As indicated in the preceding chapter, it had been thought that each of the primary modalities of cutaneous sensation is subserved by a morphologically distinct end organ, each with its separate peripheral nerve fibers. These can be broadly categorized as cutaneous and subcutaneous mechanoreceptors, muscle and joint mechanoreceptors, thermal receptors, and pain receptors (nociceptors). According to this formulation, postulated by von Frey and still largely correct but modified as noted further on,  there is a degree of specificity for each receptor and nerve fiber type, each type of end organ responding preferentially to a modality of sensory stimulus.
There are several cutaneous mechanoreceptors:
Meissner corpuscles (named after Georg Meissner), touch; Merkel discs (named after Friedrich Sigmund Merkel), pressure; Ruffini plumes (named after Angelo Ruffini), skin stretch; and Pacini (pacinian) corpuscles (named after Filippo Pacini), vibration, and several others that are more specialized. Likewise, several receptors subserve muscle force, length, and joint angle: the intrafusal muscle spindle, stretch and speed of contraction; Golgi tendon organ, muscle length; joint capsule receptors; joint angle; stretch sensitive-free endings, which is sensitive to force. There are also, as mentioned in the previous chapter, specialized nerve endings for temperature (cool and warm), for extremes of temperature (cold and hot) such as Krause end bulbs (named after Wilhelm Krause and also called bulboid receptors) for cold; and for pain, nerve endings that are not associated with a transducer (“free nerve endings”). The last of these are also termed “naked” because they are surrounded by Schwann cells but are not myelinated (Table 8-1 and Fig. 8-2).
The specificity theory, expressed in the preceding paragraph, has been modified in respect to some somatosensory modalities. For example, Merkel discs and Meissner corpuscles and free nerve endings can all be activated by moving or stationary tactile stimuli. The concept of specificity has held up best in relation to peripheral mechanisms for pain, insofar as certain primary afferent fibers, namely the C and A-δ fibers and their free nerve endings, respond maximally to noxious stimuli. Even these freely branching receptor endings and their pain fibers convey considerable non-noxious information; that is, their specificity as pain fibers is not absolute (Chap. 7). Lele and Weddell found that with appropriate stimulation of the cornea, each of the four primary modalities of somatic sensibility (touch, warmth, cold, and pain) could be recognized, even though the cornea contains only free nerve endings. In the outer ear, which is also sensitive to these four modalities, only two types of receptors—freely ending and perifollicular—are present. The lack of organized receptors in the cornea and ear makes it evident that these types of receptors are not essential for the recognition of cold and warmth as von Frey and other anatomists had postulated.
Instructive refinements of the specificity theory have emerged, based on observations by Kibler and Nathan, who studied the responses of warm and cold spots to different stimuli. Warm and cold spots are those small areas of skin that respond most consistently to thermal stimuli with a sensation of warmth or cold. They found that a cold stimulus applied to a warm spot gave rise to a sensation of cold and that a noxious stimulus applied to a warm or cold spot gave rise only to a painful sensation; they also noted that mechanical stimulation of these spots gave rise to a sensation of touch or pressure. These experiments indicate that cutaneous receptors, some not distinguishable from each other on morphologic grounds, are probably endowed with only relative degrees of specificity, in the sense that each responds preferentially (i.e., has a lower threshold) to one particular form of stimulation.