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SUMMARY Physiological studies have been made of extensor digitorum brevis muscles in 28 healthy subjects aged between 60 and 96. Within this elderly population there was evidence of muscle wasting and weakness. These changes were shown to result from a loss of functioning motor units. The surviving motor units were often enlarged and tended to have relatively slow twitches. In some subjects the maximum impulse conduction velocities were reduced in motor nerves; there was evidence that slowing of impulse conduction could be especially marked in distal regions of axons. The findings are considered to indicate the presence of motoneurone dysfunction in old age.

A wide variety of terrestrial rocks has been investigated to determine the range of variation of the high-frequency electrical properties. Both solid rocks and powders of various types have been measured at frequencies of 450 MHz and 35 GHz; some measurements have been made at elevated temperatures. Solid materials show wide variations in permittivity and absorption length, but apart from a small trend with silica content it is not clear that there is any pattern to the variations, and it is unlikely that measurements limited to these radar frequencies will be useful for identification. With powdered rocks, on the other hand, the variation from rock type to rock type is much smaller, but depends on the powder density in • quite predictable way. Consequently, radar information can be employed to make precise estimates of density and of density profiles even in the absence of information about composition. The long absorption lengths in powdered rocks indicate that radar reflections may occur at considerable depths in a regolith of the type found on the moon. INTRODUCTION Notwithstanding the growing body of planetaw radar and radiometric measurements, there are available few reliable determinations of the electrical properties of terrestrial rocks and minerals in the frequency range between 100 and 100,000 MHz. The relatively extensive compilations of electrical characteristics at low frequencies [see, for example, Parkhomenko, 1967] rarely include measurements above 10 MHz. Measurements on a few materials over a very wide frequency range have been made by the Massachusetts Institute of Technology Laboratory for Insulation Research [von Hippel, 1954, 1957; Westphal, 1963]. Griffin and Marovelli [1967] have published data on a large number of samples of rocks of six types, in the frequency range 20 to 100 MHz. Data on tektites and chondrites between 420 and 1800 MHz are included in a paper by Fenslet et al. [1962]. The only radar frequency determinations with any claim to comprehensiveness are those of Krotikov [1962], who studied both solid and powdered rocks at 3.0, 9.4, and 38 GHz. We are unaware of any data at higher frequencies X Now at School of Physics, University of Sydney, Australia. Copyrlght ¸ 1969 by the American Geophysical Union. than Krotikov's, although measurements have been made on isolated materials in the far infrared by using quantitative spectroscopic techniques.In this paper we report measurements of the dielectric constant and loss tangent of a wide variety of terrestrial rocks and minerals at two frequencies, 450 MHz and 35 GHz. These frequencies, corresponding to wavelengths of 67 cm and 8.6 mm respectively, lie near the extremes of the range spanned by current radar and radiometric observations of the moon and planets. Although our chosen frequencies themselves delimit a range of almost two decades, it will appear from the results we report that the electrical properties, permittivity and loss tangent, at the higher frequency differ as a rule only slightly from those at the lower frequency...

SUMMARY In patients with various types of chronic motor denervation, the numbers of surviving motor units have been compared with the twitch tensions developed by the same muscle (extensor digitorum brevis). It was found that functional compensation in partially denervated muscles was often marked; in most patients abnormally small twitches occurred only when fewer than 10 % of motor axons remained. The factors responsible for this compensation are considered. The twitch speeds of partially denervated muscles differed markedly, even among patients with the same disorder; there was evidence to suggest that the twitches of some motor units might become slower than those found in normal muscles.

SUMMARY A comprehensive electrophysiological study has been made of the extensor digitorum brevis muscle and its motor innervation in 17 patients with dystrophia myotonica. The mean contraction and half-relaxation times were prolonged in the isometric twitches of dystrophic muscles. Decremental responses to repetitive motor nerve stimulation were found in two patients. All the terminal latency measurements were normal and only one patient had a reduced nerve conduction velocity. As the patients aged their muscles became weaker, due to a progressive loss of motor units. This finding, and the normal sizes of many surviving motor units, suggested that the muscle changes resulted from a primary defect of motor innervation.The aim of the present study has been to obtain detailed information concerning nerve and muscle function in dystrophia myotonica. Our approach has been to make most of the observations on a single muscle and its motor nerve supply, and then to correlate the experimental findings as fully as possible. The study has involved not only conventional procedures, such as the determination of motor nerve conduction velocity and the neuromuscular responses to repetitive stimulation, but also two more specialized techniques. One of these permits the characteristics of the isometric muscle twitch to be investigated , while the other enables an estimate to be made of the number of motor units in a muscle (McComas, Fawcett, Campbell, and Sica, 1971). The most important finding of this study was that the increasing weakness resulted from the loss of successive motor units. The significance of this finding has been considered in relation to the pathogenesis of dystrophia myotonica. A preliminary account of this work has been givenelsewhere (Campbell, McComas, and Sica, 1970). METHODS SUBJECTS There were six female and 11 male patients. Their ages ranged from 17 to 63 and in all cases the clinical and electromyographic findings were characteristic of dystrophia myotonica. In general, the youngest patients were affected least by the disease; the two oldest women, aged 59 and 63 respectively, were severely incapacitated and three men were unable to work because of their disabilities. A population of 27 male and 21 female subjects aged 17 to 58 and with no evidence of disease affecting the legs was drawn upon for control observations. STIMULATING AND RECORDING TECHNIQUES Three types of electrophysiological study involved the extensor digitorum brevis (EDB) muscle.1. The number of motor units was estimated by the method of McComas, Fawcett, Campbell, and Sica (1971).2. The isometric twitch of the extensor hallucis brevisthat is, the most medial part of EDB-was investigated using the technique of .3. The conduction velocities of motor nerve fibres in the deep peroneal nerve were determined from measurements of the latencies of potentials evoked in EDB after nerve stimulation at the ankle and at the head of the fibula. The stimuli were rectangular voltage pulses, 50 LAsec in duration, and were delivered through two chl...