Regulation of soleus muscle spindle sensitivity in decerebrate and spinal cats during postural and locomotor activities.

Abstract

1. In order to study fusimotor control in reduced preparations, soleus muscle spindle afferents were recorded in premammillary decerebrate cats (n = 15) during crossed extensor reflexes and, after spinalization, during locomotion produced by either clonidine or L-l-3,4-dihydroxyphenylalanine (L-DOPA). The soleus muscle was oscillated sinusoidally (025 mm, 4 Hz) and the afferent mean firing rate and modulation were calculated. An increase in firing rate was assumed to arise from activity in dynamic y-motoneurones (dynamic y-drive) when associated with an increase in modulation to stretching, and in static y-motoneurones (static y-drive) when modulation decreased. 2. At rest in all preparations the firing rate and modulation in primary muscle spindle afferents were generally much higher than after de-efferentation (ventral root section), suggesting a predominant dynamic y-drive. Clonidine decreased and even eliminated this presumed resting y-drive in many afferents, both in the decerebrate (7 of 8) and the spinal (6 of 18) state. This effect on y-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 afferents generally fired with much higher mean rates (+121 %) and lower sensitivities (-32 %), suggesting a large increase in static y-drive (possibly accompanied by a small decrease in dynamic y-drive). These high rates were usually maintained tonically throughout the step cycle. However, a third of the afferents were silenced during locomotor contractions, and de-efferentation had no significant effect on their firing rates. Thus, for some spindles a-activity can occur without significant y-drive. 4. During locomotion in L-DOPA-treated spinal cats the inferred static y-drive only occurred phasically, coactivated with the EMG, though it could precede the EMG by 100-500 ms. In the flexion phase both the afferent rate and modulation were lower than before locomotion, suggesting a lack of effective y-drive. 5. Crossed extensor reflexes in decerebrate cats also produced a substantial increase in primary afferent firing rate (+187 %) and decrease in sensitivity (-37 %), again suggesting increased static y-drive (n = 18). This y-drive was largely independent of EMG activity and often occurred without a-activity. The mean firing rate of secondary muscle spindle afferents increased significantly during locomotion (with L-DOPA) and crossed extensor reflexes, again indicating increased static y-drive. Clonidine reduced or eliminated the y-drive in seven of eight afferents during crossed extensor reflexes. 6. In conclusion, although there are some common features, such as a predominant static y-drive in all walking preparations, the pattern of static and dynamic y-drive is not closely linked to a-activity under the conditions studied. As well as y-drive without a-activity, we have shown for the first time that a-motoneurones can be activated without significant y-drive to many spindles during behavioural tasks.

Ensemble firing of muscle afferents recorded during normal locomotion in cats

“…á-ãs linkage). This is in agreement with Gottlieb & Taylor (1983), Taylor & Donga (1989) and some of the conclusions of Bennett, De Serres & Stein (1996). We cannot rule out á_linked dynamic ã action (Murphy, Stein & Taylor, 1984;Greer & Stein, 1990;Murphy & Martin, 1993), because our technique of subtracting the predicted from actual afferent firing only identifies biasing.…”

Locomotor Training Maintains Normal Inhibitory Influence on Both Alpha- and Gamma-Motoneurons after Neonatal Spinal Cord Transection

“…Furthermore, studies in clonidine-treated spinal cats suggested that primary afferents fired at an increased mean rate, suggesting a large increase in static γ -drive, with the elevated rates usually being maintained tonically throughout the step cycle (Bennett et al, 1996). Although the long-term effects of a neonatal SCI on the discharge patterns of γ -MNs is still unknown, the above studies demonstrated that the physiological properties of γ -MNs are changed acutely postlesion.…”

Fusimotor Influence on Jaw Muscle Spindle Activity during Swallowing‐Related Movements in the Cat

“…Some of the earliest observations of spindle discharge in hindlimb and forelimb muscles in decerebrate animals (Perret & Busser, 1972;Perret & Berthoz, 1973) strongly suggested the presence of static fusimotor modulation in parallel with á_activity. Later, more detailed studies involving recording from hindlimb extensor ã-axons (Murphy, 1982;Murphy et al 1984;Murphy & Hammond, 1990) or spindles (Taylor, Stein & Murphy, 1985;Bennett, De Serres & Stein, 1996) have supported the idea that one group of ã_motoneurones fires tonically whilst another group fires in a modulated fashion, but identified the latter as dynamic. Evidence from chronic recordings from cat hindlimb spindles has been interpreted as indicating that the observed behaviour could be explained by essentially tonic levels of static and dynamic fusimotor firing, though with the possibility of some additional static modulation approximately in parallel with á-activity (Prochazka et al 1987).…”