Journal club: Phylogenic plasticity of crustacean stomatogastric circuits

by Pierre Meyrand and Maurice Moulins

J. exp. Biol. 138, 107-132, 133-153 (1988)

This old twin papers describe a neural network that has a very similar designs to that of related species but produces distinct patterns of output. They concluded that the differences in the pattern of motor output depend more on the action of the extrinsic neuromodulation onto individual neurons than the synaptic architecture of the network.

I. Pyloric patterns and pyloric circuit of the shrimp Palaemon serratus
http://jeb.biologists.org/content/138/1/107

To find the general rules of neural circuit function, direct comparison of different circuits may be useful. Finding a common principle or 'building blocks' may help understanding the fundamental mechanisms underlying rhythmic pattern generation. In this study, the authors investigated the stomatogastric nervous system of a shrimp and compared it with those of larger crustaceans such as lobster and crab. They found that pyloric networks are very similar between this shrimp and large crustaceans.

The pattern of their rhythmic outputs are quite different.  In large decapods, the pyrolic circuit generate triphasic rhythm. The pacemaker AB fire in antiphase with the constrictor motoneurones (LP and PY). In shrimp, AB fires in phase with the constrictor neurones because of electrical connection; endogenous oscillatory property was only found in LP.

II. Pyloric patterns and pyloric circuit of the shrimp Palaemon serratus

http://jeb.biologists.org/content/138/1/133

The previous study (above) showed that the fundamental network architectures are almost identical between large decapods and the shrimp. This study investigate how extrinsic modulatory inputs contribut to produce the motor outputs of different patterns.

They found that the shrimp neurons responded differently to muscarinic agonist and dopamin than those in large decapods. For example, oxotremorine activated PY, while dopamine activated PD. In large decapods, muscarinic agnonist causes oscillation in PDs while inhibit PDs. In shrimp, AB neuron is driven by and returns an inhibitory feedback to the commissural pyrolic oscillator. In large decapods, AB is the conditional oscillator and act as the pacemaker.

   Altogether, the phylogenetic plasticity in motor pattern production does not derive from structural differences in the corresponding central neuronal circuits themselves. Rather it is due to the difference in the modulatory system controlling these circuits.