Physiologically, the contractile characteristics and electromyography (EMG) of cat inferior oblique (IO) muscle fibers supplied by the medial and lateral 10 muscle nerve branches were studied by direct nerve stimulation. Anatomically, the brain stem locations and sizes of 10 motoneuron soma were evaluated after retrograde labeling by horseradish peroxidase (HRP) through whole 10 muscle nerves and/or through each medial or lateral 10 muscle nerve branch. Stimulation of the lateral nerve branch elicited significantly (p < 0.005) slower twitch contraction times (8.0 ± 1.5 ms) and lower fusion frequencies (217 ± 46 Hz) than when the medial branch of the 10 nerve was stimulated (average twitch contraction time = 6.8+1.1 ms; average fusion frequency = 260 ± 34 Hz). The EMG wave shape responses in the global and orbital layers could be differentiated when the individual nerve branches were stimulated, but the response differences were not consistent among animals. The average diameter of 10 motoneuron soma with axons in the lateral branch of the nerve were significantly smaller (p < 0.001) than the average diameter of those IO motoneuron soma associated with the medial branch of the nerve (27.9 ± 7.2 vs. 32.9 ± 7.2 µm). Regardless of which nerve branch was labeled, the full range of motoneuron soma sizes was found, and these were distributed throughout the IO subdivision of the oculomotor nucleus. These findings showed that muscle contraction time and motoneuron soma diameter were correlated with the IO nerve branch subjected to stimulation or exposed to HRP. But no topographic organization of motoneurons was found within the IO division of the oculomotor nucleus.

Keywords:
Extraocular muscles, Contractile properties, Oculomotor nucleus, Motor unit, Electromyography, Fatigue, Cat
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© 1995 S. Karger AG, Basel
1995
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