Presynaptic selection of afferent inflow in the spinal cord

The synaptic effectiveness of sensory fibers ending in the spinal cord of v ertebrates can be centrally controlled by means of specific sets of GABAerg ic interneurons that make axo-axonic synapses with the terminal arborizatio ns of the afferent fibers. In the steady state, the intracellular concentra tion of chloride ions in these terminals is higher than that predicted from a passive distribution, because of an active transport mechanism. Followin g the release of GABA by spinal interneurons and activation of GABA(A) rece ptors in the afferent terminals, there is an outwardly directed efflux of c hloride ions that produces primary afferent depolarization (PAD) and reduce s transmitter release (presynaptic inhibition). Studies made by intrafiber recording of PAD, or by measuring changes in the intraspinal threshold of s ingle afferent terminals (which is reduced during PAD), have further indica ted that muscle and cutaneous afferents have distinctive, but modifiable PA D patterns in response to segmental and descending stimuli. This has sugges ted that PAD and presynaptic inhibition in the various types of afferents i s mediated by separate sets of last-order GABAergic interneurons. Direct ac tivation, by means of intraspinal microstimulation, of single or small grou ps of last-order PAD-mediating interneurons shows that the monosynaptic PAD elicited in Ia and Ib afferents can remain confined to some sets of the in traspinal collaterals and not spread to nearby collaterals. The local chara cter of PAD allows cutaneous and descending inputs to selectively inhibit t he PAD of segmental and ascending intraspinal collaterals of individual mus cle spindle afferents. It thus seems that the intraspinal branches of the s ensory fibers are not hard wired routes that diverge excitation to spinal n eurons, but are instead dynamic pathways that can be centrally controlled t o address information to selected neuronal targets. This feature appears to play an important role in the selection of information flow in muscle spin dles that occurs at the onset of voluntary contractions in humans. (C) 1999 Editions scientifiques et medicales Elsevier SAS.