Cutaneous sensation

Cutaneous sensation

 

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Organisms interpret their surroundings through differential processing of signals that are both externally and self generated. This state of fact has been observed and proven across a variety of sensory systems as well as in different species.  In human beings, tactile sensations that are self are self generated are usually perceived as weaker compared to a similar stimuli imposed externally. It has generally been thought that the sensory attenuation result from a mechanism that is predictive.

There is strong evidence to suggest that predictive mechanism causes attenuations that are self generated tactile sensation. The sensation was only attenuated on the passive finger when a tapping movement was made by one finger above another finger of the other hand, and when the two fingers were expected to make contact. The same attenuation levels were observed when there was an anticipation of contact regardless of whether there was actual contact or not. The results are indeed consistent with the predictive mechanism action where self generated sensory events are predicted on the basis  of planned activity of the motor and the incoming sensory stream are attenuated for purposes of mitigating the events sensory salience (Bays et al, 2006).

There is a distinction between the attenuation observed for the study from the increase in the threshold of cutaneous stimuli   in a moving effector in the sense that it is synchronized to the time that is expected as well as present in a passive digit of self contact. In addition, there is an observation made in advance of increased threshold of the passive movement that takes place at a time that is unpredictable. This could only suggest that it involves a mechanism that is masking or predictive rather than predictive mechanism that this study demonstrated.

The study further indicates that for predictive cancellation to occur in a human being, a set of events that are highly specific must take place in the somatosensory system. Self-generated tactile sensation attenuation does not culminate from postdictive reconstruction or predictable stimuli that are arbitrary but rather from a mechanism that relies on anticipation of an event that is specific (Kenneth, 2001). The nervous system ability to predict sensory results of the activity of a motor may be used to examine the ability of a human being to rehearse movements mentally before executing them.

In study conducted by Bays et al, (2006) human psychophysical data are equally consistent with a postdictive or reconstructive mechanism the same way they are, with a predictive mechanism. The percept of a sensory event in a postdictive mechanism is constructed from the information of the sensory received just about the time of the episode. If the responsible mechanism for tactile sensory attenuation is postdictive, a particular sensation will be easily identified as externally or self generated sensation based on all the activities that have taken place at about the same period.

When there is contact between the fingers, the tactile sensation that follows is perceived as weaker compared to the same externally imposed stimulus. An outcome such as this could emanate from a predictive process that is capable altering the sensory perception that is evaluated subsequent to the activity.  Attenuation is observed in the study even when there is no contact of the fingers and this is a clear indication of a predictive process.

According to Iggo,  (1958) Cutaneous thermal pain and sensations can also be caused by cooling of even heating the skin. These sensations may be carried by some fibers known as unmyelinated afferent. The leg, lower thigh, and the foot were used in the observation of the receptor behavior. In the afferent fibres conduction velocities 10 out of 17 fibers that were measured ranged from about 0,5- 17m./sec. All the condition velocities were a little less than 1.2m./sec except for fibre number 5. This led to the assumption that other than fibre number 5, all other fibres were unmyelinated. The size of the receptive fields was assessed through mechanical and thermal stimuli were used. The part of the skin from which impulses discharge could be aroused by the stimulus that was weakest though effective. Receptor fields were 5 by 5 mm, and the mechanical as well as thermal stimuli fields were coincident. It should also be noted that mechanical stimulation could excite all the receptors nonetheless; the required intensity of stimulation was always high. A pressure of 3g was required by units that were more sensitive before an impulse could be discharged (Iggo, 1958).

A number of other receptors could be exited only by pulling hairs, by pressing the skin using the forceps, or by pressing in to the skin a sharp metal pin. It should be noted that when mechanical stimuli were used, it was not easy to find the receptors. The receptors sensitivity to mechanical stimulation increased after frequent cooling and heating even though sensitivity was not constant.

References

 

Bays, P., et al. (2006). Attenuation of Self-Generated Tactile Sensations is Predictive, not             Postdictive. Attenuation of Self-Generated Sensations.4(2), 1-4

 

Iggo, A. (1958). Cutaneous heat and cold receptors with slowly conducting (c) afferent    fibres. Thermal Receptors with C Fibres 1(1), 362- 370

 

Kenneth, O. (2001) The roles and functions of cutaneous mechanoreceptors. Sensory         systems 1(1), 455- 460

 

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