Figure 4.

Summary of current understanding of spinal and supraspinal plasticity associated with H-reflex conditioning. The shaded ovals indicate the sites of definite (red) or probable (pink) spinal or supraspinal plasticity associated with H-reflex conditioning. Abbreviations: MN, motoneuron; CST, main corticospinal tract; IN, spinal interneuron; and GABA IN, GABAergic interneuron. Open synaptic terminals are excitatory, solid ones are inhibitory, half-open ones could be either, and the subdivided one is a cluster of C terminals. Dashed pathways imply the possibility of intervening spinal interneurons. The monosynaptic and probably oligosynaptic H-reflex pathway from Ia and Ib inputs to the motoneuron is shown. The sites of definite or probable plasticity include: the motoneuron membrane (firing threshold and axonal conduction velocity); motor unit properties; GABAergic terminals and C terminals on the motoneuron; the Ia afferent synaptic connection; interneurons and their terminals conveying oligosynaptic group I inhibition or excitation to the motoneuron; and sensorimotor cortex. The essential roles of the corticospinal tract (originating in sensorimotor cortex) and of cerebellar output to cortex are indicated. The spinal cord plasticity that is directly responsible for H-reflex conditioning appears to be induced and maintained by cortical plasticity that itself depends for its long-term survival on the cerebellum.