Novel neurobiological properties of elements in the escape circuitry of the shrimp.

Escape behaviors in penaeid shrimp are mediated by large myelinated medial giant fibers which course from the brain to the last abdominal ganglion in the ventral nerve cord. In each abdominal segment, the medial giant axons make synaptic connections with paired myelinated motor giant axons that excite the abdominal deep flexor muscles and drive the tailflips that constitute the escape behavior. I examined (1) anatomical features of the abdominal motor giant fibers and (2) electrical properties of both the medial and motor giant axons in the pink shrimp, The motor giant axons in the paired third roots of shrimp abdominal ganglia emerge from a single fused neurite that originates from two clusters of cell bodies within the ganglion. Injection of large positive currents into the abdominal medial giant fibers generates action potentials that are transmitted to the opposite medial giant axon through putative collateral synapses within the ganglia. Transmission across the medial-to-motor giant synapse is fast and resistant to fatigue, with synaptic delays equal to or less than those previously documented at the lateral-to-motor giant electrical synapse in crayfish. Transmission was found to be extremely reliable even with presynaptic spike frequencies as high as 250 Hz. While action potentials within the medial giant fibers are transmitted across the medial-to-motor giant synapse with a large safety factor, neither prolonged positive nor prolonged negative currents pass through the synaptic nexus, irrespective of the site of injection. The lack of DC current passage along with the inability of neurobiotin or biocytin to spread through the synaptic nexus raises the possibility that the synaptic mechanism may be capacitative.