Local control of information flow in segmental and ascending collaterals of single afferents

In the vertebrate spinal cord, the activation of GABA (gamma-aminobutyric acid)-releasing interneurons that synapse with intraspinal terminals of sensory fibres leading into the central nervous system (afferent fibres) produces primary afferent depolarization and presynaptic inhibition(1-3). It is not known to what extent these presynaptic mechanisms allow a selective control of information transmitted through specific sets of intraspinal branches of individual afferents(4-7). Here we study the local nature of the presynaptic control by measuring primary afferent depolarization simultaneously in two intraspinal collaterals of the same muscle spindle afferent. One of these collaterals ends at the L6-L7 segmental level in the intermediate nucleus, and the other ascends to segment L3 within Clarke's column, the site of origin of spinocerebellar neurons(8). Our results indicate that there are central mechanisms that are able to affect independently the synaptic effectiveness of segmental and ascending collaterals of individual muscle spindle afferents. Focal control of presynaptic inhibition thus allows the intraspinal branches of afferent fibres to function as a dynamic assembly that can be fractionated to convey information to selected neuronal targets. This may be a mechanism by which different spinal postsynaptic targets that are coupled by sensory input from a common source could be uncoupled.