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The Gill and Siphon Withdrawal Reflex in Aplysia Californica

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The Gill and Siphon Withdrawal Reflex in Aplysia Californica The Gill and Siphon Withdrawal Reflex inAplysia californica Eman F. Khwaja Aplysia californica is a principal model organism in the field of neurobiology due to its famously known gill and siphon withdrawal reflex. It was first used in the 1960s by neurobiology researcher Eric Kandel and has since gained popularity due to its easily distinguishable neurons and simple nervous system. Since then, Aplysia californica has been the model organism for research studies across the globe and has contributed to increasing knowledge of ion channels, neurotransmitters, mutations, and learning and memory. The gill and siphon reflex that made this organism popular is a defensive mechanism that results in the gill and siphon of the organism to retract upon exposure to disruptive and threatening stimuli. The unique aspect of this reflex is its synaptic plasticity and learning abilities. The organism can display several types of learning in association with this specific reflex, which include habituations, sensitization, classical conditioning, and operant conditioning. This reflex ofAplysia cali- fornica utilizes a number of aspects of its nervous system and has provided to be extremely influential on studies associated with memory and learning, molecular biology, behavior, genetics, synaptic plasticity, and even aging. plysia californica, more commonly known presence of disrupting stimuli (Moroz, et al., 2006). as the California Sea Hare, is a species of There is a two-step process that occurs when the re- Asea slugs and is part of the phylum Mollus- flex is triggered through a stimulus being applied to ca. Since the 1960s, this Aplysia californica has be- either the siphon or the mantle shelf of the organisms come increasingly popular as a lab model due to the (Moroz, et al., 2006). First, there is a siphon-with- structure of its central nervous system and presence drawal and following the siphon-withdrawal, there of its gill and siphon withdrawal reflex (Rankin, et is the gill-withdrawal. These two components will al., 2008). It was first studied by Eric Kandel, the occur extremely close in time, producing a single father of modern neuroscience, and his colleagues reflex termed the gill and siphon withdrawal reflex. when they set out to determine the biochemical and Due to the limited number of neurons, roughly neurological basis of learning (Walter, et al., 1983). 10,000 neurons, within the nervous system of this In the 1970s, they discovered that cyclic AMP and organism, it is possible for researchers to observe serotonin were both necessary for short-term mem- experimental questions on this organism. There are ory formation and that cAMP-dependent protein ki- even less neurons associated with the gill and si- nase and potassium channels were integral to long- phon withdrawal reflex, and even then only a few term memory (Walter, et al., 1983). Since then, are integral to producing this behavior and its plas- Aplysia California has been significantly studied ticity during various forms of learning. Some of the across the world and across a variety of fields. Stud- integral neurons necessary to producing this reflex ies on this organism have led to a variety of scien- in Aplsyia californica include mechanosensory neu- tific advancements and even to two Nobel prizes. rons, interneurons, and motor neurons (Carew, et al., Within the nervous system of Aplysia californica 1983). Mechanosensory neurons are neurons that in- are nine separate ganglia and about 10,000 neurons nervate the siphon and are activated by mechanical (Rankin, et al., 2009). Each of the neurons are accessi- stimuli to collect information for processing (Frost, ble from the surface of the ganglia and are distinctive et al., 1985). Interneurons are located between neu- enough from each other that researchers are able to be rons that receive input and neurons that supply an identify each one individually based on their synaptic output, and these neurons receive input from sensory connections, function within the nervous system, and neurons (Frost, et al., 1985). Finally, the motor neu- physiological structure (Rankin, et al., 2008). Despite rons innervate the muscles in the gill of the organism the simple structure of this organism’s nervous sys- and receive processed information to elicit a response tem, countless complex behaviors are highly associ- (Frost, et al., 1985). In spite of the seemingly simple ated with it, such as the gill and siphon withdrawal structure of the nervous system of Aplysia californi- reflex. This gill and siphon reflex behavior is known ca, it has become a common lab model and integral as a defensive reflex mechanism that causes the si- organism to neuroscience research due to the com- phon and gill of the organism to be retracted upon the plex processes associated with its nervous system Vol. 4, No. 2, Spring 2019 67 THE GILL AND SIPHON WITHDRAWAL REFLEX are repeatedly presented to the organism to trigger Current Advancements the gill and siphon withdrawal response can lead to Aplysia californica first became a lab model in a decreased responsiveness to that particular stimuli. the 1960s and has since contributed to significant Sensitization occurs when there is an increase in re- neuroscience advancements through the work of Eric sponsiveness to a stimulus because it is a novel stimu- Kendal, Ph.D., and others who studied the gill and lus (Houwer, et al., 2018). In studies with Aplysia cal- siphon withdrawal reflex. Since then, research con- ifornica as a lab model, operant conditioning is used ducted on this organism has included a wide variety to allow the model organism to learn the association of topics within neuroscience, including genomics, between the gill and siphon withdrawal reflex and re- brain function, toxicology, and behavior. Morez, et ceiving food. Operant conditioning occurs when the al., (2006) found that Aplysia californica has 104 organism learns the association between a behavior counterpart genes that has aided in the study of neu- and a consequence which increases or decreases a rodegenerative diseases and treatments, such as Alz- specific behavior, such as a reward (Houwer, et al., heimer’s, Parkinson’s, and dementia. Greer, et al., 2018). In the case of this organism, operant condition- (2018) completed a study on gene expression which ing can occur when the organism receives a positive suggested a mechanism for which aging in sensory consequence when it completes its reflex and protects and motor neurons affected the development of mu- itself from a predator. Classical conditioning is anoth- tations that impact learning in Aplysia californica. er type of learning that Aplysia californica is capable These researchers have continued to develop their of, which is when organisms learn the association be- research by assessing the ideal time to develop long- tween two stimuli and can anticipate events, therefore term memories during human development based eliciting a response (Houwer, et al., 2018). Aplysia cal- on expression of certain genes, neurotransmitters, ifornica uses this type of learning to change their rate and neuronal development. Akhmedov, et al., (2013) of responsiveness to normalized stimuli with their gill completed research on the topic of aging in Aplysia and siphon withdrawal reflex. The ability of Aplysia californica which also suggested that mutations in californica to learn and adapt a reflex signifies height- genetic information that influence action potentials ened cognitive control and also provides a mecha- have led to a change in the responsiveness to stim- nism for energy reduction by reducing the organism’s uli and neurotransmitters, such as acetylcholine. focus on the production of an unnecessary behavior. One of the behavioral biological topics Aplysia californica is most known for is the study of learning Genetics and the gill and siphon withdrawal reflex. Research According to Cotman, et al., (1999) Aplysia cal- completed on this reflex found that it is not only a be- ifornica has 10,000 active genes, many of which en- havioral reflex, but relies heavily on neurotransmitters, code behaviors, such as the gill and siphon withdrawal ion channels, action potentials, among other aspects of reflex. These behaviors can be influenced by learned the nervous system of Aplysia californica (Agranoff, responses and repetitive stimuli, thus allowing the et al., 1999). Changes in these components of the cen- organism to adapt its own gill and siphon withdraw- tral nervous system can seriously impact the gill and al reflex when presented with repeated stimuli. This siphon withdrawal reflex. Mutations in aspects of the research has also suggested that the development of central nervous system can also impede on the organ- these learned adaptations to encoded behaviors are a ism’s ability to complete this behavior properly, such results of protein synthesis and methylation involve- as mutations to the ion channels or to the neurotrans- ment, as well as storage of the memories in the nucle- mitter receptors on the postsynaptic neuron. The gill us of the organism. Research completed by Tamva- and siphon withdrawal reflex is also capable of be- kis, et al., (2018) has also suggested that RNA that is ing adapted through a number of learning variations, transferred from one organism to another can induce including habituation, sensitization, operant condi- long-term sensitization and epigenetic nonsynaptic tioning, and classical condition (Bailet, et al., 2008). changes which result in a adapted behavioral effects Habituation occurs when a repeated stimulus is implicated in the gill and siphon withdrawal reflex. presented to an organism, resulting in a decrease reac- These behavioral implications can include a decreased tion or response to the particular stimulus (Houwer, et response without going through the same situations to al., 2018). In Aplysia californica, certain stimuli that induce habituations as other organisms might have to. 68 The Onyx Review E. F. Khwaja Esdin, et al., (2010) found that long-term habituation term memory is more associated with changes in the of the gill and siphon withdrawal reflex required gene density of synaptic connections (Dash, et al., 1990).
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