REGULATION OF SOLEUS MUSCLE-SPINDLE SENSITIVITY IN DECEREBRATE AND SPINAL CATS DURING POSTURAL AND LOCOMOTOR ACTIVITIES

1. In order to study fusimotor control in reduced preparations, soleus muscle spindle afferents were recorded in premammillary decerebrate c ats (n = 15) during crossed extensor reflexes and, after spinalization , during locomotion produced by either clonidine or L-beta-3,4-dihydro xyphenylalanine (L-DOPA). The soleus muscle was oscillated sinusoidall y (0.25 mm, 4 Hz) and the afferent mean firing rate and modulation wer e calculated. An increase in firing rate was assumed to arise from act ivity in dynamic gamma-motoneurones (dynamic gamma-drive) when associa ted with an increase in modulation to stretching, and in static gamma- motoneurones (static gamma-drive) when modulation decreased. 2. At res t in all preparations the firing rate and modulation in primary muscle spindle afferents were generally much higher than after de-efferentat ion (ventral root section), suggesting a predominant dynamic gamma-dri ve. Clonidine decreased and even eliminated this presumed resting gamm a-drive in many afferents, both in the decerebrate (7 of 8) and the sp inal (6 of 18) state. This effect on gamma-drive may account, at least in part, for its suppressive effect on spasticity in humans. 3. When locomotion commenced in clonidine-treated spinal cats, primary afferen ts generally fired with much higher mean rates (+121%) and lower sensi tivities (-32%), suggesting a large increase in static gamma-drive (po ssibly accompanied by a small decrease in dynamic gamma-drive). These high rates were usually maintained tonically throughout the step cycle . However, a third of the afferents were silenced during locomotor con tractions, and de-efferentation had no significant effect on their fir ing rates. Thus, for some spindles alpha-activity can occur without si gnificant gamma-drive. 4. During locomotion in L-DOPA-treated spinal c ats the inferred static gamma-drive only occurred phasically, coactiva ted with the EMG, though it could precede the EMG by 100-500 ms. In th e flexion phase both the afferent rate and modulation were lower than before locomotion, suggesting a lack of effective gamma-drive. 5. Cros sed extensor reflexes in decerebrate cats also produced a substantial increase in primary afferent firing rate (+187 %) and decrease in sens itivity (-37 %), again suggesting increased static gamma-drive (n = 18 ). This gamma-drive was largely independent of EMG IG activity and oft en occurred without alpha-activity. The mean firing rate of secondary muscle spindle afferents increased significantly during locomotion (wi th L-DOPA) and crossed extensor reflexes, again indicating increased s tatic gamma-drive. Clonidine reduced or eliminated the gamma-drive in seven of eight afferents during crossed extensor reflexes. 6. In concl usion, although there are some common features, such as a predominant static gamma-drive in all walking preparations, the pattern of static and dynamic gamma-drive is not closely linked to alpha-activity under the conditions studied. As well as gamma-drive without alpha-activity, we have shown for the first time that alpha-motoneurones can be activ ated without significant gamma-drive to many spindles during behaviour al tasks.