Properties Of Extraocular Muscles Research Articles

The preferential involvement of extraocular muscles by myasthenia gravis

Extraocular muscles are preferentially involved by myas-thenia gravis, affecting nearly all patients during the course of the illness. Ocular manifestations of ptosis and diplopia may be the sole abnormality in 10–15% of patients. The anatomical and physiological properties of ocular muscles appear to form the primary basis of their susceptibility compared to other skeletal muscles. The autoimmune process differs among patients with ocular myasthenia and may contribute to the limitation of symptoms to the ocular muscles. This review highlights the unique properties of extraocular muscles and the levator palpebrae in order to provide insights in the involvement of these muscles by myasthenia gravis.

Expression of a novel combination of fast and slow troponin T isoforms in rabbit extraocular muscles.

The properties of extraocular muscles (EOMs) are quite different from those of the trunk and limb. Here we show that there is a novel pattern of troponin T (TnT) expression in EOMs which most likely contributes to the fine control of ocular movement and may reflect their innervation by cranial motoneurons. Three regions of the muscle were analysed to distinguish the TnT isoforms present in the fast singly-innervated fibres from those in the multiply-innervated fibres. More than 95% of the TnT in the singly-innervated fibres is TnT3f, which exhibits the most graded response to changes in calcium concentration during activation (Schachat et al., J. molec. Biol. 198, 551-4). In multiply-innervated fibres, which exhibit tonic contractures, the slow troponin T TnT2s is expressed. While neither TnT3f nor TnT2s is unique to EOM, this pattern is unusual in two respects: first, both TnT3f and TnT2s are minor components of the trunk and limb musculature, and second, most muscles express several fast and both slow TnT species. Although EOM occupies a highly specialized physiological niche, its unusual physiology is not reflected in the presence of new TnT isoforms but in the expression of a different ratio of the known species of TnT.