Structure, distribution and innervation of muscle spindles in avian fast and slow skeletal muscle

Muscle spindles in 2 synergistic avian skeletal muscles, the anterior (ALD) and posterior (PLD) latissimus dorsi, were studied by light and electron m icroscopy to determine whether morphological or quantitative differences ex isted between these sensory receptors. Differences were found in the densit y, distribution and location of muscle spindles in the 2 muscles. They also differed with respect to the morphology of their capsules and intracapsula r components. The slow ALD possessed muscle spindles which were evenly dist ributed throughout the muscle, whereas in the fast PLD they were mainly con centrated around the single nerve entry point into the muscle. The muscle s pindle index (number of spindles per gram wet muscle weight) in the ALD was more than double that of its fast-twitch PLD counterpart (130.5+/-2.0 vs 5 5.4+/-2.0 respectively, n = 6). The number of intrafusal fibres per spindle ranged from 1 to 8 in the ALD and 2 to 9 in the PLD, and their diameters v aried from 5.0 to 16.0 mu m and 4.5 to 18.5 mu m, respectively. Large diame ter intrafusal fibres were more frequently encountered in spindles of the P LD. Unique to the ALD was the presence of monofibre muscle spindles (12.7 % of total spindles observed in ALD) which contained a solitary intrafusal f ibre. In muscle spindles of both the ALD and PLD, sensory nerve endings ter minated in a spiral fashion on the intrafusal fibres in their equatorial re gions. Motor innervation was restricted to either juxtaequatorial or polar regions of the intrafusal fibres. Outer capsule components were extensive i n polar and juxtaequatorial regions of ALD spindles, whereas inner capsule cells of PLD spindles were more numerous in juxtaequatorial and equatorial regions. Overall, muscle spindles of the PLD exhibited greater complexity w ith respect to the number of intrafusal fibres per spindle, range of intraf usal fibre diameters and development of their inner capsules. It is postula ted that the differences in muscle spindle density and structure observed i n this study reflect the function of the muscles in which they reside.