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Euopisthobranchia (Aplysia), Nudipleura (Tritonia, Hermissenda, Pleurobranchaea), and Panpulmonata (Lymnaea, Helix, Limax) gastropod mollusks exhibit a variety of reflex, rhythmic, and motivated behaviors that can be modified by elementary forms of learning and memory. The relative simplicity of their nervous systems and behavioral repertoires has allowed the uncovering of processes of neuronal and synaptic plasticity underlying non-associative learning, such as habituation, sensitization, and different forms of associative learning, such as classical and operant conditioning. Decades of work on these simpler and accessible animal systems have almost uniquely yielded an understanding into the mechanistic basis of learning and memory spanning behavior, neuronal circuitry, and molecules. Given the conservative nature of evolutionary processes, the mechanisms deciphered have also provided valuable insights into the neural basis of learning and memory in other metazoans, including higher vertebrates.

Article

James S.H. Wong and Catharine H. Rankin

The nematode, Caenorhabditis elegans (C. elegans), is an organism useful for the study of learning and memory at the molecular, cellular, neural circuitry, and behavioral levels. Its genetic tractability, transparency, connectome, and accessibility for in vivo cellular and molecular analyses are a few of the characteristics that make the organism such a powerful system for investigating mechanisms of learning and memory. It is able to learn and remember across many sensory modalities, including mechanosensation, chemosensation, thermosensation, oxygen sensing, and carbon dioxide sensing. C. elegans habituates to mechanosensory stimuli, and shows short-, intermediate-, and long-term memory, and context conditioning for mechanosensory habituation. The organism also displays chemotaxis to various chemicals, such as diacetyl and sodium chloride. This behavior is associated with several forms of learning, including state-dependent learning, classical conditioning, and aversive learning. C. elegans also shows thermotactic learning in which it learns to associate a particular temperature with the presence or absence of food. In addition, both oxygen preference and carbon dioxide avoidance in C. elegans can be altered by experience, indicating that they have memory for the oxygen or carbon dioxide environment they were reared in. Many of the genes found to underlie learning and memory in C. elegans are homologous to genes involved in learning and memory in mammals; two examples are crh-1, which is the C. elegans homolog of the cAMP response element-binding protein (CREB), and glr-1, which encodes an AMPA glutamate receptor subunit. Both of these genes are involved in long-term memory for tap habituation, context conditioning in tap habituation, and chemosensory classical conditioning. C. elegans offers the advantage of having a very small nervous system (302 neurons), thus it is possible to understand what these conserved genes are doing at the level of single identified neurons. As many mechanisms of learning and memory in C. elegans appear to be similar in more complex organisms including humans, research with C. elegans aids our ever-growing understanding of the fundamental mechanisms of learning and memory across the animal kingdom.

Article

Donald Edwards

Crayfish are decapod crustaceans that use different forms of escape to flee from different types of predatory attacks. Lateral and Medial Giant escapes are released by giant interneurons of the same name in response to sudden, sharp attacks from the rear and front of the animal, respectively. A Lateral Giant (LG) escape uses a fast rostral abdominal flexion to pitch the animal up and forward at very short latency. It is succeeded by guided swimming movements powered by a series of rapid abdominal flexions and extensions. A Medial Giant (MG) escape uses a fast, full abdominal flexion to thrust the animal directly backward, and is also followed by swimming that moves the animal rapidly away from the attacker. More slowly developing attacks evoke Non-Giant (NG) escapes, which have a longer latency, are varied in the form of abdominal flexion, and are directed initially away from the attacker. They, too, are followed by swimming away from the attacker. The neural circuitry for LG escape has been extensively studied and has provided insights into the neural control of behavior, synaptic integration, coincidence detection, electrical synapses, behavioral and synaptic plasticity, neuroeconomical decision-making, and the modulatory effects of monoamines and of changes in the animal’s social status.