STRUCTURE AND COMPOSITION OF PERIPHERAL-NERVES AND NERVE ROOTS IN SPRAWLING MOUSE

Abstract

The Sprawling mouse is a neurological mutant which shows marked ataxia, a spastic gait and inability to place the paws and grip. The condition has an autosomal dominant inheritance and affects sensory pathways predominantly. Hindlimb muscles in particular show a deficiency in muscle spindles. Peripheral nerves and lumbar nerve roots of Sprawling and normal mice were examined with light microscopy and EM. The peripheral nerves and the dorsal roots were thin and grey and were composed predominantly of small myelinated and unmyelinated axons. No evidence of axonal or myelin degeneration was found. Quantitative studies showed a marked reduction in the total number of myelinated axons, most severe in the dorsal roots. In the ventral roots there was a deficiency in the contribution formed by myelinated axons of small caliber, probably indicating a deficiency of .gamma. fibers. Examination of the myelinated axons in nerves and roots showed a normal relationship between fiber size and internodal lengths and number of myelin lamellae. The genetic defect in Sprawling is probably responsible for a failure of myelination of sensory axons. The deficiency of large sensory axons and of small motor axons could be correlated with the deficiency of muscle spindles.