A new autosomal-recessive mouse mutant with progressive motor neuronopathy (pmn) is described. Homozygotes develop paralysis of the hindlimbs during the third week of life. Soon thereafter the forelimbs also become weak, and all mice die six to seven weeks after birth. Heterozygotes are normal. Skeletal muscles show neurogenic atrophy without histological signs of reinnervation. Axonal degeneration apparently starts at the endplates and is prominent in the sciatic nerve and its branches and the phrenic nerve. Axonal sprouts are abundant. There is no evidence of demyelination, and unaffected nerve fibers are normally myelinated. Sensory axons are spared. Almost all distal motor axons have disappeared in four to five weeks after birth. Ventral nerve roots show a reduced diameter of the largest fibers but no fiber deficits. The ventral horn cells show slight chromatolysis. The corticospinal tract is normal, but in terminally ill animals the fasciculus gracilis, the rubrospinal tract and possibly also reticulospinal fibers degenerate. The brain is histologically normal. The disease manifests itself in a dying-back fashion in the distal portion of the motor neurons and may represent an animal model of hereditary motor neuron diseases in man.

Electromyography (EMG), histology, and histochemistry were related in 264 patients with neuromuscular disorders classified according to history and clinical and other laboratory findings. Electromyography and histological and histochemical abnormalities were divided in specific and nonspecific criteria. Specific histochemical criteria alone identified 28% of neurogenic lesions. Criteria of myopathy, obtained from the pattern of electrical activity during 30% of maximal effort, helped to delineate a myopathy when the only abnormality was an increased incidence of polyphasic potentials together with a pattern of full recruitment during maximal effort. Histology, histochemistry, or both, and EMG were concordant with clinical findings in 77% of 188 patients with myopathy and in 91% of 64 patients with neurogenic lesions. The electromyogram was concordant with the clinical classification in 87% of patients with myopathy and in 91% of patients with neurogenic impairment. The biopsy was in agreement with or contributed to the classification in 79% of patients with myopathy and in 92% of patients with neuropathy.

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