Internal structure of cat extraocular muscle

Abstract

Teasing preparations of cat extraocular muscles (EOM) were used to study the arrangement of muscle fibers and the distribution of the different cholinesterase-positive sites, i.e. (1) large motor endplates, (2) small motor endings of the 'en grappe' type, (3) myotendinous junctions and (4) myomyous junctions. The distribution of these cholinesterase-positive structures gives clear evidence of a complex muscle architecture of cat EOM. In the global layer of cat EOM, only multiply innervated muscle fibers run the whole length of the muscle. The focally innervated muscle fibers are generally shorter; they are usually arranged in series of two to three fibers being interconnected by myomyous junctions. Moreover, muscle fiber splitting is frequently present resulting in a netlike arrangement of muscle fibers. Most of the myomyous junctions occur between focally innervated muscle fibers, but also end-to-side connections of focally to multiply innervated muscle fibers are observed; multiply innervated muscle fi0ers have not been found connected to each other. In this layer, large motor endplates are distributed in several bands between origin and insertion. In the orbital layer all muscle fibers run from tendon to tendon, focally as well as multiply innervated ones. Here, large motor endplates are confined to a band in the middle of the muscle, and myomyous junctions are generally absent. Some functional implications of this complex architecture of cat EOM are discussed.

Magnetic Resonance Imaging of Human Extraocular Muscles in Convergence

“…Finally, it is not clear if EOM contractility is uniform along the anteroposterior lengths of EOMs, and it is further doubtful if EOM fiber tensions are uniformly transmitted from origin to insertion. While Mayr reported that the feline OL fibers run the entire extent of the EOM (Mayr et al 1975), Alvarado found a variation in fiber number along the length of the feline IO muscle (Alvarado and Van Horn 1975). Gradual distal reduction in the number of IR and LR fibers has been reported in rabbit (McLoon et al 1999).…”

Extraocular Motor Unit and Whole-Muscle Responses in the Lateral Rectus Muscle of the Squirrel Monkey

“…It has been shown that the cat lateral rectus muscle is composed of serially arranged and branching muscle fibers rather than having all its fibers arranged in parallel arrays (Mayr et al, 1975;Alvarado-Mallart and Pinçon-Raymond, 1976). Other investigators have speculated that such an arrangement could lead to a loss of expected force when the muscle contracts (Katz, 1939;Demieville and Partridge, 1980;Goldberg et al, 1997b).…”

“…It remains to be seen whether primate extraocular muscle will show nonparallel arrays of muscle fibers as found in the cat (Mayr et al, 1975;Alvarado-Mallart and Pinçon-Raymond, 1976) and whether the summation of motor unit forces will sum to equal the observed whole-muscle forces. In addition, we also may need to account for the pronounced hysteresis in single motor unit force.…”

Polyneuronal Innervation of Single Muscle Fibers in Cat Eye Muscle: Inferior Oblique

“…When the major wiring of the neuromuscular system has been accomplished as in normal adults and only minor adjustments may be required, the electrophysiological tests for polyneuronal innervation failed to produce positive results (Brown and Matthews 1960). It may be argued that the tension excess observed here in EOM and in previous studies in other skeletal muscles could be due to the complex muscle architecture (Alvarado-Mallart and Pinçon-Raymond 1976;Goldberg et al 1997;Mayr et al 1975;Trotter et al 1995). The internal properties of the muscle can result in nonlinear summation of motor-unit forces (Goldberg et al 1997;Monti et al 2001;Perreault et al 2003;Shall et al 2003;Troiani et al 1999).…”