Morphometry of intramuscular nerves in amyotrophic lateral sclerosis

Selective loss of alpha motoneurons innervating the medial gastrocnemius muscle in a mouse model of amyotrophic lateral sclerosis

Mutations in the superoxide dismutase gene 1 (SOD-1) are found in patients with familial amyotrophic lateral sclerosis (FALS). Overexpression of a mutated human SOD-1 gene in mice results in neurodegenerative disease as result of motoneuron loss in lumbar spinal cord (10). Using this mouse model of FALS, we have established a quantitative assay utilizing the retrograde tracer Fluorogold (FG) to determine the number of motoneurons innervating one skeletal muscle in mice with ongoing disease. In adult wild-type mice, the number of alpha motoneurons retrogradely labeled by an injection of FG into medial gastrocnemius muscle is 50 +/- 7 and this number remains constant from 7 to 18 weeks of age. In mutant mice, the number of alpha motoneurons retrogradely labeled by FG is the same as in wild-type mice at 7 and 9 weeks, but then declines to 36% of that in normal mice at 18 weeks. This decline also correlates positively to severity of motor impairments in these mice as assessed by the hindlimb splay test. In contrast, the number of FG-labeled gamma motoneurons remains relatively unchanged in both wild-type and mutant mice up to 18 weeks. At 18 weeks of age, this apparent alpha motoneuron denervation is paralleled by an average of 55% reduction of MG-muscle mass and 40% weaker performance in the hindlimb splay test. These data suggest that alpha motoneurons are the most vulnerable neuronal subtype in this mouse model of ALS and it is primarily their loss that leads to functional motor deficits. This quantitative bioassay also will be valuable for evaluating novel therapeutics for ALS.

Extrafusal and intrafusal muscle effects in experimental botulinum toxin-A injection

The effects of botulinum toxin-A was compared on both extrafusal and intrafusal muscle fibers in the biceps femoris of Wistar rats. Four days after injection no action potentials were elicited with stimulation single-fiber electromyography on the injected side. Fourteen days after injection, jitter became measurable and these values were increased on the injected side. Extrafusal muscle fibers began to atrophy on the 4th day and this continued to the 14th day postinjection. Atrophy was also evident and progressive in intrafusal muscle fibers. Increased terminal innervation ratios, end-plate spread of cholinesterase, and increased density of very small myelinated fibers in large intramuscular nerves were observed 14 days postinjection. Both extrafusal and intrafusal fibers are cholinergically innervated, and both were progressively affected by botulinum toxin, perhaps varying in degree only. In addition to partial denervation, Botulinum toxin effects in dystonia may also be related to modified spindle afferent discharge.

Motor neuron disease (amyotrophic lateral sclerosis)

Amyotrophic lateral sclerosis is an insidiously developing, adult-onset, progressive anterior horn cell degeneration with associated degeneration of descending motor pathways. It has been recognized as an important clinical syndrome since the middle of the 19th century. Despite increasing clinical and research interest in this condition, its cause remains obscure, even in the broadest terms. Epidemiologic characteristics of the disease have been interpreted as evidence of both genetic and environmental causes. A major change in the view of this disease is the widely developing perception that it is a disease of elderly persons more than of middle-aged adults as was previously taught. Etiologic hypotheses encompass a broad range of postulated pathophysiologic mechanisms, and we review these in detail. The clinical limits of the disease can now be better defined by using modern diagnostic techniques. Although interest in supportive symptomatic therapy is growing, no intervention has yet been shown to modify the biologically determined motor system degeneration.

Peripheral nerve involvement in Werdnig-Hoffmann disease

The number of large myelinated axons was markedly decreased in almost all the intramuscular nerve bundles included in 32 muscle biopsies from patients with Werdnig-Hoffmann disease compared to that in normals. The morphometric analysis of peripheral nerves in 5 epon-embedded sections also showed a selective loss of larger myelinated fibers. The ultrastructural findings of the nerves were similar to those seen in Wallerian degeneration including axonal degeneration, myelin breakdown with phagocytosis, Schwann cell proliferation forming Schwann cell columns, axonal sprouting and probable remyelination. The earlier and more striking peripheral nerve involvement than that previously believed was not different from that seen in amyotrophic lateral sclerosis (ALS). The earlier damage to the peripheral nerves probably resulted from a degeneration of the anterior horn cells or anterior spinal roots as in ALS rather than from a dying-back process.