The Gastropyloric Receptor (GPR) neurons are muscle receptors in the stomatogastric ganglion of decapod crustaceans (Katz et al., 1989). These neurons have been most extensively studied in the crab (Cancer borealis), but they have been found in all decapods that have been examined (Katz, Thesis, 1989).
The GPR cells are muscle stretch receptors. In the crab, they are activated by contraction of the medial tooth muscle of the gastric mill (Katz et al, 1989).
Peripheral innervation pattern
In crabs, there are two distinct types of GPR cells. GPR1 innervates the gm8a muscle. GPR2 innervates the gm9 and cpv3a muscles. In lobsters (Homarus), there are 4 GPR cells, which all innervate the same set of muscles. In crayfish, there are two GPR cells, which also co-innervate the same set of muscles.
Central projection pattern
Each GPR cell has an axon that projects to the stomatogastric ganglion (STG), where it branches. The axon continues through the stomatogastric nerve and branches at the superior oesophageal nerve (son), sending a branch to each commissural ganglion (CoG).
The GPR cells are serotonergic in crabs, hermit crabs, lobsters (Homarus), and shrimp. But in the spiny lobster (Panulirus) they are not serotonergic. Although it has not been conclusively demonstrated, it is likely that serotonin (5-HT) is a borrowed transmitter in the GPR cells. In other words, serotonin is not synthesized in these neurons, but is taken up by transporters. The evidence for this is that serotonin immunoreactivity is often very weak unless the preparation is bathed in serotonin.
The GPR cells elicit rapid EPSPs that are blocked by a variety of nicotinic acetylcholine antagonists (Katz and Harris-Warrick, 1989).
The GPR cells were found to be allostatin-immunoreactive in a number of species (Skiebe, 1999).
The GPR cells make monosynaptic connections onto neurons in the stomatogastric ganglion (STG) and in the commissural ganglia (CoGs). In the STG, the GPR cells excite DG with monosynaptic cholinergic EPSPs and induce plateau potential properties with 5-HT.
In the crab the GPR cells display two modes of activity in response to muscle movement: spiking and bursting (Katz et al, 1989). In the latter mode the neuron exhibits endogenous bursting properties that might play a role in coding sensory information (Birmingham et al, 1999). Spiking and bursting appear to originate from distinct spike initiations zones. The evidence for this is the observation of different spike sizes in extracellular recordings measured close to the innervated muscles.
A number of neuromodulatory substances including allatostatin and 5-HT can modify the response of GPR2 to muscle stretch (Birmingham et al, 2003).
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