Is there clear evidence that action potentials travelling in a parent axon reach all of their terminals? In spinal afferents, many fibres extend their arborizations beyond the area in which cells can be shown to respond to the afferents. In addition, impulses fail to propagate in the long-range reach of myelinated fibres in dorsal columns when a GABA-operated Cl- shunt in the terminals is operating. When the mechanism is immobilized by antagonists of GABAA receptors, impulse blockade is relieved. This suggests an additional control mechanism, which is located in axons proximal to the synaptic area. Such a control could focus or defocus those parts of a terminal arborization that affect postsynaptic cells.