Elastic fibres and reticulin of mammalian muscle spindles and their functional significance.

Abstract

The distribution of elastic fibres and reticulin of muscle spindles of man, cat and rat was studied by light and electron microscopy. The elastic fibres form a continuous network, arranged longitudinally around the intrafusal fibres; this is connected to a much finer elastic fibre network among the extrafusal muscle fibres. In general there are many more elastic fibres around nuclear bag fibres than around nuclear chain fibres and it seems unlikely that the ‘elasticity’ of nuclear chain fibres observed experimentally can be due to the few elastic fibres which surround them. A comparison of the arrangement of elastic fibres on nuclear bag fibres in their extracapsular regions and in the lymph space suggests that nuclear bag fibres are more easily extensible within the lymph space: different portions of the same nuclear bag fibre may therefore also differ mechanically. In human muscle spindles two types of nuclear bag fibre are distinguished: in one type the elastic fibres are concentrated on either side of the equatorial region, whereas in the other type they are more evenly distributed along the length of the fibre. This suggests that these bag fibres may be subjected to different mechanical stresses, possibly due to different motor innervation. Most of the elastic fibres in the region of the primary sensory ending ramify among the inner capsule cells immediately surrounding the intrafusal bundle. This may be a mechanism of bridging the coils of the primary sensory ending while maintaining or increasing the mechanical strength of the region. Each individual intrafusal fibre has a reticulin covering along its length and the sensory nerve terminals lie between the intrafusal fibre and its reticulin covering. In the equatorial region the entire intrafusal bundle is bound together by an additional covering of reticulin. It is suggested that when intrafusal fibres are stretched the sensory endings may be compressed between the intrafusal muscle fibre and the reticulin and/or elastic fibres with resultant distortion and depolarization of the sensory endings.