The topography of muscle spindles and Golgi tendon organs in the rotator cuff and surrounding shoulder muscles of a small laboratory marsupial (monodelphis domestica) were studied using light microscopy of serial sections. The shoulder joint of monodelphis has a large degree of freedom of movement allowing this animal to use the upper extremities for a wide range of activities like climbing and manipulating food. Thus, similar to the situation in man the shoulder joint is mainly secured by muscles. Silver stained serial paraffin sections were examined under the light microscope and the distribution of muscle spindles and Golgi tendon organs was reconstructed using three-dimensional image processing. In the two animals examined 113 and 131 muscle spindles respectively were found within the 4 rotator cuff muscles. In addition, 76 and 40 Golgi tendon organs respectively were seen at the musculo-tendinous junctions of these muscles preferentially close to the insertion at the humerus head. Also the surrounding shoulder muscles contain both muscle spindles and Golgi tendon organs in large numbers, but the ratio of Golgi tendon organs per muscle spindle appears to be lower. Number and localization of muscle spindles and Golgi tendon organs suggest, that these receptors are important for both reflex control of shoulder muscle tone as well as monitoring of static position and movement in the shoulder joint.
