Augmentation of postural muscle tone induced by the stimulation of the descending fibers in the midline area of the cerebellar white matter in the acute decerebrate cat.

In a reflexively standing acute decerebrate cats, the cerebellar white matter was systematically stimulated and the effects on the level of postural muscle tone were studied. A stimulating microelectrode was placed systematically at 0.1-0.5 mm increments from H + 2 to H - 2 at levels ranging from P7.0 to P8.0 rostrocaudally and mediolaterally from LR0 to L1.5 or R1.5. Stimuli delivered to the restricted region of the cerebellar white matter along its midline resulted in simultaneous and bilateral augmentation of tonic activities in the neck, lumbar back, fore- and hindlimb extensor muscles along with increased levels in the forces exerted by each of the left and the right fore- and hindlimbs. Effective stimulus regions were located in the cerebellar white matter rostral and ventral to the most rostral part of the fastigial nucleus. Microinjection of a retrograde neural tracer, cholera-toxin b subunit conjugated horseradish peroxidase (CTb-HRP), into the lesioned effective stimulus sites resulted in a retrograde labeling of cells in the fastigial nuclei, bilaterally. All these results suggest that the augmentation of postural muscle tone was evoked by a selective activation of fastigiofugal fibers which course through the 'hook bundle'.