30 35 4( Skin temperature I
Responses of cold and warm receptors in 'the skin. The skin temperature was held at different values while nerve impulses were recorded from representative fibers leading from each receptor type. (Modified from Ken-shalo. In: Zotterman Y. Sensory Functions of Skin in Primates. Oxford: Pergamon, 1976.)
their normally high thresholds, both cutaneous and deep pain receptors show little adaptation, a fact that is unpleasant but biologically necessary. Deep and visceral pain appear to be sensed by similar nerve endings, which may also be stimulated by local metabolic conditions, such as ischemia (lack of adequate blood flow, as may occur during the heart pain of angina pectoris).
The free nerve endings mediating pain sensation are anatomically distinct from other free nerve endings involved in the normal sensation of mechanical and thermal stimuli. The functional differences are not microscopically evident and are likely to relate to specific elements in the molecular structure of the receptor cell membrane.
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