Re-afferent effects of individual static and dynamic gamma-stimuli during maintained fusimotor stimulation

The ability of maintained dynamic and static fusimotor stimulation to modulate the primary afferent response of the muscle spindle in the rhythm of gamma-stimulation was investigated using a highly sensitive method for modulation detection. The effect of 41 gamma-fibers (13 dynamic; 28 static) on 38 primary afferents obtained from the tibialis anterior muscle of the cat was studied. It was found that maintained stimulation of 10 out of the 13 dynamic (77%) and of 25 out of the 28 static (89%) gamma-fibers could evoke significant modulations of the primary afferent response in the rhythm of fusimotor stimulation at a minimum of one stimulation rate. Moreover, both static and dynamic gamma-stimulations could evoke significant primary afferent modulations almost over the entire range of stimulation rates studied (30-300 stimuli per second). These results show that both gamma-systems can modulate the primary afferent response in the rhythm of fusimotor stimulation over a wide range of stimulation rates; thus the central nervous system may be provided with re-afferent information about the effect of each individual gamma-motoneuron discharge. Some hypotheses for the internal spindle mechanism responsible for the afferent modulations are discussed.