Experimental brain research. Experimentelle Hirnforschung. Expérimentation cérébrale
-
The present review examines the experimental evidence supporting the existence of central mechanisms able to modulate the synaptic effectiveness of sensory fibers ending in the spinal cord of vertebrates. The first section covers work on the mode of operation and the synaptic mechanisms of presynaptic inhibition, in particular of the presynaptic control involving axo-axonic synapses made by GABAergic interneurons with the terminal arborizations of the afferent fibers. This includes reviewing of the ionic mechanisms involved in the generation of primary afferent depolarization (PAD) by GABAergic synapses, the ultrastructural basis underlying the generation of PAD, the relationship between PAD and presynaptic inhibition, the conduction of action potentials in the terminal arborizations of the afferent fibers, and the modeling of the presynaptic inhibitory synapse. ⋯ In the final section, we examine the non-synaptic presynaptic modulation of transmitter release, including the possibility that the intraspinal endings of primary afferents also release colocalized peptides in a similar way as in the periphery. The outcome of the studies presently reviewed is that intraspinal terminals of sensory fibers are not hard-wired conductors of the information generated in their peripheral sensory receptors, but dynamic systems that convey information that can be selectively addressed by central mechanisms to specific neuronal targets. This central control of information flow in peripheral afferents appears to play an important role in the generation of integrated movements and processing of sensory information, including nociceptive information.