Gill and Siphon Withdrawal Reflex (GSWR)
A two-component reflex is triggered when a weak or moderate stimulus is applied to the siphon or the mantle shelf. These two components consist of two reflex acts, the siphon-withdrawal reflex and the gill-withdrawal reflex. Together they form a reflex pattern with short latency which protects the animals gill and siphon to potentially threatening stimuli.
Both central ganglia and peripheral neurons are often involved in the neural control of behavior in molluscs. In molluscs such as Aplysia californica the peripheral motor neurons are more extensive, opposed to vertebrates, and innervate somatic (locomotor and appendageal) muscles. Central pathways are activated by weak stimuli applied at some distance from the target effector structure and peripheral pathways are activated when the stimuli is applied at a distance or directly on the target effector structure.
A stimulus to the siphon (weak or moderate) is mediated by abdominal ganglion (55%) and by peripheral motor neurons (45%) and is activated simultaneously.
By using preparations of Aplysia californica six central motor neurons have been found in the abdominal ganglion that produce movements of the gill. Stimulation of the cells named L7, LDG1, LDG2 and RDG results in large gill contractions and stimulation of L9G1 and L9G2 produces smaller contractions.
In the abdominal ganglion has seven central motor neurons been found that also produce movements of the siphon. LDS1, LDS2, LDS3, RDS, LBS1, LBS2, and LBS3 control contraction and constriction of the siphon. The siphon is additionally innervated by about 30 peripheral motor neurons.
Kandel and colleagues used preparations of Aplysia californica where individuals were restrained in small aquariums in a manner that the gill was exposed. A tactile stimulus was administered to the siphon and elicited the gill and siphon withdrawal reflex. A photocell was placed under the gill to record amplitude and duration of the response elicited by the stimulus.
Habituation was observed when the stimulus was delivered repeatedly to the siphon. Stimulus every 90 seconds resulted in a rapidly declined response. By delivering an electric shock to the tail the response was rapidly restored, dishabituation occurred. Sensitization was observed when a strong stimulus was administered to the tail, this enhanced a completely rested reflex in Aplysia californica.
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