Behavioral neurobiologists in the field of
neuroethology have researched this behavior extensively for over fifty years in the crayfish species
Procambarus clarkii. The type of
escape response depends on the region of the crayfish that is stimulated but all forms require abdominal contractions. When a strong, unpleasant tactile stimulus is presented, such as a burst of water or the prod of a probe, a stereotyped behavior occurs in which the muscular tail and wide tail fan region of the
telson are used like a paddle to propel the crustacean away from harm using powerful abdominal flexions. The entire process occurs in a fraction of a second as movements are generated within two hundredths of a second (20 milliseconds) from the original trigger stimulus and the period of latency after a flexion is a hundredth of a second (10 milliseconds).
Anatomy involved Like other
decapod crustaceans, the crayfish possesses a hard, segmented
exoskeleton that reflects muscular and neural segmentation. The anterior portion of the crayfish is the cephalothorax region. The region rostral to the cephalic groove, which separates the head and thorax region, is characterized by the presence of eyes, antennae and claws while the region caudal contains four pairs of walking legs. This is the crayfish's primary mode of locomotion. Their projections extend through the third root in each ganglion, and Furshpan and Potter found that the synapses they subsequently made with the MoG passed depolarizing currents in a direct and unidirectional manner. These
electrical synapses account for the speed of the escape mechanism and display some features of chemical synapses such as LTP and LTD. Non-giant escape often occurs during situations where lateral or medial giant-mediated escape may not be beneficial or during times where those behaviors are suppressed. This allows the crayfish to attack offenders, escape during feeding, or wriggle free when it has been restrained by the carapace.
Lateral giant-mediated escape The lateral giant (LG)-mediated escape mechanism is the most extensively analyzed form of the tail flip. The LG is not actually one neuron, but rather a group of closely associated neurons arranged end to end and connected by
electrical synapses (also called septate synapses). As a result, the LGI functions as one giant, continuous neuron, such as the MG.
3 – Swimming Non-giant-mediated responses are initiated after the tail flip, creating cycles of flexion followed by extension. This non-giant system is activated parallel to the LGI circuit when the hair cells receive input. However, this behavior has a longer delay that allows the onset of swimming to occur after the completion of the tail flip. The non-giant swimming occurs independently of the LGI response since direct stimulation of the LGI with the electrodes results in a tail flip but not the subsequent non-giant swimming. This swimming response seems to be a fixed action pattern mediated by a
central pattern generator since it does not require
sensory feedback for physical and temporal maintenance.
Learning Repeated tapping of the abdomen leads to habituation of the tail flip mechanism. However, self–habituation is prevented by command neuron–derived inhibition because when a tail flip is begun, the mechanisms that induce habituation are repressed. The habituation occurs at the level of the A type and C type interneurons, which experience synaptic depression. The habituation process is also mediated further up the circuit through the buildup of tonic inhibition, brought on by the repeated stimulation.
Social status The feedback from social situations affect a crayfish's ability to perform a tail flip. Serotonin levels are affected by social status. High levels are associated with aggressive behavior and a reduction in the frequency of tail flips performed. This is because the excitability of the LGI is decreased. Aggressive dominant males have a moderate reduction in tail flips while the subordinates have a much lower occurrence of tail flips. This presents a paradox since this means that subordinates are more likely to get killed. However, it was found that subordinates are more likely to use non-giant-mediated escape, indicating that the reduction in tail flips and enhancement of non giant escape is adaptive. Changes in social status should correlate with changes in serotonin levels, resulting in changes in the escape strategies used by the crayfish. However this is only true when submissive crayfish become dominant, and not the reverse. The neuromodulatory processes of facilitation and inhibition seem to be mediated by different cell receptors. Differences in the effects of serotonin on the behavior of the crayfish seem to be the result of differences in the populations of these receptors. It is unknown how these modulation processes convey the information to the LG, and how the behavioral changes are precipitated.
Evolution of the tail flip escape mechanism It has been hypothesized that the tail flip is derived from an ancient limb protraction driven (as opposed to tail flexion-driven) mechanism. This is because the SGs appear to be modified limb motor neurons whose peripheral axons affect the legs and swimmerets, but end blindly without any known function. It is known that another effect of Giant Fiber excitation is limb promotion which suggests that the premotor limb interneurons may be ancestors of the Giant Fibers. It has been speculated that the ancestral escape mechanism was most likely a backwards jump due to the simultaneous protraction of the legs driven by the ancestors of the Giant Fibers. This behavior was probably similar to the escape system found in a
mantis shrimp called
Squilla that diverged from the crayfish lineage very early on. It is likely that this mechanism was replaced by the tail flip since the wide surface area of the tail made this behavior more selectively advantageous. This most likely occurred when the ancestral flexor motor neurons in each segment formed connections with one of these limb motor neurons. The
Squilla mechanism seems to be similar to this ancestral state because a large diameter axon in the dorsal nerve chord facilitates limb promoter motor neurons. This seems to match the ancestral condition, but it is not known for sure whether the circuitry is
homologous. == Etymology ==