Presynaptic selection of afferent inflow in the spinal cord

The synaptic effectiveness of sensory fibers ending in the spinal cord of vertebrates can be centrally controlled by means of specific sets of GABAergic interneurons that make axo-axonic synapses with the terminal arborizations of the afferent fibers. In the steady state, the intracellular concentration of chloride ions in these terminals is higher than that predicted from a passive distribution, because of an active transport mechanism. Following the release of GABA by spinal interneurons and activation of GABA(A) receptors in the afferent terminals, there is an outwardly directed efflux of chloride ions that produces primary afferent depolarization (PAD) and reduces transmitter release (presynaptic inhibition). Studies made by intrafiber recording of PAD, or by measuring changes in the intraspinal threshold of single afferent terminals (which is reduced during PAD), have further indicated that muscle and cutaneous afferents have distinctive, but modifiable PAD patterns in response to segmental and descending stimuli. This has suggested that PAD and presynaptic inhibition in the various types of afferents is mediated by separate sets of last-order GABAergic interneurons. Direct activation, by means of intraspinal microstimulation, of single or small groups 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 intraspinal collaterals and not spread to nearby collaterals. The local character of PAD allows cutaneous and descending inputs to selectively inhibit the PAD of segmental and ascending intraspinal collaterals of individual muscle spindle afferents. It thus seems that the intraspinal branches of the sensory fibers are not hard wired routes that diverge excitation to spinal neurons, but are instead dynamic pathways that can be centrally controlled to address information to selected neuronal targets. This feature appears to play an important role in the selection of information flow in muscle spindles that occurs at the onset of voluntary contractions in humans.