In the vertebrate nervous system, axon calibers correlate positively with myelin sheath dimensions and electrophysiological parameters including action potential amplitude and conduction velocity. Neurofilaments, a prominent component of the neuronal cytoskeleton, are required by axons to support their normal radial growth. To distinguish between fiber features that arise in response to absolute axon caliber and those that are under autonomous control, we investigated transgenic mice in which neurofilaments are sequestered in neuronal cell bodies. The neurofilament deficient axons in such mice achieve mature calibers only 50% of normal and have altered conduction properties. We show here that this primary axonal defect also induces multiple changes in myelin sheath composition and radial dimensions. Remarkably, other fundamental fiber features, including internodal spacing and the architecture and composition of nodes of Ranvier, remain unaltered. Thus, many fiber characteristics are controlled through mechanisms operating independently of absolute axon caliber and the neurofilament cytoskeleton.

A case of hemifacial atrophy (Parry-Romberg disease) is discussed. Electrophysiological and immunological studies were performed. Electromyography, blink reflex and trigeminal evoked potential abnormalities indicate that the brain stem may be implicated in the aetiology of the disease. Immunological evidence favoured this possibility and demonstrated possible involvement of the noradrenergic system. Hyperactivity of the brain stem sympathetic centres, possibly caused by an autoimmune process, may be the primary cause of the cutaneous and subcutaneous atrophy in Parry-Romberg disease.

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