Withdrawal reflex and habituation in the garden snail - Essay Example

Describe an investigation that could be conducted to assess habituation of the withdrawal reflex in the garden snail - Introduction Learning represents an important attribute of all living things and it is essential to the functioning and survival of all living organisms. Learning can easily be defined as a "process where a living organism experiences certain relationships between events and is able to recognize an association between events, and as a consequence the subject's behavior changes because of that experience."1 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 coeruleus2, 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. Habituation 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. However, after persistent stimulation that does not cause harm to the garden snail, the process of habituation is observed, as the garden snail no longer retracts its body or its eye stalk. Although these processes appear simple, the underlying mechanisms that govern of these processes are more complicated. 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 motor neuron C3. In their paper, they conclude that the additional motor neurons mediating tentacle retraction are important for maximizing the rate of muscle contraction, while those mediating tentacle bending most likely have a nondefensive behavior. They also state that even though these neurons are arranged in parallel with C3, each of these neurons innervates only a single effector or a portion of the tentacle. C3 on the other hand, has a direct innervation of multiple effectors and its sufficiency to evoke strong responses in those effectors, therefore supporting their conclusion that C3 is essential in creating and coordinating tentacle withdrawal. These studies are at the moment on the front in the research concerning the withdrawal reflex and habituation in the garden snail. The methods used are scientifically sound and the findings are generally accepted in the scientific community. - Method Like in any scientific experiment, a group of specimens must be obtained which will be used to perform the experiment. Snails are usually collected locally, during warm and wet conditions. It should also be noted that the best time to look for garden snails is in the early morning or evening hours, in grassy and leafy areas. After the snails are collected, they should be kept alive for a couple of days in a container, with controlled feeding and watering. This is done in order to eliminate any toxins that the snails might have ingested and affect their nervous system. Then using coffee stirrers, or any other object that will not do any physical harm to the body of the garden snail, the body is stimulated and the results are documented. It is necessary to pay attention to several parameters, such as whether or not there is a reaction, and how fast the reaction has occurred. After the snail has been stimulated, it is left to return to its former state. The experiment is then repeated several times. - Expected results As many experiments that have previously shown, the first couple of times the garden snail will produce the expected withdrawal reflex. As the experiment progresses, we will notice that reaction time might be slower and less intense and in the end, completely disappear. That is actually the point when habituation has occurred. It is also important to notice that, if stimulation is performed again some time after habituation has developed, we will se the appearance of the withdrawal reflex, supporting the short-term memory bases of habituation. The experiment will merely coincide with the many others performed on this issue. - Summary The withdrawal reflex and habituation are two essential physiological processes in all living organisms, including the garden snail. Although simple in nature, the underlying principles are still being investigated, and hopefully one day will help us understand more about the nature of the nervous system and the processes of learning and perception, both in animals and in humans References: 1. http://nyny.essortment.com/learningtypesd_rmjt.htm 2. Wood, D. C. (1988). Habituation in Stentor produced by mechanoreceptor channel modification. Journal of Neuroscience, 2254 (8). 3. Thompson, R.F. and Spencer, W.A. 1966. Habituation: A model phenomenon for the study of neuronal substrates of behavior. Psych. Rev. 73:16 -43. 4. Steven A. Prescott, Nishi Gill, and Ronald Chase; 1997; Neural Circuit Mediating Tentacle Withdrawal in Helix aspersa, With Specific Reference to the Competence of the Motor Neuron C3; The Journal of Neurophysiology Vol. 78 No. 6 5. Steven A. Prescott, and Ronald Chase; 1999 Sites of Plasticity in the Neural Circuit Mediating Tentacle Withdrawal in the Snail Helix aspersa: Implications for Behavioral Change and Learning Kinetics; Vol. 6, No. 4, pp. 363-380, Learning memory Read More