Cercus

Abstract: Summary1.The pattern of connections between cereal sensory neurons and a single sensory interneuron, the medial giant interneuron (MGI), was examined using intracellular methods.2.Two types of sound sensitive hairs are located on each cercus. Each type was examined for its ability to excite, or inhibit the MGI.3.The effect of stimulating a large population of one hair type was examined by blocking movement of all other types, recording intracellularly from MGI, and stimulating the preparation with tones.4.T-hairs on the ipsilateral cercus powerfully excited MGI initiating a train of action potentials. L-hairs on either cercus weakly excited MGI. T-hairs on the contralateral cercus inhibited MGI.5.The effect of stimulating individual receptor hairs was examined by blocking the movement of all but a single hair. Intracellular recordings from MGI exhibit subthreshold synaptic potentials when L-or ipsi-T hairs are stimulated. When the hair was removed the potentials disappeared. Ipsilateral T-hairs excite MIG with a high probability (approximately 75%) while ipsilateral L-hairs excite MGI with a lower probability (approximately 50%).6.Thus the strengths of connection between hair type and MGI can be ranked as follows: Ipsilateral T-hairs provide the strongest excitatory input and therefore determine the overall directional characteristics of MGI. Ipsilateral L-hairs provide weaker, often subthreshold excitatory inputs and contralateral L-hairs provide the weakest excitatory input to MGI. Finally the contralateral T-hairs inhibit MGI.

Abstract: 1 The embryonic development of the abdominal cerci of the house cricketAcheta domesticus is described from scanning and transmission electron microscope data 2 A staged description of externally visible events in embryogenesis is tabulated as a context for describing the chronology of embryonic development of the abdominal cerci 3 Three phases of cercal development are distinguished: differentiation of the cercal anlagen and secretion of the first embryonic cuticle; elongation of the cercus culminating in the secretion of the second embryonic cuticle after completion of a continuous epidermis at the time of dorsal closure; and differentiation of functional sensilla on the third embryonic (first instar) cuticle 4 The first axon profiles appear in the cercus immediately before elongation of the cercus These axons have dendrites with ciliary configuration in the lumen of the cercus Glial cells associated with the pioneer axons may precede the axons in occupying dorsal and ventral luminal midlines of the cerci 5 Trichoid sensilla appear in the integumet following apolysis of the second embryonic cuticle 6 Axons are added to the dorsal and ventral pioneer fibre bundles shortly before sensilla become apparent 7 The majority of sensory axons traverse the cercus during the final 15% of embryonic development 8 The sensilla of the first instar cercus do not achieve their final orientation until the cercal cuticle is expanded following eclosion from the second embryonic cuticle that encloses the hatchling until it reaches a free surface 9 The role of the pioneer fibres in establishing a pathway for the functional sensillar neurons is discussed in relation to other studies of sensillar development in insects

Abstract: Summary1.The pattern of functional connections between the receptors on the surface of the cerci of crickets and the two largest interneurons (MGI and LGI) of their ventral nerve cord was studied, with special attention given to groups of receptors defined by theirlocation on the cercal surface.2.Selectively blocking the transversely vibrating T-hairs on the dorsal and ventral surfaces of the cercus severely reduces the response of both interneurons in the ipsilateral connective to sound stimuli.3.Selectively blocking the longitudinally vibrating L-hairs on the medial and lateral surfaces leaves the response of the MGI unaffected and enhances the response of the LGI.4.Selectively stimulating the T-hairs of the contralateral cercus markedly reduces the response of both giant fibers to ipsilateral stimulation. This effect is tonic.5.We interpret these results to mean that ipsilateral T-hairs provide the main excitatory input to both the MGI and LGI, and thus determine the transversely oriented directional sensitivity curves of these cells. The contralateral T-hairs provide inhibition to both cells. Ipsilateral L-hairs excite the MGI weakly and apparently inhibit the LGI; there is also indirect evidence that contralateral L-hairs are weakly excitatory.6.Progressively blocking both groups of hairs starting at the tip of the ipsilateral cercus has very little effect on the giant fiber response until the proximal 1 or 2 mm are reached. Blocking the receptors starting at the base of the cercus drastically reduces the response and elevates the threshold even when the great majority of receptors are still free.7.Thus, there is variation in the effectiveness of inputs to the MGI and LGI both along the circumference of the cercus and along its length. There are several possible bases for such variation; we present one explicit model based on position-dependent probability of connection between receptors and interneurons. Finally, we suggest a causal link between the position of a receptor cell body on the surface of the cercus and the nature of the connections formed by its axon within the central nervous system.

Abstract: The hypothesis that pioneer fibers, which develop relatively early in the differentiation of insect appendages, serve to organize the peripheral sensory nerves was tested by ablating apical regions of the cercal rudiments in embryos of Acheta domesticus. Multiple nerve bundles rather than the normal middorsal and midventral pair of nerves were formed within the cercus following ablation of the cercal tip before pioneer fiber differentiation, but the cercal nerve was normal when lesions were made after formation of the pioneer fiber tracts and associated glia. These results indicate a necessary morphogenetic role for the pioneer fibers.