Generation of a fixed motor pattern: II. Electrical properties and synaptic characteristics of pyloric neurons in the stomatogastric ganglion of the crab,Cancer pagurus

Neuronal morphology and neuropil structure in the stomatogastric ganglion of the lobster, Homarus americanus

The stomatogastric nervous system (STNS) has long been used as a model system for the study of central pattern generation, neuromodulation, and network dynamics. Anatomical studies of the crustacean stomatogastric ganglion (STG) in different species have mostly been restricted to subsets of neurons and/or general structural features. For the first time, we describe the morphology of all STG neurons belonging to the two circuits that produce the well-described pyloric and gastric rhythms in the lobster, Homarus americanus. Somata sit on the dorsal and lateral surface of the STG and send a single primary neurite into the core of the neuropil, which is mostly made up of larger lower order branches. The perimeter of the neuropil consists mostly of finer higher order branches. Immunohistochemical labeling for synaptic proteins is associated with the small diameter branches. Somata positions are not constant but show preferred locations across individuals. The number of copies is constant for all neuron types except the PY and GM neurons (PY neuron number ranges from 3 to 7, and GM neuron number ranges from 6 to 9). Branch structure is largely nondichotomous, and branches can deviate substantially from cylindrical shape. Diameter changes at branch points can be as large as 20-fold. Clearly, the morphology of a specific neuron type can be quite variable from animal to animal.

Voltage clamp analysis of intact stomatogastric neurons

Two-electrode voltage clamp of intact, identified pyloric neurons of the spiny lobster stomatogastric ganglion reveals two major outward currents. A rapidly inactivating, tetraethylammonium- (TEA) insensitive, 4-aminopyridine- (4AP) sensitive, outward current resembles IA of molluscan neurons; it activates rapidly on depolarizations above rest (e.g. -45 mV), delaying both the axonal-sodium and the neuropil-calcium spikes which escape voltage-clamp control. We infer that A-current is distributed both in a space clamped region (on or near the soma) and in a non-space clamped region with access to the generators for sodium and calcium spikes. A calcium-dependent outward current, IO(Ca), activates rapidly at clamp steps above -25 mV and inactivates at depolarizing holding voltages. Increasing depolarization results in an increase in both IO(Ca) and firing rate but a reduction in the amplitude of the sodium spike current. Blockage of IO(Ca) with Cd2+ causes little change in spike firing pattern. These observations are consistent with IO(Ca) being activated primarily in the soma and nearby regions which are under good control with a soma voltage clamp (and distant from the Na(+)-spike trigger zone). While the lack of space clamp limits resolution of charging transients and tail currents, the identification of the major current subgroups can still be readily accomplished, and inferences about the location and function of currents can be made which would not be possible if the cells were space clamped or truncated.

The innervation of the pyloric region of the crab, Cancer borealis: homologous muscles in decapod species are differently innervated

The muscles of the pyloric region of the stomach of the crab, Cancer borealis, are innervated by motorneurons found in the stomatogastric ganglion (STG). Electrophysiological recording and stimulating techniques were used to study the detailed pattern of innervation of the pyloric region muscles. Although there are two Pyloric Dilator (PD) motorneurons in lobsters, previous work reported four PD motorneurons in the crab STG (Dando et al. 1974; Hermann 1979a, b). We now find that only two of the crab PD neurons innervate muscles homologous to those innervated by the PD neurons in the lobster, Panulirus interruptus. The remaining two PD neurons innervate muscles that are innervated by pyloric (PY) neurons in P. interruptus. The innervation patterns of the Lateral Pyloric (LP), Ventricular Dilator (VD), Inferior Cardiac (IC), and PY neurons were also determined and compared with those previously reported in lobsters. Responses of the muscles of the pyloric region to the neurotransmitters, acetylcholine (ACh) and glutamate, were determined by application of exogenous cholinergic agonists and glutamate. The effect of the cholinergic antagonist, curare, on the amplitude of the excitatory junctional potentials (EJPs) evoked by stimulation of the pyloric motor nerves was measured. These experiments suggest that the differences in innervation pattern of the pyloric muscles seen in crab and lobsters are also associated with a change in the neurotransmitter active on these muscles. Possible implications of these findings for phylogenetic relations of decapod crustaceans and for the evolution of neural circuits are discussed.