Corticomotoneuronal cells are “functionally tuned”

Abstract

Corticomotoneuronal (CM) cells in the primary motor cortex (M1) have monosynaptic connections with motoneurons. They are one of the few sources of descending commands that directly influence motor output. We examined the contribution of CM cells to the generation of activity in their target muscles. The preferred direction of many CM cells differed from that of their target muscles. Some CM cells were selectively active when a muscle was used as an agonist. Others were selectively active when the same muscle was used as a synergist, fixator, or antagonist. These observations suggest that the different functional uses of a muscle are generated by separate populations of CM cells. We propose that muscle function is one of the dimensions represented in the output of M1.

Primary motor cortex neurons classified in a postural task predict muscle activation patterns in a reaching task

“…Task-dependent processing in M1 has also been observed in previous reaching and posture comparisons (Kurtzer et al 2005). While the latency between atypical and typical activity was much shorter than the latency between the muscular agonist and antagonist, a recent study by Griffin et al (2015) also showed that cells that were functionally tuned for antagonist activity show activity only shortly after those tuned for agonist activity. The antagonist activity was relatively weak in the present study because of the speed of reaching and the contribution of friction from the exoskeleton assisting the deceleration of the limb.…”

“…It has been shown that neurons strongly active during precision grip were much less active for power grip that required more muscle force (Muir and Lemon 1983). As well, load representations can change between posture and movement, but their directional preference remains relatively constant (Kurtzer et al 2005) and can be only active when the muscle is used for a specific phase of movement (Griffin et al 2015). Even when neural activity reflects features such as the direction of the spatial goal, these neurons likely remain associated with a specific body part such as the wrist (Kakei et al 1999) or elbow/shoulder (Sergio and Kalaska 2003).…”

“…While the target innervation, or muscle field, of some neurons, such as CM cells, can be identified by STA of EMG Fetz 1980, 1985), this does not necessarily define the portions of the body to which the neuron fires. Even when limiting to CM cells that have monosynaptic connections to motoneuron pools, Griffin et al (2015) showed how directional tuning, or motor field, of some CM cells does not necessarily align with the muscle field, or connectivity. As well, CM cells represent only a small fraction of corticospinal neurons that predominantly target the distal musculature in primates (Buys et al 1986;Palmer and Ashby 1992) and are localized mainly in the caudal portion of M1 or "new M1" (Rathelot and Strick 2009).…”

“…As well, motor cortical neurons reflect aspects of motor function beyond patterns of motor output, such as correlates of movement kinematics (Kalaska et al 1989) and behavioral goals (Kakei et al 1999). Neural activity is also context (Hepp-Reymond et al 1999), phase (Griffin et al 2015), and task (Buys et al 1986;Kurtzer et al 2005) dependent. These complexities will necessarily influence the relationship between motor cortical activity across postural and movement tasks and likely increase variability in the difference between their PRD and PTD.…”

The Neuropsychology of Movement and Movement Disorders: Neuroanatomical and Cognitive Considerations

“…The direct access of New M1 to motoneurons enables it to bypass some spinal cord circuits and sculpt novel patterns of motor output, essential for highly skilled movements (Griffin, Hoffman, & Strick, 2015;Lemon, 2008). This result suggests that certain types of motor performance rely heavily on M1.…”