When confronted with threatening stimuli and predators, the crayfish responds with an innate escape machanism called the startle reflex. Also known as tailflipping, this stereotyped behaviour involves rapid flexions of the abdominal muscles which produce powerful swimming strokes that thrust the small crustacean through the water and away from danger. In the struggle for existence, the speed of this response can mean the difference between life and death, and the crayfish has evolved an incredibly fast escape mechanism which can be initiated within well under one-hundredth of a second.
This mechanism depends on a process called coincidence detection, whereby the electrical impulses inputs from sensory organs on different parts of the body arrive simultaneously at a specific location in the central nervous system. Although this reflex has been studied intensively, the mechanism by which nervous impulses arrive in synchrony at the central nervous system was poorly understood. DeForest Mellon, Jr. and Kate Christison-Lagay of the University of Virginia now describe the simple yet ingenious and beautiful mechanism underlying this phenomenon.
The crayfish Procambarus clarkii has two long antennae and two smaller appendages called antennules on its head. Each antennule gives rise to two flagella, along which are distributed up to 10 prominent sensory organs called feathered sensilla (below). These organs act as near-field hydrodynamic sensors – in other words, they are specialized for detecting the changes in mechanical pressure that occur when something near the crayfish moves and displaces the water surrounding it. They are highly sensitive to disturbances in the surroundings, responding to water displacements in the order of several hundred-thousandths of a millimeter.
Scanning electron micrograph of a feathered sensillum on the flgellum of P. clarkii. Scale bar = 100 micrometers (From Mellon & Christison-Lagay, 2008).
At the base of each sensillum is a pair of neurons at its base, which are sensitive to displacements in opposite directions. These cells project into the central nervous system and form connections with a set of giant interneurons located in the ventral nerve cord, which in turn send axons to the motor neurons that control the muscles which produce flexion of the abdomen. The rapidity of the tailfip reflex is due in large part to the structure of these cells. They have very large diameter axons which conduct nervous impluses at a high speed, but multiple simultaneous inputs are needed to activate the large fibres.
Mellon and Christison-Lagay performed electrophysiological recordings to measure the conduction velocities of nervous impulses generated at different points along the flagella. They mechanically stimulated individual sensilla using a probe attached to the cone of an audio speaker, and found that the velocity of the impulses depended upon the position of the sensillum – axons attached to sensilla at the tips of the flagella, which are furthest from the central nervous system, conducted impulses faster than those attached to sensilla at the base, by a factor of six.
The crayfish is an invertebrate, and therefore does not produce myelin, the fatty tissue which surrounds axons in the vertebrate nervous system and increases their conduction velocity. The velocity at which an impulse is propagated is therefore directly proportional to the circumference of the fibre, and velocity increases with diameter. The researchers therefore sought to determine whether there was an anatomical basis for the differences in conduction they observed. They took transverse sections of axons from several points along the flagella, and used electron microscopy to measure their diameters.
The preliminary data showed axon diameter gradually increases with distance from the base of the flagella. This suggests that conduction velocities are staggered as a result of the graduation of fibre diameters, so that nervous impulses generated at progressively further points along the flagella are propagated more quickly. The velocities of the axons are precisely calibrated, so that impulses generated from arrays of sensilla along the length of the flagella are delivered simultaneously to the giant interneurons. In this way, the tailflip reflex of the crayfish is optimized so that a rapid response to threatening stimuli can be generated.
Mellon, D. & Christison-Lagay, K. (2008). A mechanism for neuronal coincidence revealed in the crayfish antennule. Proc. Nat. Acad. Sci. 105: 14626-14631. DOI: 10.1073/pnas.0804385105