FELINE CAUDOFEMORALIS MUSCLE - MUSCLE-FIBER PROPERTIES, ARCHITECTURE,AND MOTOR INNERVATION

Feline caudofemoralis (CF) is a promising preparation in which to stud y the properties of mammalian fast-twitch skeletal muscle, but little is known about its muscle fiber properties, architecture, and motor in nervation. We used histochemical techniques to confirm that it contain ed predominantly type IIB fibers (95+/-2%, n=8, with six of eight musc les composed exclusively of type IIA and IIB fibers), but physiologica l experiments showed less fatiguability than for the type IIB componen t of medial gastrocnemius. This may be related to the surprisingly str ong and regular recruitment of CF during repetitive tasks such as walk ing and trotting, which we demonstrated electromyographically. We meas ured muscle length over the anatomical range of motion for CF (similar to 0.6-1.2 L-0) and estimated working length during walking and trott ing (similar to 0.95-1.15 L-0). The specific tension was similar to th at of the exclusively slow-twitch soleus muscle (31.2 +/- 4.7 N/cm(2) compared with 31.8+/-4.1 N/cm(2); P>0.8). Single fiber dissections of CF revealed a series-fibered architecture with a mean of 2.3 fibers, e ach 2.5 cm long, required to span the fascicle length. We identified t wo neuromuscular compartments in CF by cutting one of the two nerve br anches innervating CF and depleting the glycogen stores in the intact motor units. These compartments were in parallel and extended the leng th of the muscle; their electromyographic activity was similar during various natural behaviors. CF and gluteus maximus motoneurons were lab eled concurrently with a combination of fluorescent, retrograde tracer s including Fluororuby, Fluorogold and Fast Blue. The CF motor nucleus was located in L7-S1, overlapping and intermingling extensively with the nucleus of the adjacent gluteus maximus muscle. Distributions of C F motoneuron diameter revealed one large peak around 50-55 mu m, with relatively few small-diameter (less than 35 mu m) cells. Using estimat es of the total number of fibers in three muscles and the estimated nu mber of alpha-motoneurons for those same muscles, we calculated a mean innervation ratio of similar to 270, which is at the low end of the i nnervation ratios for type IIB motor units from other feline muscles a nd more similar to type IIA motor units. in general, CF appears to be a useful preparation in which to study the properties of fast-twitch m uscle, but these properties may vary somewhat from type IIB fibers fro m different muscles.