Alan M. Hirahara, MD, FRCSC
Orthopaedic SurgeonSpecialist in Sports Medicine

Spatial Distribution of Type II Fibers in the Soleus of Spinal Isolated Cats

Roy RR, Garfinkel A, Ounjian M, Payne J, Hirahara A, Hsu E, Edgerton VR
Muscle Nerve
DOI: 10.1002/mus.880181015
PMID: 7659113

October 1st 1995 - December 31st 1991

Senior thesis evaluating the spatial distribution of type II fibers in the soleus of spinal isolated cats.


The motor unit is the basic unit for force production in a muscle. However, the position and shape of the territory of a motor unit within the muscle have not been defined precisely. The territories of five motor units in the cat tibialis anterior muscle were reconstructed three-dimensionally (3-D) from tracings of the glycogen-depleted fibers belonging to each unit. The motor unit territories did not span the entire length of the muscle and their cross-sectional areas tapered along the proximodistal axis producing a conical shape. In addition, the position of the territory of each unit shifted in an anterior-posterior plane along the longitudinal axis of the muscle, presumably as a consequence of the pinnation of the fibers. The area of the motor unit territory at any given level along the proximodistal axis was highly correlated with the number of fibers within the territory at that level. Connective tissue boundaries (outlining fascicles) appeared to have a strong influence on the shape of the territory, territories showed abrupt changes at connective tissue boundaries as groups of motor unit fibers within a fascicle often terminated together while motor unit fibers in neighboring fascicles did not terminate. It is likely that the mechanical impact of the recruitment of a motor unit is affected by the location and shape of motor units within the same muscle area.(ABSTRACT TRUNCATED AT 250 WORDS)