Static and dynamic γ‐motor output to ankle flexor muscles during locomotion in the decerebrate cat

Abstract

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 γ-motor firing patterns in the ankle extensor medial gastrocnemius (MG) have therefore been repeated in the flexor tibialis anterior (TA). One or more single γ-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 γ-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 γ 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 γ 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 γ-motor control is common to both flexors and extensors. The static γ 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 γ pattern sensitizes the primary afferents to detect the onset of muscle lengthening and to detect departures from the intended movement trajectory.

Timing-Specific Transfer of Adapted Muscle Activity After Walking in an Elastic Force Field

“…For example, by pulling the foot forward, the force field changed the expected muscle elongation/shortening pattern. This likely changed spindles afferent discharge (Taylor et al 2006). In addition, by pulling the foot up, the force field tended to reduce loading of the experimental leg and to shift body weight to the other leg more rapidly at the end of stance.…”

The time course of the tonic oculomotor proprioceptive signal in area 3a of somatosensory cortex

“…These components resemble the dynamic and static ␥-motor firing patterns, respectively, of a fusimotor response (Taylor et al 2006). The onset of the tonic signal, which reflects the position of the eye in the orbit, is masked by the phasic component during a saccade task, making it difficult to determine when area 3a has access to stable eye position information.…”

Fusimotor control of spindle sensitivity regulates central and peripheral coding of joint angles

“…It was observed that static gamma activity formed a fusimotor template of intended movement (Taylor et al, 2006), and may carry kinematic information of joint angles. In the periphery, direct recording of Ia afferents from the dorsal ganglion cells of decerebrated cats suggested a similar conclusion that Ia afferents carried joint angle information (Stein et al, 2004).…”