CONTRACTILE PROPERTIES AND TEMPERATURE SENSITIVITY OF THE EXTRAOCULAR-MUSCLES, THE LEVATOR AND SUPERIOR RECTUS, OF THE RABBIT

1. Contractile and fatigue-resistance characteristics, temperature sen sitivity (10-37-degrees-C) of contraction, and histochemical fibre typ es were determined for two of the extraocular muscles, the superior re ctus and levator palpebrae superioris (levator), of the rabbit. 2. The levator displayed similar contractile characteristics (time to peak, half-relaxation time of twitch response, and twitch-tetanus force rati o) to mammalian fast-twitch limb muscle at room temperature (20-degree s-C). However, normalized twitch and tetanic force levels were signifi cantly less than those found in limb muscle. The superior rectus displ ayed the characteristics of even faster contraction than the levator a t 20-degrees-C, but generated lower maximum force levels than the leva tor. 3. The twitch response of the superior rectus showed a biphasic r elaxation phase. This response was not due to non-twitch (tonic) fibre s present in the superior rectus as it was unaffected by propranolol a pplication during muscle stimulation. 4. The superior rectus and levat or displayed significantly less fatigue in the tetanic force response than fast-twitch limb muscles did in response to a fatiguing electrica l stimulation protocol. The levator was significantly more fatigue res istant than the superior rectus. 5. The force responses of both extrao cular muscles displayed a similar dependence on temperature (10-37-deg rees-C) to limb skeletal muscles. 6. The superior rectus and levator e xhibited a high proportion of fast-twitch muscle fibres (type II) as s hown by myosin ATPase staining. Succinate dehydrogenase activity indic ated that these muscles showed a high oxidative capacity, with a stain ing intensity typical of type I or type II A fibres of limb muscles. 7 . The results emphasize the morphological and functional complexity of mammalian extraocular muscles. The combination of very fast contracti le properties with high oxidative capacity make these muscles well sui ted to their role in eye/eyelid movement.