SUMMARY1. The effects of the muscle-depolarizing drug succinylcholine (SCh) on the stretch responses of jaw-closer muscle spindle afferents were studied in the anaesthetized cat. Using ramp and hold stretches repeated every 6 s the basic measurements made were: initial frequency (IF), peak frequency (PF) and static index (SI), the frequency 0-5 s after the end of the ramp of stretch. Derived from these were: dynamic difference (DD) = PF -IF, dynamic index (DI) = PF -SI and static difference (SD) = SI -IF. Increases in these measures caused by a single i.v. dose of SCh (200 gtg kg-') are symbolized by the prefix A.2. In a population of 234 units, ADD and AIF were each distributed bimodally, but were uncorrelated, thus defining four subgroups.3. ADD was argued to be an index of the effect of bag1 intrafusal fibre contraction and AIF to be an index of the effect of bag2 fibre contraction. On this basis it is proposed that units can be divided into four groups according to the predominant influences of the bag1, bag2 and chain fibres as blc (6-8%), b1b2c (22-2%), b2c (54-3%) or c (16&7%).

Simultaneous recordings were made from gamma (γ) motor axons and from muscle spindle afferents of the medial gastrocnemius (MG) muscle during locomotion in decerebrate cats. The γ‐neurons were identified as static or dynamic (γs or γd) by correlating their behaviour during midbrain stimulation with changes in muscle spindle afferent responses to muscle stretch. On the basis of their behaviour during locomotion, γs neurons could be divided into two groups. One group (type‐1) showed strongly and smoothly modulated discharge increasing in parallel with the active muscle shortening in ankle extension, but with phase advance. The other group (type‐2) also showed a modulated pattern, but with increased firing centred on the flexion phase. The proportions of the two were 13 type‐1 and 7 type‐2. The type‐1 firing pattern accurately predicted the difference in firing frequency for secondary afferents obtained by subtracting from the recordings made during active movements the response of the same units to the movements repeated passively in the absence of fusimotor activity. The type‐2 pattern also became consistent with the difference signal, when operated on by a phase lag appropriate to the effects of bag2 intrafusal fibres. These results suggest that there may be some degree of separate control of chain and bag2 intrafusal fibres. The discharge of γd axons was also found to fluctuate with the locomotor cycle, with a pattern very distinct from that of the γs records. The γd firing frequency rose very suddenly from zero to a maximum at the onset of muscle shortening and continued into the beginning of lengthening. The term ‘interrupted’ discharge is suggested as a useful description. The timing of this discharge was shown to be appropriate for sensitising the primary afferents to detect the onset of stretch.

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...

Mammals may exhibit different forms of locomotion even within a species. A particular form of locomotion (e.g. walk, run, bound) appears to be selected by supraspinal commands, but the precise pattern, i.e. phasing of limbs and muscles, is generated within the spinal cord by so-called central pattern generators. Peripheral sense organs, particularly the muscle spindle, play a crucial role in modulating the central pattern generator output. In turn, the feedback from muscle spindles is itself modulated by static and dynamic fusimotor (gamma) neurons. The activity of muscle spindle afferents and fusimotor neurons during locomotion in the cat is reviewed here. There is evidence for some alpha-gamma co-activation during locomotion involving static gamma motoneurons. However, both static and dynamic gamma motoneurons show patterns of modulation that are distinct from alpha motoneuron activity. It has been proposed that static gamma activity may drive muscle spindle secondary endings to signal the intended movement to the central nervous system. Dynamic gamma motoneuron drive appears to prime muscle spindle primary endings to signal transitions in phase of the locomotor cycle. These findings come largely from reduced animal preparations (decerebrate) and require confirmation in freely moving intact animals.