The ''Aplysia'' gill and siphon withdrawal reflex (GSWR) is an involuntary, defensive

reflex In biology, a reflex, or reflex action, is an involuntary, unplanned sequence or action and nearly instantaneous response to a stimulus. Reflexes are found with varying levels of complexity in organisms with a nervous system. A reflex occurs ...

of the

sea hare The clade Anaspidea, commonly known as sea hares ('' Aplysia'' species and related genera), are medium-sized to very large opisthobranch gastropod molluscs with a soft internal shell made of protein. These are marine gastropod molluscs in the ...

Aplysia californica The California sea hare (''Aplysia californica'') is a species of sea slug in the sea hare family, Aplysiidae.Rosenberg, G.; Bouchet, P. (2011). Aplysia californica J. G. Cooper, 1863. Accessed through: World Register of Marine Species at http:// ...

'', a large shell-less sea snail or sea slug. This reflex causes the sea hare's delicate

siphon A siphon (from grc, σίφων, síphōn, "pipe, tube", also spelled nonetymologically syphon) is any of a wide variety of devices that involve the flow of liquids through tubes. In a narrower sense, the word refers particularly to a tube in a ...

and

gill A gill () is a respiratory organ that many aquatic organisms use to extract dissolved oxygen from water and to excrete carbon dioxide. The gills of some species, such as hermit crabs, have adapted to allow respiration on land provided they are ...

to be retracted when the animal is disturbed.Carew, T. J. (2000). Behavioral Neurobiology: The Cellular Organization of Natural Behavior. Sinauer Associates, Inc. ''Aplysia californica'' is used in

neuroscience Neuroscience is the scientific study of the nervous system (the brain, spinal cord, and peripheral nervous system), its functions and disorders. It is a multidisciplinary science that combines physiology, anatomy, molecular biology, development ...

research for studies of the cellular basis of behavior including:

habituation Habituation is a form of non-associative learning in which an innate (non-reinforced) response to a stimulus decreases after repeated or prolonged presentations of that stimulus. Responses that habituate include those that involve the intact org ...

dishabituation Dishabituation (or ''dehabituation'') is a form of recovered or restored behavioral response wherein the reaction towards a known stimulus is enhanced, as opposed to habituation. Initially, it was proposed as an explanation to increased response fo ...

, and

sensitization Sensitization is a non-associative learning process in which repeated administration of a stimulus results in the progressive amplification of a response. Sensitization often is characterized by an enhancement of response to a whole class of stim ...

, because of the simplicity and relatively large size of the underlying

neural circuit A neural circuit is a population of neurons interconnected by synapses to carry out a specific function when activated. Neural circuits interconnect to one another to form large scale brain networks. Biological neural networks have inspired the ...

ry.

Eric Kandel Eric Richard Kandel (; born Erich Richard Kandel, November 7, 1929) is an Austrian-born American medical doctor who specialized in psychiatry, a neuroscientist and a professor of biochemistry and biophysics at the College of Physicians and Surge ...

, recipient of the

Nobel Prize in Physiology or Medicine The Nobel Prize in Physiology or Medicine is awarded yearly by the Nobel Assembly at the Karolinska Institute for outstanding discoveries in physiology or medicine. The Nobel Prize is not a single prize, but five separate prizes that, accord ...

in 2000 for his work with ''Aplysia californica'', was involved in pioneering research into this reflex in the 1960s and 1970s.

Nonassociative learning

Nonassociative learning is a change of the behavior of an animal due to an experience from specific kinds of stimuli. In contrast to

associative learning Learning is the process of acquiring new understanding, knowledge, behaviors, skills, values, attitudes, and preferences. The ability to learn is possessed by humans, animals, and some machines; there is also evidence for some kind of learnin ...

the behavioral change is not caused by the animals learning that a particular temporal association occurs between the stimuli. There are three different forms of nonassociative learning examined in ''Aplysia'': habituation, dishabituation and sensitization. Eric Kandel and colleagues were the first to demonstrate that Aplysia californica is capable of displaying both habituation and dishabituation. Habituation in ''Aplysia californica'' occurs when a stimulus is repeatedly presented to an animal and there is a progressive decrease in response to that particular stimulus. Dishabituation in ''Aplysia californica'' occurs when the animal is presented with another novel stimulus and a partial or complete restoration of a habituated response occurs. Sensitization in ''Aplysia californica'' is the increase of a response due to the presentation of a novel, often noxious, stimulus.

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 animal's gill and siphon from potentially threatening stimuli.Kandel, E. R. (1976). Cellular Basis of Behavior, an introduction to behavioral neurobiology. W. H. Freeman and Company. 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, as opposed to

vertebrate Vertebrates () comprise all animal taxa within the subphylum Vertebrata () ( chordates with backbones), including all mammals, birds, reptiles, amphibians, and fish. Vertebrates represent the overwhelming majority of the phylum Chordata, ...

s, 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, LD_G1, LD_G2 and RD_G results in large gill contractions and stimulation of L9_G1 and L9_G2 produces smaller contractions. In the abdominal ganglion has seven central motor neurons been found that also produce movements of the siphon. LD_S1, LD_S2, LD_S3, RD_S, LB_S1, LB_S2, and LB_S3 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, and dishabituation occurred. Sensitization was observed when a strong stimulus was administered to the tail, thus enhancing a completely rested reflex in ''Aplysia californica''.

Nonassociative learning

Gill and siphon withdrawal reflex (GSWR)

References

Further reading