Morphological variability and specializations in bovine extraocular muscle spindles.
Bovine extraocular muscles were examined to determine whether the structure of their muscle spindles was notably different from those commonly encountered in mammalian limb muscles. Extraocular muscle spindles on the whole were 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 expressed more neonatal/fast myosin heavy chain and less embryonic myosin heavy chain than large fibers. When incubated for myosin ATPase, about 70% of the large 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 number of large intrafusal fibers in extraocular spindles was on average greater than the ratio of nuclear chain fibers to nuclear bag fibers that is typical for limb spindles of rodents and cats. Structural modifications at muscle spindle sensory regions, extrafusal-like fibers and intrafusal-like fibers with few equatorial nuclei and many myofibrils, may produce distinct afferent signals that are appropriate for sensorimotor integration in the specialized extraocular muscles.
Maier
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Department of Cell Biology, University of Alabama at Birmingham 35294-0019, USA.
D000818:Animals; D002415:Cats; D002417:Cattle; D008322:Mammals; D018485:Muscle Fibers, Skeletal / Q000166:cytology*; D018995:Myosin Heavy Chains / Q000032:analysis; D009218:Myosins / Q000032:analysis; D009801:Oculomotor Muscles / Q000166:cytology*; D012377:Rodentia; D013045:Species Specificity; D013385:Succinate Dehydrogenase / Q000032:analysis
D013385:Succinate Dehydrogenase; D018995:Myosin Heavy Chains; D009218:Myosins