Fig. 2

Illustration of the spinal circuit mechanisms investigated in this study. The excitability of the monosynaptic Ia afferent-motoneuron (MN) of the flexor carpi radialis muscle (FCR) is modulated by the activity in many spinal circuits. A Post activation depression of previously activated synapses plays a role in modification of synaptic transmission at the Ia–MN synapse and is probably related to the readily releasable neurotransmitters. B Reciprocal inhibition of the antagonist muscle group (extensor carpi radialis muscle (ECR)) on the wrist flexors is mediated by inhibitory interneurons which are activated by the ECR Ia afferents. C Presynaptic inhibition (D1) inhibition modulates the release of transmitters from Ia-MN presynaptically. D H reflex and M wave recruitment curve example. The amplitude of the H reflexes (dark gray circles) and the direct muscle responses (M waves: light gray circles) are plotted against the stimulation intensity in milliampere (mA). The intensity of stimulation during the experiments was set to produce an H reflex of the amplitude Hmax/2 (red circle). The maximum muscle response (Mmax) is the amplitude recorded when the M wave amplitude plateaus with increased stimulation intensity. E Example EMG responses to stimulation with different interstimulus intervals (ISI) during post-activation depression measurement. The red overlaid M waves and H reflexes were recorded at an ISI of 8 s, while the blue traces were recorded at an ISI of 2 s