Cerebral Control of Fusimotor Activity in the Tenuissimus Muscle of the Cat: A Qualitative Exteriorized Study

Fusimotor neurons innervate intrafusal muscle fibers within muscle spindles. They are of at least three different types: static and dynamic axons innervating static and dynamic bag fiber respectively. Different areas in central nervous system are known to influence their activity. In present study, cortical control of the gamma motoneurons of tenuissimus muscle spindle in eight anaesthetized cats was investigated using direct visual method. Cats of either sex, weighing 2-3.8 kg were anaesthetized with sodium pentobarbitone (45 mg/kg), tracheotomized and intubated, obturator and femoral nerves cut and all the hindlimb muscles except tenuissimus denervated (right sciatic nerve also eat). Muscle spindles uncovered microsurgically and placed in the bath overviewed by a phillips camcorder. Right sensorimolor cortex was then exposed and a paraffin pool was made using skull skin. 0.3 to 3 mA current were applied, both anodal and cathodal, within a five second period of a 12 second stimulation window. Spontaneous and evoked intrafusal movements were monitored and taped for furthur analysis. The findings confirmed the evidence for independent cortical control of different types of static gamma motoneurons and revealed a topographical mapping of the sensorimotor cortex in relation to the type of gamma motoneurons recruited, either static or dynamic. Static effects were elicited following stimulation of a wide area across the sensorimotor cortex, the postcruciate dimple being almost at the center. Within that region, a "dynamic area" was identified from which dynamic effects were clearly elicited during stimulation. Different types of static gamma axons and dynamic gamma axon can be controlled independently on cerebral activation. The state of anaesthesia was crucial in obtaining reproducible results and its variation could alter the fusimotor effect from static to dynamic or even from excitation to inhibition.