Contractile properties of single skinned fibres from the extraocular muscles, the levator and superior rectus, of the rabbit.

1. The superior rectus and levator palpebrae superioris (levator), two of the extraocular muscles, were dissected from the rabbit and stored in a glycerol-based solution at -20 degrees C in order to prepare single, skinned fibres. 2. The Ca(2+)- and Sr(2+)-activated isometric contractile properties were determined for individual extraocular muscle fibres. Fibres were separated into discrete groups or fibre types on the basis of their physiological characteristics. The superior rectus and levator muscles were both found to consist of fibres which exhibited similar contractile characteristics to fast- and slow-twitch fibres from other mammalian muscle, including type I, type IIA and type IIB fibres. 3. As well as the existence of the normal, classical fibre types in extraocular muscle there were also a large number of fibres from both muscles which exhibited mixed fast- and slow-twitch contractile characteristics within the single contracting unit. Of the fibres sampled, the mixed fibres comprised the second largest population (7/19, 37%) in the levator and the largest fibre population in the superior rectus (11/31, 35.5%). These results are consistent with histochemical and immunohistochemical reports in the literature which suggest the co-existence of fast and slow myosin along the length of the extraocular muscle fibres. 4. Extraocular muscle fibres exhibited lower absolute maximum forces compared with other mammalian limb muscle fibres. However, when corrected for fibre cross-sectional area, the maximum tension development was within the normal range for mammalian limb muscle fibres, except for one group (type IIA) of fast-twitch fibres which exhibited significantly lower maximal tension. 5. The existence of a large proportion of fibres with composite fast- and slow-twitch characteristics highlights the functional and morphological complexity of these muscles. It is postulated that the functional significance of these mixed fibres may be to provide or enhance the resolution for subtle, precise movements of the eye and eyelid.