Action potential reflection and failure at axon branch points cause stepwise changes in EPSPs in a neuron essential for learning

Action potential reflection and failure at axon branch points cause stepwis e changes in EPSPs in a neuron essential for learning. J. Neurophysiol. 83: 1693-1700, 2000. In leech mechanosensory neurons, action potentials revers e direction, or reflect, at central branch points. This process enhances sy naptic transmission from individual axon branches by rapidly activating syn apses twice, thereby producing facilitation. At the same branch points acti on potentials may fail to propagate, which can reduce transmission. It is n ow shown that presynaptic action potential reflection and failure under phy siological conditions influence transmission to the same postsynaptic neuro n, the S cell. The S cell is an interneuron essential for a form of nonasso ciative learning, sensitization of the whole body shortening reflex. The P to S synapse has components that appear monosynaptic (termed "direct") and polysynaptic, both with glutamatergic pharmacology. Reflection at P cell br anch points on average doubled transmission to the S cell, whereas action p otential failure, or conduction block, at the same branch points decreased it by one-half. Each of two different branch points affected transmission, indicating that the P to S connection is spatially distributed around these branch. points. This was confirmed by examining the locations of individua l contacts made by the P cell with the S cell and its electrically coupled partner C cells. These results show that presynaptic neuronal morphology pr oduces a range of transmission states at a set of synapses onto a neuron ne cessary for a form of learning. Reflection and conduction block are activit y-dependent and are basic properties of action potential propagation that h ave been seen in other systems, including axons and dendrites in the mammal ian brain. Individual branch points and the distribution of synapses around those branch points can substantially influence neuronal transmission and plasticity.