Fiber analysis of an insect peripheral nerve: light vs. electron microscopy

Development of bilateral asymmetry in sensory innervation to lobster claws

Sensory innervation to the paired claws of the lobster. Homarus americanus, was examined during their differentiation from a bilaterally symmetric state to an asymmetric state of a slender cutter and a stout crusher claw. This was done by estimating the total number and size distribution of axons in the second nerve root which provides most (approximately 90%) of the innervation to the claws and has few, bilaterally distributed motor axons. The paired claws which are undifferentiated and resemble each other in the 1st larval stage correspondingly have nerve roots that are bilaterally symmetric. In early juvenile (4th and 5th) stages when claw type is determined, as well as in subsequent (6th, 7th, 8th, 16th) juvenile stages when the claws gradually differentiate into cutter and crusher types, the paired homologous roots are also similar. It is only in adults that asymmetry in sensory innervation is seen with more axons in the crusher root than in its cutter counterpart. The difference in number of axons between the dimorphic claws is related to differences in surface area between the claws. Thus, bilateral asymmetry in sensory innervation is acquired by the continual but differential addition of axons to the paired claws.

Cellular morphology of chronic spinal cord injury in the cat: analysis of myelinated axons by line-sampling

A systematic line-sampling method is described for counting and mapping myelinated axons in transverse sections of the spinal cord. Its advantages over random sampling of small areas are considered. The technique was applied to quantifying experimental weight-drop contusion injuries of cat spinal cord, from several months to more than a year after injury. Contusion of the mid-thoracic cord with a 20 g weight dropped 20 cm was usually sufficient to produce chronic hindlimb paralysis whilst allowing the survival of significant numbers (40,000-110,000) of myelinated axons passing through the lesion site. The axons which survived were concentrated towards the pial surface. There was a proportionally greater loss of larger diameter axons, but this was independent of distance from the pia, indicating that at least two independent factors contribute to selective axonal death following injury, one related to depth within the cord, the other to axon diameter. Myelin sheath thickness was decreased from normal and this deficit also increased with depth. There was overlap in all these quantitative morphological characteristics between animals showing some recovery of hindlimb locomotion and those with maintained spastic paralysis at more than six months after injury. Effective locomotion was found to recover in some cases with the maintenance of a small proportion (5-10%) of the original axonal population, largely concentrated in a rim only 200-300 microns thick. Morphological correlates of paralysis in chronic injuries included severe reduction of axonal number, selective elimination of large fibers, and sustained dysmyelination. Any one or combination of these may be responsible for chronic paralysis in individual animals.

The fine structure of the cockroach subgenual organ

This paper describes the fine structure of the cockroach subgenual organ, a complex ciliated mechanoreceptor that detects vibrations in the substrate upon which the animal stands. Located beneath the knee in each walking leg, the cockroach subgenual organ is a thin, fan-shaped flap of tissue slung across the dorsal blood space of the tibia at right angles to the leg's long axis. It is innervated by approximately 50 chordotonal sensilla. The fine structure of the chordotonal sensilla is is described in detail; possible transducer sites are discussed.