Mental rehearsal of motor tasks recruits α‐motoneurones but fails to recruit human fusimotor neurones selectively

Abstract

As mental rehearsal of movements activates multiple cortical areas associated with movement, we assessed whether this increases fusimotor drive and whether enhanced muscle spindle activity could contribute to the improvement in skill that accompanies mental rehearsal. Microneurographic recordings were made from six muscle spindle afferents innervating extensor muscles in the forearm or tibialis anterior, which were selected because their discharge increased during very weak contractions. Activity was monitored while subjects imagined performing a range of activities including simple and complex movements involving the relevant muscles. No activation of muscle spindle afferents occurred during imagined motor tasks without EMG. When the relevant muscles contracted during mental rehearsal, spindle discharge increased, much as in weak contractions. Mental rehearsal increased background EMG in the involved muscles and also increased H reflex amplitude independently of EMG changes. Although there was no evidence for selective fusimotor activation during imagined movement, skeletomotor activity and reflex excitability increased. Similar changes occur with preparation for movement following a cue. It is likely that mental rehearsal usually involves unintentional performance of the planned motor task.

Mental Practice With Motor Imagery: Evidence for Motor Recovery and Cortical Reorganization After Stroke

“…16,17 Alternatively, mental rehearsal of a particular motor skill may partially activate the descending corticospinal pathway, the spinal machinery, and effector muscles. 18,19 In line with this hypothesis are the observations that spinal circuits are activated by transcranial magnetic stimulation in a similar manner during motor imagery and motor execution. 20-27 This finding, however, is challenged by other studies, showing modulation of the motor cortical excitability without evoking descending volleys to the spinal cord.…”

Mental imaging of motor activity in humans

“…This will require a complete description of the state of the motor system, which seems difficult with the presently available techniques. For example, standard EMG recordings may miss the activity of deep muscular fibers; the degree of activity of muscle spindle afferents remains unknown [32]. This approach should bridge the gap between the study of cognitive phenomena such as the covert states of action generation, thought to be possible only in man, and the detailed study of the underlying neural mechanisms accessible only in animals.…”

The Effect of Motor Imagery on Spinal Segmental Excitability

“…Even without changes in the excitability of motoneurons and interneurons, the effect of an external stretch would thus be magnified and could trigger a stretch reflex during motor imagery. However, this is not likely to happen, because Gandevia et al (1997) reported no change in spindle afferent discharge (and therefore the ␥-motor system) during motor imagery. In contrast, their results showed increased motoneuronal activity and reflex excitability.…”

“…First, as indicated above, it could be because of subthreshold activation of motoneurons during motor imagery (cf. Gandevia et al, 1997). Because of the existence of direct corticomotoneuronal connections, especially for finger muscles (Porter and Lemon, 1993), it is possible that the excitability of spinal motoneurons is modulated directly by the motor cortex outflow.…”