In a series of studies to investigate the basic structural features and characteristics of the biological apatite crystals using a transmission electron microscope, we examined the ultrastructure of the human enamel, dentin, and bone crystals through the cross and longitudinal sections at near atomic resolution. Subsequently, using the same approach, we have been able to directly examine the images of the lattice imperfections in the crystal lattices of the human tooth and bone crystals, and the images of the fusion of the crystals. In this research, furthermore, using transmission and scanning electron microscopes, we examined the dissolution of the enamel crystals caused by the carious enamel from the same viewpoint. The material used for the observation of the dissolution of the enamel crystals was obtained from the region which corresponds to the middle layer of the enamel at the portion near the wall of a carious cavity caused by the fissure caries on the occlusal surface of the lower first molars. Small cubes of the materials used for the observation by transmission electron microscope were fixed in glutaraldehyde and osmium tetroxide and embedded in epoxy resin using the routine methods. The ultrathin sections we1 e cut with a diamond knife without decalcification. The sections were examined with the HITACHI H-800H type transmission electron microscope operated at 200kV. Each crystal was observed at an initial magnification of 300,000 times and at a final magnification of 10,000,000 times and over. The material used for the observation by the scanning electron microscope was the fractured surface obtained from the carious enamel. The fractured carious enamel surfaces were coated with carbon and gold and observed with the HITACHI HHS-2R type scanning electron microscope operated at 25kV. The crystals were observed at a final magnification of 50,000 times As a result, we have confirmed that the dissolution of the enamel crystals caused by a caries occurs in the units of “hexagonal cell”. We sincerely believe that the electron micrographs shown in this report are the first to show the images of the dissolution of the enamel crystals caused by a caries at near atomic resolution.

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To clarify the effect of the change in motoneuronal excitability on the F wave, we studied the persistence, mean size, and minimum latency of the F wave in nine normal subjects while awake and asleep. Recordings were made from the abductor pollicis brevis muscle by stimulating the median nerve at the wrist. The persistence and size of the F wave markedly decreased during sleep, especially in stage REM. The mean size in stage REM was less than 5% of that in stage W in most subjects, and the F wave entirely disappeared in one subject. The minimum latency during sleep was longer than during wakefulness. Prolongation was within 2.0 ms when the persistence was more than 10%. A decrease in the number of motoneurons that elicit the F wave may be the major cause of prolongation. We conclude that the decreased motoneuronal excitability can cause the F wave to disappear without conduction block in the peripheral motor nerve and that the prolongation of the F wave for more than 2 ms provides a marker for proximal conduction delay in the clinical nerve conduction studies.

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