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Integrative |
-motor output to ankle flexor muscles during locomotion in the decerebrate cat
1 Department of Clinical Neuroscience, Division of Neuroscience and Mental Health, Imperial College London, Charing Cross Campus, London W6 8RP, UK
In locomotion, the flexor muscles of the leg are mainly concerned with the relatively constant task of raising the foot, whereas the extensors have the more variable task of support and propulsion at different speeds. This suggests that the way in which the fusimotor system works may differ between the two muscle groups. Observations previously made of the static and dynamic 
-motor firing patterns in the ankle extensor medial gastrocnemius (MG) have therefore been repeated in the flexor tibialis anterior (TA). One or more single -motor axons, dissected from a small filament of TA nerve, were recorded simultaneously with a number of single spindle afferents in dorsal rootlets. Cats were decerebrated and locomoted spontaneously on a treadmill. Identification of each -motor axon depended on relating the changes in firing caused by midbrain stimulation to the changes in static and dynamic behaviour of the spindle afferents in response to repetitive ramp and hold stretches. Static axons all showed a smooth modulation in frequency, increasing in phase with muscle shortening, superimposed on a minimum frequency of about 20–30 impulses s–1. Dynamic axons showed interrupted firing with the frequency rising abruptly from zero at the onset of shortening, and falling again to zero shortly after the onset of lengthening. The frequency during the active periods was relatively constant, even when movement amplitudes varied. The basic similarity in the static and dynamic gamma discharge patterns for the two muscles suggests that the strategy of -motor control is common to both flexors and extensors. The static pattern is thought to be a ‘temporal template’ of the expected movement, effectively expanding the dynamic response range of the spindles in active movements. The dynamic pattern sensitizes the primary afferents to detect the onset of muscle lengthening and to detect departures from the intended movement trajectory.
(Received 9 November 2005;
accepted after revision 18 January 2006;
first published online 19 January 2006)
Corresponding author A. Taylor: Department of Clinical Neuroscience, Division of Neuroscience and Psychological Medicine, Imperial College London, Charing Cross Campus (Rm 10L05), St Dunstan's Road, London W6 8RP, UK. Email: t.taylor{at}imperial.ac.uk
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