Laminar organization of the extraocular muscles of the rabbit

The functional organization of the inferior rectus muscle of the rabbit was studied with electrophysiological, mechanical, and anatomical techniques. The activity of single inferior rectus muscle fibers was recorded with a microelectrode while the isometric force of the inferior rectus muscle was modulated by vestibuloocular reflex activation. The active isometric force of the inferior rectus muscle at which a muscle fiber was recruited into activity was defined as the recruitment threshold for that particular muscle fiber. Recruitment thresholds were measured for 100 muscle fibers. Fibers with low recruitment thresholds were scattered throughout the inferior rectus, but concentrated most densely in the orbital rim. Multiple-unit recordings at various depths within the inferior rectus muscle demonstrated an almost exclusive activity of the orbital rim fibers at active isometric forces of less than 1 g. The development of active tension in the orbital layer of a surgically divided inferior rectus muscle precedes the development of tension in the nonorbital layer. Histochemically, the orbital layer of the inferior rectus muscle has a high oxidative metabolism as evinced by succinic dehydrogenase, diphosphopyridine nucleotide diaphorase, and acid-preincubated ATPase stains. The entire inferior rectus muscle stained intensely for ATPase with alkali preincubation. An analysis of serial cross sections of the inferior rectus muscle near its insertion revealed that the fibers of the orbital rim have the most lateral and the most anterior insertions. These data suggest that muscle fibers of the orbital rim of the inferior rectus muscle maintain the stability of the eye during horizontal eye movements by moving the "effective insertion" of the muscle in the opposite direction to the movement of the "anatomic insertion.".