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Sensory Modalities and Adaptation The receptors that provide information for somesthetic senses come in a variety of anatomical and functional types. Each specific receptor will respond to only one type of functional signal (such as mechanical or chemical, but not both). If the information is transmitted all the way to the brain’s cortex, we perceive a sensation. The type of perception that this information leads to is called a modality. There are four main modalities typically recognized as part of the somesthetic senses. These include temperature, touch, pain (nociception), and position and movement (proprioception). Yet each of these can be further subdivided into sub-modalities (sometimes called stimulus modalities). For example, the modality of pain can be subdivided into sharp, dull and aching. There is not general agreement over how sensory modalities should be categorized and subcategorized. In some classification schemes the general sense of touch is replaced with pressure. In others, there may be differences in the subcategories of touch. One classification scheme is presented in the table below. One Scheme for Categorizing Sensory Modalities In subdivision 2, touch is a sensation produced under conditions of very little skin displacement, while pressure requires displacement of the skin and underlying tissues. Flutter is a sensation produced by a stimuli acting at a lower frequency than vibration. Main Modality Subdivision 1 Touch Crude touch Discriminative touch Subdivision 2 Touch Pressure Vibration One Scheme for Categorizing Sensory Modalities Flutter Pain (Nociception) Sharp Dull Aching Temperature Hot Cold Proprioception Position Muscle length Muscle tension Joint pressure Movement Muscle length Muscle tension Joint pressure Joint angle The stimulus modalities shown in the subdivision 1 and subdivision 2 columns in the table might contribute to more complex sensations when combined under certain conditions. For example, there is evidence that a tickle occurs with simultaneous activation of certain touch and pain receptors – as long as other conditions are appropriate. As you are probably aware, others can tickle you, but you generally cannot tickle yourself, even if you can activate the same sensory receptors. Thus input from other systems seems to be able to affect our sensations. There is still a lot for scientists to learn about sensory perception. Sensory Adaptation When first jumping into cool water, you may endure a wave of sensory information “reminding you” that the water is cool. Yet minutes later, you may be “used to” the water temperature, or have adapted to it. This change in perception did not occur because the water warmed up, but because the sensory receptors that originally responded to the change in temperature are no longer sending signals to the CNS, or are sending them at a decreased rate. This example indicates how a rapidly adapting receptor might function; it provides significant signals to the CNS about the original change in temperature that the body experiences, but then adapts such that it sends fewer signals thereafter. Not all sensory receptors are rapidly adapting. Certain pain receptors seem to have little adaptation, or are very slowly adapting, so that as long as the pain stimulus is applied, the person continues to “get the message”! The general rates of adaptation for specific sensory receptors are indicated in the above table.