Tactile Sensation:Defination,Types & Examples
Tactile sensation is divisible into-
1.Non-discrimination (crude) touch
2.Two-point discriminative (fine) touch.
Non-discrimination (crude) touch
Non-discriminative or Light (crude) touch and pressure ascend in the anterior spinothalamic tract. Crude touch is sensed following gentle stroking of the skin with a fine cotton strand, but this sensation does not include detailed information about the stimulus. Tactile examination of an object is dependent on discriminative (fine) touch sense, which enables one to detect fine detail regarding the location, size, shape, and texture of an object even when the eyes are closed.
Destruction of this tract produces contralateral loss of light touch and pressure sensibilities below the level of the lesion. Remember that discriminative touch will still be present because this information is conducted through the fasciculus gracilis and fasciculus cuneatus. The patient will not feel the light touch of a piece of cotton placed against the skin or feel pressure from a blunt object placed against the skin.
Two-point discriminative (fine) touch
Two-point discriminative (fine) touch—that is, the ability to localize accurately the area of the body touched and also to be aware that two points are touched simultaneously, even though they are close together (two-point discrimination)— ascends in the posterior white columns. Also ascending in the posterior white columns is information from muscles and joints pertaining to movement and position of different parts of the body. In addition, vibratory sensations ascend in the posterior white column.
Destruction of these tracts cuts off the supply of information from the muscles and joints to consciousness; thus, the individual does not know about the position and movements of the ipsilateral limbs below the level of the lesion. With the patient's eyes closed, he or she is unable to tell where the limb or part of the limb is in space. For example, if you passively dorsiflex the patient's big toe, he or she is unable to tell you whether the toe is pointing upward or downward. The patient has impaired muscular control, and the movements are jerky or ataxic.
The patient also has loss of vibration sense below the level of the lesion on the same side. This is easily tested by applying a vibrating tuning fork to a bony prominence, such as the lateral malleolus of the fibula or the styloid process of the radius.
There will also be a loss of tactile discrimination on the side of the lesion. This is tested most easily by gradually separating the two points of a compass until the patient can appreciate them as two separate points, not as one when they are applied to the skin surface. Tactile discrimination varies from one part of the body to another. In a normal individual, the points have to be separated by about 3 to 4 mm before they are recognized as separate points on the tips of the fingers. On the back, however, the points have to be separated by 65 mm or more before they can be recognized as separate points. The sense of general light touch would be unaffected, as these impulses ascend in the anterior spinothalamic tracts.