DO NERVE IMPULSES PENETRATE TERMINAL ARBORIZATIONS - A PRE-PRESYNAPTIC CONTROL MECHANISM

Is there clear evidence that action potentials travelling in a parent axon reach all of their terminals? In spinal afferents, many fibres ex tend their arborizations beyond the area in which cells can be shown t o respond to the afferents. In addition, impulses fail to propagate in the long-range reach of myelinated fibres in dorsal columns when a GA BA-operated Cl- shunt in the terminals is operating. When the mechanis m is immobilized by antagonists of GABA, receptors, impulse blockade i s relieved. This suggests an additional control mechanism, which is lo cated in axons proximal to the synaptic area. Such a control could foc us or defocus those parts of a terminal arborization that affect posts ynaptic cells.