Structure and localization of synaptic complexes in the cardiac ganglion of a portunid crab

Abstract

The cardiac ganglion of Portunus sanguinolentus exhibits spontaneous rhythmic activity when isolated. The ganglion contains five large and four small intrinsic neurons and is innervated by three pairs of fibres originating in the thoracic ganglia. We have identified the processes of the large neurons in electron micrographs by injecting these cells with two electron-dense markers, horseradish peroxidase (HRP) and Procion Rubine (PR). In addition we have studied the processes of the four smaller neurons by light microscopy serial reconstructions and by electron microscopy of selected regions. Both markers were found only in neuronal processes and not in glial cells nor in the extracellular space, except close to the soma of the injected cell. We found contacts between the small secondary (collateral) processes of the large cells but not between their somata or their primary processes (axons and dendrites). Two specialized structures present at the contacts between the collateral processes were small membrane close appositions, possibly the site of electrotonic junctions, and chemical synapses. Contacts between processes marked by HRP and those marked by PR were common, as were contacts between processes marked by either HRP or PR and those of the other intrinsic neurons. Adjacent processes stained by PR could contain PR deposits of different densities, but it is unclear whether this finding was due to intercellular diffusion of the dye or to its diffusing at different rates into branches of the same process. Identified processes of all the intrinsic neurons contained the same type of vesicles, which were different from those found in processes of the extrinsic fibres. Chemical synapses were present at contacts between processes of the extrinsic and intrinsic neurons, as well as at contacts between processes of the intrinsic neurons. The axons of three small cells made a series of contacts at which extensive arrays of membrane close appositions, but not chemical synapses, were found. These three axons also formed contacts, either directly or through their collateral branches, with processes of the large cells, at which both membrane close appositions and chemical synapses were present. The axon of the fourth small cell could not be followed in our series.

Modulation of an Integrated Central Pattern Generator–Effector System: Dopaminergic Regulation of Cardiac Activity in the Blue CrabCallinectes sapidus

“…2D, arrows). Impulses corresponding to these EPSPs were not detected in the motor trunks, indicating that they originate from the small interneurons, the projections of which are confined to the CG (Mirolli et al 1987;Tazaki andCooke 1979a, 1983a,b).…”

“…2C). In other decapods, electrical coupling among CG motor neurons and common synaptic input is thought to underlie such coordinated bursting and thus simultaneous contraction of the entire myocardium (Hagiwara 1961;Mirolli et al 1987;Tazaki and Cooke 1979a). Indeed, as in other decapods (Friesen 1975a,b;Hartline 1979;FIG. 2.…”

“…In the crab cardiac ganglia that have been investigated to date, the spatiotemporal properties of the electrical coupling and synaptic signaling result in precise synchrony of the motor neuron firing (Berlind 1982;Mirolli et al 1987;Tazaki 1972;Tazaki andCooke 1979a,b, 1983a,b). Simultaneous recording from an anterior motor neuron, a posterior motor neuron, a branch of the anterior connective, and a branch of the posterior connective revealed that such synchrony of firing is also a property of the Callinectes CG (Fig.…”

“…In other crab species, the synchrony is thought to reflect highly effective electrical coupling close to the region of synaptic integration and impulse initiation of all 5 motor neurons. This critical integrative area is located within the anterior portion of the CG (Mirolli et al 1987;Tazaki andCooke 1979a, 1983a). The synchrony appears to be unique to crab cardiac systems.…”

“…Direct modulation of the posterior motor neurons cannot be excluded, however, because their synaptic input and impulse initiation occur in the anterior portion of the ganglion, in a region that receives substantial THli innervation ( Fig. 7A; see Cooke 2002;Mirolli et al 1987;Tazaki and Cooke 1983a). It is also notable that the THli projections are confined to the ganglion itself.…”