Dendritic bottlenecks of crustacean motoneurons

Cobalt-labelled fast flexor motoneurons of the crayfish (Procambarus) were studied by electron microscopy after treatment with diaminobenzidine. The neurons were traced into the abdominal ganglion to locations at which they made contacts with the lateral giant fibres of the nerve cord. Fine secondary dendritic branches extended from the primary dendrites of the fast flexor motoneurons to the lateral giant fibre. These fine branches had bottlenecks at various places along their lenghts and also at their junctions with primary dendrites. Chemical synapses occurred at the bottlenecks and at other locations on the fine branches. It is postulated that chemical synapses at dendritic bottlenecks could act to modify the effectiveness of the excitatory drive provided by the lateral giant fibres to the fast flexor motoneurons, most likely by 'gating' electrical signals conveyed by the fine dendrites.