Withdrawal reflex and habituation in the garden snail Essay

Withdrawal reflex and habituation in the garden snail - Essay Example There is many types of learning, the most basic form of which is non-associative learning. Surprisingly noticed even in primitive life forms, such as the protozoan Stentor coeruleus, it represents a type of learning where the organism becomes aware that some events are unrelated and irrelevant. The most common example of such learning is habituation. According to Thompson and Spencer, it is defined simply as "the gradual waning of an evoked behavioral response with repeated stimulation."3. In essence if an animal first responds to a stimulus that is neither rewarding nor harmful the animal reduces subsequent responses. There are two basic characteristics that separate habituation from the other types of learning. One is that it does not require conscious motivation or awareness to occur, and that it is stimulus specific and that does not cause a cause a general decline in responsiveness.The garden snail (Helix aspersa) is no exception when the process of habituation is concerned. Hab ituation in the Helix aspersa is well documented and easily observable, particularly in the withdrawal reflex of both the entire body of the garden snail, as well as the eye stalk. When the body or the eye stalk of the garden snail is stimulated, the Helix aspresa instinctively withdraws the entire body towards its shell or the eye stalk towards the body, respectively. This very simple and instinctive action, with the purpose of maintaining the structural integrity of the body of the garden snail, represents the withdrawal reflex in the Helix aspresa. ... When the garden snail is presented with a novel stimulus, the sensory neurons generate large excitatory postsynaptic potentials in the interneurons and motor cells. These potentials cause the motor cells to discharge strongly, leading to the rapid withdrawal of the body or eye stalk. As the stimulus is repeatedly presented, the synaptic potentials produced by the sensory neurons in the interneurons and in the motor cells become progressively smaller. In time, fewer action potentials are generated in the motor cells, and the behavior is reduced. Finally, the postsynaptic potentials become very small and fail to produce action potentials in the motor neurons, and therefore no behavior is produced. The reduction in the effectiveness of the synaptic connections between the sensory and motor neurons is what maintains the state of habituation that the garden snail has acquired. - Background Although the effects of the withdrawal reflex and the subsequent habituation are easily observable, the underlying principles have been scrutinized by the scientific eye. There are many scientific papers that deal with this issue. One of them, written by Steven Prescott and Ronald Chase explains the sites of plasticity in the neural circuit mediating tentacle withdrawal in the garden snail. With their research, they have managed to show that the different parts of the nervous system in the snail mediate different reactions when the withdrawal reflex is concerned. In an another paper, the same others try to explore the matter even further, with analyzing the neural circuit mediating tentacle withdrawal in garden snail, with specific reference to the competence of the