Activity-Dependent Plasticity of Spinal Circuits in the Developing and Mature Spinal Cord

Figure 1

Presynaptic inhibition to Ia fibers. (i) Schematic diagram of different inputs to Ia afferents and alpha motoneurons. Proprioceptive input from Ia fiber can be selectively suppressed by presynaptic inhibition through PAD interneurons. The interneuron which makes axoaxonic connection with Ia fiber is GABAergic and regarded as last-order PAD IN. This interneuron is under the influence of an excitatory interneuron which is referred to as first-order PAD IN. This first-order PAD IN receives inputs from both descending tracts and from peripheral afferents [15]. In such a case, different inputs can interact to control the Ia input to motoneurons without affecting the intrinsic properties of motoneurons. (ii) During presynaptic inhibition, the normal activity of the muscle can remain unchanged, while the reflex gain reduces. In this example, standing with prism goggles (PV condition) suppressed the H-reflex in comparison to normal vision (NV) standing condition (a), while there was no change in the soleus and tibialis anterior muscle EMG activity (b). This is most likely due to the presynaptic inhibition of Ia fibers which spares the background activity of motoneurons. Part II adapted with permission from [16].