Sustained contractions produced by plateau‐like behaviour in human motoneurones

Abstract

Electrical stimulation over human muscle can generate force directly by activation of motor axons and indirectly by ‘reflex’ recruitment of spinal motoneurones. These experiments were designed to define the properties of the centrally generated ‘reflex’ force, including the optimal stimulus conditions for producing it in tibialis anterior (TA) and triceps surae (TS), and its interaction with volition. Subjects (n= 21) were seated with their foot strapped to an isometric myograph. Surface EMG was recorded from TS and TA. High‐frequency electrical stimulation (100 Hz) of TS and TA with wide pulse widths (1 ms) was most effective to evoke the sustained centrally generated forces. The maximal force evoked by this mechanism during stimulation of TA for 40 s was ∼42 % of that produced by a maximal voluntary contraction. For both muscle groups, ramp increases and decreases in stimulus frequency (from ∼4 to 100 Hz and back to 4 Hz over 6 s) resulted in marked hysteresis in the force‐frequency plot. After a single ‘burst’ of 100 Hz stimulation during prolonged stimulation at 25 Hz, force remained elevated. Repeated bursts often generated progressively larger force increments. These behaviours were abolished by an anaesthetic nerve block proximal to the stimulation site, confirming the central origin for the ‘extra’ force. After a brief voluntary contraction was performed during 25 Hz stimulation, force remained elevated, and this showed some gradation with voluntary contraction amplitude. Sometimes voluntary contractions alone initiated the sustained central motor output. Involuntary contractions often persisted for many seconds after electrical stimulation ceased. These were not terminated by brief inhibitory inputs to the active motoneurones but could be stopped by the voluntary command to ‘relax completely’. Overall, these centrally generated contractions are consistent with activation of plateau potentials in motoneurones innervating the ankle dorsiflexors and plantarflexors. Large forces can be produced through this mechanism. The interaction with volitional drives suggests that plateau behaviour may contribute significantly to the normal output of human motoneurones.

High-frequency submaximal stimulation over muscle evokes centrally generated forces in human upper limb skeletal muscles

“…The patterns of stimulation were similar to those used in previous studies (1,8,9,41). In the first pattern, there was a 3-s "100-Hz burst" in a 7-s stimulation train.…”

“…In fact, the development of extra torques was similar for the biceps and FPL despite their different Ia monosynaptic connectivity. Furthermore, the enhanced torques had similar profiles to those in lower limb muscles (8,9,33) and wrist flexors (1). The extra torque development in FPL shows that strong Ia monosynaptic projections are not required for the development of enhanced force of central origin (see Ref.…”

“…One technique frequently used to elicit sustained muscle contractions in humans is through high-frequency trains of electrical stimuli (1,8,9,33,41). High-frequency (100-Hz) burst of electrical stimulation can reliably induce enhanced contractions in lower limb muscles of able-bodied and some spinal cord-injured subjects (9,41).…”

“…High-frequency (100-Hz) burst of electrical stimulation can reliably induce enhanced contractions in lower limb muscles of able-bodied and some spinal cord-injured subjects (9,41). These enhanced contractions in lower limb muscles are thought to be an effective mechanism to perform large muscle contractions or maintain tonic activity associated with postural tasks (22).…”

Beginning at the end: Repetitive firing properties in the final common pathway

“…Studies have also been undertaken on humans, with an emphasis on recording the discharge of single motor units (e.g., Kiehn and Eken, 1997). In selected instances, the data have suggested that plateau potentials are involved in the observed firing patterns (Eken and Kiehn, 1989;Kiehn and Eken, 1997;Gorassini et al, 1999Gorassini et al, , 2000Collins et al, 2001Collins et al, , 2002aWalton et al, 2002). To study the mechanisms (i.e., PICs and their modulation) underlying repetitive firing during behavior, however, it is necessary to use intracellular recording techniques.…”