Task-related changes in propriospinal excitation from hand muscles to human flexor carpi radialis motoneurones.

This study addresses whether there is excitation from human hand muscles to flexor carpi radialis (FCR) motoneurones mediated through propriospinal circuits and, if so, whether it is used in specific motor tasks. Electrical stimuli to the ulnar nerve at wrist level produced an excitation in FCR motoneurones with characteristics typical of a propriospinally mediated effect: low threshold (0.6 x motor threshold (MT)), a group I effect that was not reproduced by purely cutaneous stimuli, long central delay (4.

Mediation of late excitation from human hand muscles via parallel group II spinal and group I transcortical pathways.

This study addresses the question of the origin of the long-latency responses evoked in flexors in the forearm by afferents from human hand muscles. The effects of electrical stimuli to the ulnar nerve at wrist level were assessed in healthy subjects using post-stimulus time histograms for flexor digitorum superficialis and flexor carpi radialis (FCR) single motor units (eight subjects) and the modulation of the ongoing rectified FCR EMG (19 subjects). Ulnar stimulation evoked four successive peaks of heteronymous excitation that were not produced by purely cutaneous stimuli: a monosynaptic Ia excitation, a second group I excitation attributable to a propriospinally mediated effect, and two late peaks.

Increase in group II excitation from ankle muscles to thigh motoneurones during human standing.

In standing subjects, we investigated the excitation of quadriceps (Q) motoneurones by muscle afferents from tibialis anterior (TA) and the excitation of semitendinosus (ST) motoneurones by muscle afferents from gastrocnemius medialis (GM). Standing with a backward lean stretches the anterior muscle pair (TA and Q) and they must be co-contracted to maintain balance. Equally, forward lean stretches the posterior muscle pair (GM and ST) and they must be co-contracted.

Perception of non-voluntary brief contractions in normal subjects and in a deafferented patient.

The accuracy of force perception by human subjects in the absence of voluntary motor command was evaluated by exploring how they perceived isometric twitches of wrist extensor muscles produced by external stimulation. Twelve normal subjects and a well-known patient lacking large-diameter afferent fibres (GL) performed estimation, production and reproduction tasks. Magnetic stimulation of the radial nerve and, in normal subjects, transcranial magnetic stimulation (TMS) of the motor cortex were used to produce weak brief non-voluntary twitches.