Morphological variability and specializations in bovine extraocular musclespindles

Bovine extraocular muscles were examined to determine whether the structure of their muscle spindles was notably different from those commonly encount ered in mammalian limb muscles. Extraocular muscle spindles on the whole we re shorter, and intrafusal fiber counts/ spindle were more variable than in somatic muscles. No pronounced nuclear bags were seen in intrafusal fibers . Based on cross-sectional areas, intrafusal fibers in extraocular muscles could be loosely categorized as small or large types. Small fibers expresse d more neonatal/fast myosin heavy chain and less embryonic myosin heavy cha in than large fibers. When incubated for myosin ATPase, about 70% of the la rge fibers and 15% of the small fibers in spindles presented profiles that were characteristic of type I extrafusal fibers, and not of nuclear bag or nuclear chain fibers. The ratio of number of small intrafusal fibers to num ber of large intrafusal fibers in extraocular spindles was on average great er than the ratio of nuclear chain fibers to nuclear bag fibers that is typ ical for limb spindles of rodents and cats. Structural modifications at mus cle spindle sensory regions, extrafusal-like fibers and intrafusal-like fib ers with few equatorial nuclei and many myofibrils, may produce distinct af ferent signals that are appropriate for sensorimotor integration in the spe cialized extraocular muscles.