Patterns of fusimotor activity during locomotion in the decerebrate cat deduced from recordings from hindlimb muscle spindles

Abstract

The part played by muscle spindles in the control of natural movements must depend on how the static and dynamic gamma (ãs and ãd) fusimotor systems are activated. However, their patterns of activity have been difficult to elucidate, because technical problems severely limit the possibilities for directly recording from ã_motoneurones. The alternative approach of deducing ã-patterns from spindle afferent records has been used in a variety of reduced preparations Journal of Physiology (2000), 522.3, pp. 515-532 515 Patterns of fusimotor activity during locomotion in the decerebrate cat deduced from recordings from hindlimb muscle spindles 1. Recordings have been made from multiple single muscle spindle afferents from medial gastrocnemius (MG) and tibialis anterior (TA) muscles of one hindlimb in decerebrate cats, together with ankle rotation and EMG signals, during treadmill locomotion. Whilst the other three limbs walked freely, the experimental limb was denervated except for the nerves to MG and TA and secured so that it could rotate only at the ankle joint, without any external load. Each afferent was characterised by succinylcholine testing with regard to its intrafusal fibre contacts. Active movements were recorded and then replayed through a servo mechanism to reproduce the muscle length changes passively after using a barbiturate to suppress ã-motor firing. 2. The difference in secondary afferent firing obtained by subtracting the discharge during passive movements from that during active movements was taken to represent the profile of static fusimotor activity. This indicated an increase before the onset of movement followed by a strongly modulated discharge in parallel with muscle shortening during locomotion. The pattern of static firing matched the pattern of unloaded muscle shortening very closely in the case of TA and with some phase advance in the case of MG. The same effects were observed in primary afferents. 3. Primary afferents with bagÔ (b1) contacts in addition showed higher firing frequencies during muscle lengthening in active than in passive movements. This indicated increased dynamic fusimotor firing during active locomotion. There was no evidence as to whether this fluctuated during the movement cycles. 4. When the mean active minus passive difference profile of firing in bagµ-chain (bµc) type primary afferents was subtracted from that for b1bµc afferents, the difference was dominated by a peak centred on the moment of maximum lengthening velocity (v). 5. The component of the active minus passive difference firing due to b1 fibre contacts could be modelled by f(t) = av (where a is a constant) during lengthening and by f(t) = 0·2av during shortening. The remainder of the difference signal matched the predictions of the static fusimotor signal derived from secondary afferents. 6. The findings are discussed in relation to the concept that the modulated static fusimotor pattern may represent a 'temporal template' of the expected movement, though the relationship of the results to locomot...

Neuromuscular Abnormalities Associated With Spasticity of Upper Extremity Muscles in Hemiparetic Stroke

“…This idea has been supported by others suggesting that CNS reorganizes its neuronal connections following lesions (Carr et al 1993) by developing new connections including sprouting and strengthening of existing connections to optimize its performance (Carr et al 1993). Another possible mechanism is increased stretch-evoked synaptic excitation of motoneurons that can arise from ␥-motoneuron hyperactivity that increases muscle spindle sensitivity (Prochazka 1989;Taylor et al 2000) and an increase in sensitivity of excitatory interneurons to muscle afferents. This high sensitivity could arise from increased afferent terminal sprouting, increased postsynaptic receptor hypersensitivity, and reduced presynaptic inhibition (Burke and Ashby 1972;Burke and Lance 1973;Burke et al 1971;Iles and Roberts 1986;Katz and Rymer 1989;Stein et al 1995;Young and Shahani 1980).…”

Heterogenic Feedback Between Hindlimb Extensors in the Spontaneously Locomoting Premammillary Cat

“…Afferents carrying length information from muscle spindles project mainly to parent motoneurons and close synergists and are thought to contribute to the regulation of muscular stiffness (Nichols and Houk 1976) and enhanced force output during stance (Mazzaro et al 2005(Mazzaro et al , 2006Stein et al 2000). Additionally, length feedback is subject to modulation during the step cycle (Ellaway et al 2002;Sinkjaer et al 1996;Taylor et al 2000) at the stance-swing transition.…”

Static Fusimotor Action During Locomotion in the Decerebrated Cat Revealed by Cross‐Correlation of Spindle Afferent Activity

“…These features with a long time course did prove to be related to stretch of the muscle. Recording the movement at the ankle and applying the same movement passively during suppression of fusimotor activity with barbiturate anaesthesia confirmed this (Taylor et al 2000a). The broad peak or oscillation in the correlogram persisted whereas the brief peak that we have associated with chain fibre activation was abolished.…”

“…This implies that g S -efferents were active before locomotion commenced which is supported by direct recordings (Murphy et al 1984;Taylor et al 2000b). It would also suggest that rate modulation rather than recruitment of g S -efferents might be the basis for the wind-up in spindle discharge rates that has repeatedly been observed prior to the start of locomotion (Prochazka et al 1976;Taylor et al 2000a). For the MG muscle during locomotion, synchrony was equally evident during ankle flexion and extension.…”