There are three different categories (modalities) of the somatosensory system. The first, discriminative touch, is the perception of pressure, vibration, and texture. This system relies on four different receptors in the skin. They are:
1) Meissner's corpuscles
2) Pacinian corpuscles
3) Merkel's disks
4) Ruffini endings
The first two are considered rapidly adapting (they quickly stop firing in response to a constant stimulus) and the second two are considered slowly adapting (they do not stop firing). To put this into an example, if you lay your pen down in your palm, the Meissner's and Pacinian corpuscles will fire rapidly as it first touches down, to let you know something has landed. If the pen lays still, they will stop firing almost right away. The Merkel's and Ruffini endings, however, will continue to fire to let you know that something is still there.
The pain and temperature system does not have specialized receptor organs. Instead, it uses free nerve endings throughout skin, muscle, bone, and connective tissue to perceive changes in temperature and pain peptides. Although pain will result from damage to a free nerve ending, in reality most pain is a result of substances released by damaged tissues: prostaglandins, histamine, and substance P. The free nerve ending has receptors for these substances and lets you know (stridently) when tissue has been damaged.
The third modality, proprioceptive sensation, relies on receptors in muscles and joints. The muscle spindle is the major stretch receptor within muscles, and just like the cutaneous receptors, it has a rapidly-adapting and slowly-adapting component. (For more on the muscle spindle see "Spinal motor structures".) There are also Golgi tendon organs and joint afferents to monitor stresses and forces at the tendons and joints.
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