Neuropilar Projections of the Anterior Gastric Receptor Neuron in the Stomatogastric Ganglion of the Jonah Crab, Cancer Borealis

Abstract

Sensory neurons provide important feedback to pattern-generating motor systems. In the crustacean stomatogastric nervous system (STNS), feedback from the anterior gastric receptor (AGR), a muscle receptor neuron, shapes the activity of motor circuits in the stomatogastric ganglion (STG) via polysynaptic pathways involving anterior ganglia. The AGR soma is located in the dorsal ventricular nerve posterior to the STG and it has been thought that its axon passes through the STG without making contacts. Using high-resolution confocal microscopy with dye-filled neurons, we show here that AGR from the crab Cancer borealis also has local projections within the STG and that these projections form candidate contact sites with STG motor neurons or with descending input fibers from other ganglia. We develop and exploit a new masking method that allows us to potentially separate presynaptic and postsynaptic staining of synaptic markers. The AGR processes in the STG show diversity in shape, number of branches and branching structure. The number of AGR projections in the STG ranges from one to three simple to multiply branched processes. The projections come in close contact with gastric motor neurons and descending neurons and may also be electrically coupled to other neurons of the STNS. Thus, in addition to well described long-loop pathways, it is possible that AGR is involved in integration and pattern regulation directly in the STG.

Robust circuit rhythms in small circuits arise from variable circuit components and mechanisms

“…That said, at a finer scale of analysis, intracellular dye-fills of identified neurons in invertebrates show clear evidence of animal-to-animal variations in soma positions and branching patterns [27, 28, 29]. For example, a recent study of the Anterior Gastric Receptor (AGR) neuron in the crab stomatogastric ganglion (STG) shows large variations in the number of branches that AGR makes in the STG neuropil [28]. …”

Control of sensory ectopic spike initiation by descending modulatory projection neurons

“…The AGR axon projects through the STG (Fig. 1A) and albeit no connections from AGR to descending projection neurons have been identified, Goeritz et al (2013) suggested putative chemical connections to an unknown modulatory projection neuron.…”

Consequences of acute and long-term removal of neuromodulatory input on the episodic gastric rhythm of the crabCancer borealis

“…Distinct input pathways have been shown at times to activate similar gastric patterns and at other times to activate different versions of gastric rhythm, which depend on feedback projections back to the CoGs (Blitz and Nusbaum 2012;Saideman et al 2007;Wood et al 2000). Sensory inputs from the stomach are also important for shaping gastric activity both in the intact animal and in vitro (Beenhakker et al 2004(Beenhakker et al , 2005Blitz et al 2004;Combes et al 1995;Goeritz et al 2013;Hedrich et al 2011;Harris-Warrick 1989, 1991). Thus decentralization might represent a more dramatic perturbation for the gastric network than for the pyloric circuit.…”