Tag Archives: insect motor neuron

Developmental Changes in Dendritic Shape and Synapse Location Tune Single-Neuron Computations to Changing Behavioral Functions

Developmental Changes in Dendritic Shape and Synapse Location Tune Single-Neuron Computations to Changing Behavioral Functions

Maurice Meseke, Jan Felix Evers, Carsten Duch
AJP – JN Physiol
July 2009 vol. 102 no. 1 41-58

The complexity of the nervous system of any animal is such that any study can exist at any level or organization. One of Cajal’s rules is that the single neuron is the basic unit of the nervous system, yet other levels of organization can be found be it in a network of neurons, as a functional unit with glial support cells or within the structure of the neuron itself. One interesting question is whether a neuron preforms computation and if yes, how. Neural coding in a single neuron can broken into two parts, what happens before the action potential fires and what happens after, the dendrites and the axons, so each can be studied separately to some extent. The focus in this study is the relationship between dendritic architecture and synapse location and how this changes as the M5 motor neuron is restructured during metamorphosis from serving one behavior i.e., crawling, to a very different one,  flying. The authors state the difference:

“Flight motor output requires high spike timing precision, but motoneuron bursting during larval crawling does not.”

Results:

  • Increase in total dendritic length
  • No difference in mean length of individual branches or total number of branches
  • No difference in total number of branches
  • In adult, more branches in lower branch orders
  • Mean radius of individual dendrites increased
  • Dendritic density has doubled (3d perimeter of endpoints within the neuropil)
  • Maximum dendritic path lengths the same
  • No change in mean branch order analysis
  • At higher branch orders, more branches in the adult
    • “… overall densities of GABAergic and other type I terminals are similar in the larval and in the adult motor neuropil surrounding the dendrites of MN5. Consequently, the observed higher density of putative GABAergic terminals contacting adult MN5 dendrites as compared with larval ones is not a consequence of terminal availability. “
  • GABAergic terminals shift distally in the dendritic tree in the adult
    • “… inhibitory inputs did not occur randomly along the run of individual thick dendrites (between 0.5 and 1 μm diam) but, by contrast, were located with a higher likelihood toward the end of thick dendrites, i.e., at the root of dendritic sub-trees arising from this dendrite but furthest away from other sub-trees.”

“… the passive architecture of the adult MN5 should be tuned for effective summation of synaptic input that occurs within narrow time windows, but the efficiency of synaptic input summation should decrease markedly if input synapse firing synchronicity decreased. “

“… the larval MN5 innervates a slowly contracting muscle. In a number of invertebrate preparations, the contraction slopes of slow muscles are mostly independent of brief changes in motoneuron spiking frequency and spike timing …. Accordingly, the larval MN5 fires bursts of multiple action potentials during crawling behavior without a clear demand on spike time precision within each burst…”

So, does the passive structure of the larval and adult dendritic trees with random synapses placed according to the discovered rules of placement show a computational difference?

“… the maximum depolarization amplitude is significantly stronger affected by input firing synchronization in passive models of the adult as compared with the larval dendritic tree.”

“Sensitivity to input firing synchronization is not caused by specific synapse distributions but by dendritic tree geometry.”

So the study finds, among other things, that the dendritic tree does tune the computation, that the neuropil connections are pretty much the same at both stages and that there seems to be subdendric specialization built in a distal to proxial manner. I hope to come back and detail some of this.