Antagonist co-contraction is observed in humans during voluntary elbow rotations (Patton and Mortensen, 1971), isometric clasping of the hand (Long et al., 1970), and walking along a balance beam (Llewellyn et al., 1990). Co-contraction will stiffen and stabilize joints, which may aid in the performance of new motor tasks,

or those subject to unpredictable perturbations. Spinal pathways have been implicated in suppressing reciprocal inhibition mediated by inhibitory group Ia interneurons in order to promote co-contraction. During voluntary co-contraction of antagonist ankle muscles, this suppression has been shown to involve enhanced recurrent inhibition of Ia interneurons as well as an increase in presynaptic inhibition of group Ia afferents that excite Ia interneurons, though the mechanisms underlying co-contraction at the wrist appear distinct ISRIB concentration (Pierrot-Deseilligny and Burke, 2006). Cortical output during voluntary co-contraction is unlikely simply to reflect the combination of separate drives for activating two antagonist muscles. Recordings from motor cortex have detected units specifically active during co-contraction (Humphrey and Reed, 1983). Some CSMNs facilitate activation of certain wrist muscles but suppress their antagonists—and these have been shown to fire during flexion and extension movements Selleckchem ABT888 but can cease during isometric clasping (Fetz and Cheney, 1987). Moreover, the suppression of group Ia inhibition during the co-contraction

of ankle antagonists is far greater than that expected based on the inhibitory activity observed during activation of either muscle alone (Nielsen and Kagamihara, 1992). Lastly, measurements of cerebral blood flow (Johannsen et al., 2001) and EEG-EMG coherence (Hansen et al., 2002) suggest that distinct corticospinal pathways may be active during co-contraction of ankle antagonist muscles compared to the separate activation of either muscle alone. If parallel descending pathways exist, how do they engage

spinal circuits? A pathway involved in co-contraction could directly target interneurons mediating recurrent and presynaptic inhibition. Exploiting genetic access to measure and perturb activity in CSMNs targeting these interneurons could implicate the involvement of particular unless spinal targets in a co-contraction pathway. It is also possible that the generation of appropriate motor neuron drive during co-contraction involves indirect pathways through other spinal interneurons or descending relay systems. Intriguingly, measurements of forelimb EMG in rats during a reach-to-target task show distinct movement phases in which antagonist muscles either alternate activation or co-contract (Hyland and Jordan, 1997). Nevertheless, it is still possible that there is substantial overlap in the CSMNs active during co-contraction and flexion-extension movements and that temporal patterning of CSMN output is critical to differential recruitment of motor neurons.