains in the exoskeleton and provide information about forces acting upon the legs. Each sensory neuron possesses a dendrite that inserts into a cuticular cap in the exoskeleton. The cap is the site of mechanotransduction. We measured the sizes and numbers of receptor caps on the cockroach hindleg at different developmental stages. Our goal was to identify morphological features that could be correlated with the range of forces that must be detected. As cockroaches increase in size through successive molts, the number of cuticular caps in each group increases. The tibial group, for example, has two sensilla in first instar animals and 10-12 in the adult. There is also an increase in the range of cap sizes within each group. Observations of animals and their molted exoskeletons suggest that this increase occurs as the caps of existing receptors increase in size and smaller ones are added with each molt. These changes may be important in increasing the range of forces the receptors can signal while retaining sensitivity to low levels of force.