Caridoid escape reaction
[1] The type of response depends on the part of the crustacean stimulated, but this behavior is complex and is regulated both spatially and temporally through the interactions of several neurons.
In 1946, C. A. G. Wiersma first described the tail-flip escape in the crayfish Procambarus clarkii and noted that the giant interneurons present in the tail were responsible for the reaction.
[6] Based on studies of P. clarkii it was discovered that the tail-flip mechanism is characterized by a decisive, all-or-nothing quality that inhibits all unnecessary behaviors while generating a fixed action pattern for escape swimming.
[7] 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.
[2][7] Finally, the caridoid escape reflex requires that neurons be able to complete the arduous task of synchronizing the flexion of several abdominal segments.
[9] 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.[6] Variations in escape response characteristic depend on the location where the crayfish body is prodded or attacked and also depend on which of the giant neurons is stimulated.
Their cell bodies and dendrites begin in the brain and collect sensory information presented by visual and tactile stimuli.
This results in the firing of all motor giant (MoG) neurons and the flexion of all the phasic fast flexor (FF) muscles in the abdomen, which rapidly curls the tail fan and last three segments underneath the crayfish.
Further studies should focus on this escape variant, paying special attention to exactly how visual information is processed and then converted into neuronal signals that produce a tail flip response.
[8] They are so named because they lack the involvement of the giant interneurons, most likely because they do not produce depolarizations in the sensory neurons that are above the thresholds required to initiate these behaviors.
However, this slower swimming behavior allows for flexibility, since the crayfish can use visual stimuli and steer itself in a selected direction.
This allows the crayfish to attack offenders, escape during feeding, or wriggle free when it has been restrained by the carapace.
[11][12] The lateral giant (LG)-mediated escape mechanism is the most extensively analyzed form of the tail flip.
The LGI only innervates the first three rostral segments of the tail and is activated within 10 ms from when a mechanical stimulus is presented to the abdomen.
Wiersma's initial experiments showed that direct stimulation of the LGI was a sufficient release for the tail flip.
It takes the information from both the alpha and the beta pathways and if the timing of the spikes is synchronous, the behavior is produced.
If the timing is asynchronous, the later input is blocked by reducing the driving force of the signal and increasing the threshold voltage.
This process is mediated by the MRO and tail fan hair receptors, which were inhibited during the flexion portion of the escape behavior.
The hair cells detect the resulting movement caused by the tail flip when activated, they would fire and excite the fast extensor motor neurons.
Evolution has allowed the crayfish to be more flexible by presenting several control systems that will prevent the tail flip in situations where it will most likely be unnecessary or ineffective.
Crayfish often find themselves in a conflicting situation where they are performing the highly motivated behavior of feeding when they suddenly receive a tail flip stimulus.
[2] When a crayfish is held by its carapace either in the water or in the air, it does not respond with a tail flip when it receives sensory stimuli that would normally elicit the response.
[2] The tail flip normally induces inhibition in an absolute fashion, such that it takes precedence over all the other tasks the segments are performing.
The habituation process is also mediated further up the circuit through the buildup of tonic inhibition, brought on by the repeated stimulation.
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 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.
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.
