A fine-structural study of the termination of intrafusal muscle fibres in the Chinese hamster.

The termination of intrafusal muscle fibres was studied in muscle spindles of the tenuissimus muscle of the Chinese hamster. Most nuclear chain fibres terminated within the spindle capsule. The nuclear bag- and nuclear chain fibres that extended beyond the limit of the spindle capsule terminated in the endomysial connective tissue of the adjacent extrafusal fibres. Three types of intracapsular terminations were found: (1) termination on the neighbouring nuclear bag fibre, (2) termination in the periaxial space without any obvious attachment site, or (3) termination on the spindle capsule. The intrafusal fibres were tapered and longitudinal sarcolemmal invaginations were poorly developed or were completely lacking. The fibre ends were characterized by their folded basal lamina, which appeared to be mechanically reinforced by associated collagen fibrils and elastic fibres.