Muscle pain receptors have been studied in man. During the advance of an EMG needle pain is most regularly experienced as the muscle fascia is pierced. Further advancing the needle is usually quite painless. However, there are scantily distributed points within the muscle (`pain spots') which may give rise to pain if encountered by the advancing needle. The nature of the responsible receptors is obscure but it is likely that pain may sometimes be caused by activation of nerve fibres (presumably from these receptors) coursing through the muscle rather than by activation of the receptors themselves. Intramuscular bleeding is unlikely to contribute often to the pain of needle electromyography. Mechanically sensitive `pain spots' are more painful than other areas when stimulated electrically or injected with hypertonic (6%) saline. This suggests a common receptor. The high electrical threshold and relative invulnerability to pressure of afferent fibres from these receptors suggest that they are mainly of small diameter. These findings are discussed in relation to experimental work on pressure-pain receptors in animals.
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