Enhancement of electrical signalling by neuronal morphology: New presynaptic and postsynaptic mechanisms

Date of Award




Degree Name

Doctor of Philosophy (Ph.D.)

First Committee Member

Kenneth J. Muller, Committee Chair


Effects of neuronal morphology on synaptic transmission and action potential initiation were demonstrated in the nervous system of the leech, where it is possible to record at separate locations from neurons with known morphologies and defined behavioral roles. Specific questions addressed were (1) How do a neuron's morphological and electrical properties influence whether action potentials reverse their direction of propagation, known as reflection? (2) What is the effect of reflection on synaptic transmission? (3) How do reflection and action potential failure influence transmission from a spatially distributed synapse? (4) How does the morphology of a postsynaptic neuron influence firing rate by producing multiple impulse initiation sites?Chapter I describes how action potentials reflect at central axon branch points of pressure mechanosensory neurons (P cells). Reflection operates as a new mechanism to enhance synaptic transmission from a subset of a neuron's branches. Reflection can change with electrical activity, making this enhancement of transmission reversible. Impulses from the periphery activated a synapse, then reflected from a central branch point, and then quickly activated the synapse a second time, greatly increasing transmission by producing facilitation. A compartmental model based on experimental data indicates that reflection can occur for a greater range of membrane morphology than previously thought.Chapter II describes how reflection and action potential failure, or conduction block, at P cell central branch points affect synaptic transmission to the S cell, a neuron essential for a type of nonassociative learning. Reflection at P cell central branch points increased transmission, and conduction block at the same branch points decreased it.Chapter III describes a new mechanism to increase the firing rate of a neuron, initiation of impulses at multiple sites. In the S cell chain, a linear group of strongly electrically coupled neurons, a single skin stimulus evoked synaptic input that initiated impulses at multiple locations. Impulses that arose at different sites initiated at different times, and therefore did not collide, but propagated throughout the chain.Neuronal structure, therefore, can affect signalling by influencing activity in separate sets of presynaptic terminals within a single neuron, and by creating multiple initiation sites that can increase postsynaptic activity.


Biology, Neuroscience; Biophysics, General

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