Aplysia gill and siphon withdrawal reflex
The Aplysia gill and siphon withdrawal reflex (GSWR) is an involuntary, defensive reflex
A reflex action, also known as a reflex, is an involuntary and nearly instantaneous movement in response to a stimulus. A true reflex is a behavior which is mediated via the reflex arc; this does not apply to casual uses of the term 'reflex'.-See also:...

 of the sea hare
Sea hare
The clade Aplysiomorpha, commonly known as Sea hares or Sea Bags , are medium-sized to very large Opisthobranchia with a soft internal shell made of protein...

Aplysia californica, a large shell-less sea snail or sea slug. This reflex causes the sea hare's delicate siphon and gill
A gill is a respiratory organ found in many aquatic organisms that extracts dissolved oxygen from water, afterward excreting carbon dioxide. The gills of some species such as hermit crabs have adapted to allow respiration on land provided they are kept moist...

 to be retracted when the animal is disturbed.

Aplysia californica is used in neuroscience
Neuroscience is the scientific study of the nervous system. Traditionally, neuroscience has been seen as a branch of biology. However, it is currently an interdisciplinary science that collaborates with other fields such as chemistry, computer science, engineering, linguistics, mathematics,...

 research for studies of the cellular basis of behavior including: habituation
Habituation can be defined as a process or as a procedure. As a process it is defined as a decrease in an elicited behavior resulting from the repeated presentation of an eliciting stimulus...

, dishabituation, and sensitization
Sensitization is an example of non-associative learning in which the progressive amplification of a response follows repeated administrations of a stimulus. An everyday example of this mechanism is the repeated tonic stimulation of peripheral nerves that will occur if a person rubs his arm...

, because of the simplicity and relatively large size of the underlying neural circuitry.

Eric Kandel, recipient of the Nobel Prize in Physiology or Medicine
Nobel Prize in Physiology or Medicine
The Nobel Prize in Physiology or Medicine administered by the Nobel Foundation, is awarded once a year for outstanding discoveries in the field of life science and medicine. It is one of five Nobel Prizes established in 1895 by Swedish chemist Alfred Nobel, the inventor of dynamite, in his will...

 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 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 is 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 is when the animal is presented to 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 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 vertebrate
Vertebrates are animals that are members of the subphylum Vertebrata . Vertebrates are the largest group of chordates, with currently about 58,000 species described. Vertebrates include the jawless fishes, bony fishes, sharks and rays, amphibians, reptiles, mammals, and birds...

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, 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.

Further reading

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