Motoneuronal innervation and mechanical properties of extraocular muscles in the catfish, (Ictalurus punctatus).

Mechanical characteristics and electrical activity were studied in the extraocular muscles of the catfish, Ictalurus punctatus. The contractile properties were determined by stimulation of the individual muscle nerve branches to lateral and medial rectii and the superior and inferior oblique muscles. The speed of contraction was higher than in most other fish muscle, with a twitch contraction time of about 12 ms and a tetanus fusion frequency of 150-170 Hz in all four eye muscles. The fatigue resistance was also high. These properties were the same in fully innervated and partially innervated muscle, largely irrespective of what part of the muscle that was activated. Although different fibre types are known to exist in fish extraocular muscle, it was not possible to obtain functional separation of the mechanical force profile even in the lateral rectus with two distinct motoneuronal innervations. We suggest that polyneuronal innervation of the muscle fibres produces the mechanical responses. Since EMG activity during spontaneous eye movements was similar in the global and the orbital parts of the muscle, all types of fibres in fish extraocular muscle are probably recruited for all types of eye movements.