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"Sensory stimuli are equally perceived equally over the body." Discuss this statement.

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Introduction

28/10/02 Minh Lam "Sensory stimuli are equally perceived equally over the body." Discuss this statement. It is helpful to distinguish between sensation, which is the neural mechanism by which data about the physical world are brought to the sensory part of consciousness and perception, which is what the mind does with this information. With this in mind the statement is clearly untrue as there is an uneven distribution in the perception of sensory stimuli. If we consider the special sensations of vision, hearing, taste and smell it is clear that these are not perceived equally throughout the body but concentrated in certain areas. For example our perception of vision is from light stimulating photoreceptors within the eye and nowhere else in the body. A stimulus has four fundamental properties, which include modality, intensity, position and timing. There are two main ways in which information about the stimulus can be transmitted to the CNS. The first is known as frequency coding and involves controlling the frequency of impulses within a single fibre (indicating the intensity and timing of the stimulus). The second method is known as labelled line coding and is achieved by which fibres of an array are activated (indicating location and modality). An example of labelled line coding is cold fibres which when activated give a cold sensation irrespective of the stimulus. ...read more.

Middle

It is thought there are many more warm receptors than there are spots and it requires the simultaneous activation of many receptors to elicit the sensation of warmth. This is known as spatial summation in the sensory system. Cooling stimulates the A? fibres and warmth stimulates a sub-population of C fibres. There is maximum firing where the two fibre types overlap (around body temperature) enhancing small changes in temperature away from body temperature. Evidence of sensory stimuli being unequally perceived is shown in the two-point discriminatory test. This shows that tactile acuity is better in certain parts of the body e.g. hands and fingers than others e.g. the back. Experiments have shown that the smallest discriminable distance between two points of contact, the two-point limen, improves twenty fold from the shoulder to the fingers. In general, tactile acuity increases with mobility of the body parts. Areas with high tactile acuity have small receptive fields. If two points contacting the skin stimulate only one receptive field it will only be processed as one point. RAI and SAI fibres have small receptive fields and the highest density is on the fingertips. Pain is different from the other sensations of the skin and is difficult to understand for a variety of reasons. It is not only a feeling, but demands both a motor and emotional reaction. ...read more.

Conclusion

The thalamus is composed of many well-defined nuclei, which are often classed into four groups: anterior, medial, ventrolateral and posterior. Sensory stimuli travel from the thalamus to the primary somatosensory cortex (SI) located in the post central gyrus (immediately posterior to the central sulcus). The homunculus is evidence of an unequal perception of sensory stimuli over the body. It maps out the somatosensory input to the cortex showing that the area of cortex devoted to processing information from a particular part of the body is not proportional to the topography of the skin, but reflects the degree of innervation in that area. Each part of the body is represented according to its importance to the organism. In humans there is a proportionally large area devoted to the hands and face, whereas in rodents whiskers are more dominant. Sensory stimuli are not equally perceived all over the body, but different stimuli are detected more strongly in some areas than others. Our perception of touch is best in our fingers, which makes sense because this is the most mobile and dextrous part of our body. Proprioceptors are essential in our ability to sense the position and location of various different parts of the body. They are located within the muscles, joints and deep connective tissue, where movement takes place. The statement is more true of pain and temperature sensations, which can be felt through most of the body but these sensations are modulated in a different way. ...read more.

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