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Pressure and pain in muscle independent of skin sensation

PainSci » bibliography » Graven-Nielsen et al 2004
Tags: chronic pain, muscle, muscle pain, neat, biology, fun, pain problems

Two articles on PainSci cite Graven-Nielsen 2004: 1. The Complete Guide to Trigger Points & Myofascial Pain2. The Complete Guide to Low Back Pain

PainSci commentary on Graven-Nielsen 2004: ?This page is one of thousands in the bibliography. It is not a general article: it is focused on a single scientific paper, and it may provide only just enough context for the summary to make sense. Links to other papers and more general information are provided wherever possible.

This study attempted to determine if painful and non-painful pressure sensations from muscles actually exist. That may seem like an odd research question to ask, but it is possible that much of what seems like “deep” muscle sensation could in fact come an illusion of depth inferred from pressure on the skin, like a “3D” image projected on a flat screen. Sensation and perception are full of oddities like this, so we cannot actually take it for granted that muscle itself is sensitive.

The authors put it this way: “Painful and non-painful pressure sensations from muscle are generally accepted to exist but the peripheral neural correlate has not been clarified.”

This is a challenging question to study, because it’s difficult to eliminate skin sensation as a factor! So these researchers went to rather a lot of trouble to do just that, with “anaesthetised skin combined with a block of large diameter muscle afferents.” And it turns out that people can still feel pressure and pain even with anaesthetized skin. Although not conclusive in itself, this evidence does strongly suggest that muscle tissue proper knows when it’s being poked!

~ Paul Ingraham

original abstract Abstracts here may not perfectly match originals, for a variety of technical and practical reasons. Some abstacts are truncated for my purposes here, if they are particularly long-winded and unhelpful. I occasionally add clarifying notes. And I make some minor corrections.

Painful and non-painful pressure sensations from muscle are generally accepted to exist but the peripheral neural correlate has not been clarified. The aim of the present human study was to assess the non-painful and painful pressure sensitivity with (1) anaesthetised skin, and (2) anaesthetised skin combined with a block of large diameter muscle afferents.

The skin was anaesthetised by a topically applied anaesthetic cream and later lidocaine was administrated subcutaneously. The pressure sensitivity was assessed quantitatively by computer-controlled pressure stimulation on the anterior tibial muscle. Thresholds to detection, pain and pain tolerance were assessed. In the first experiment, computer-controlled needle insertion depths evoking touch and pain sensations were used to assess the efficacy of cutaneous anaesthesia. Touch and pain sensations evoked during needle insertions were found to be superficial in intact skin but when anaesthetised, touch sensation was occasionally evoked at depths related to penetration of the fascia.

With the skin completely anaesthetised to brush and von Frey hair pinprick stimulation, skin indentation with the strongest von Frey hair caused a sensation described as a deep touch sensation. Simultaneously, pressure detection and pain thresholds increased but it was still possible to elicit non-painful and painful pressure sensation in all subjects. In a second experiment, a differential nerve block of group I and II afferent fibres was obtained by full-leg ischaemia simultaneously with cutaneous anaesthesia.

The efficacy of the tourniquet block was continuously assessed by a battery of somatosensory tests (heat, brush, vibration, electrical and movement detection) applied at the foot simultaneously with pressure stimulation on the anterior tibial muscle. After 20 min of ischaemia, group II afferent fibres mediating the sensations of movement detection, vibration and brush on the foot was blocked but the heat pain threshold was not affected. In this condition (anaesthetised skin and block of group I and II fibres from deep tissue) a pressure sensation was evoked in 70% of subjects although the pressure detection threshold was increased. The pressure pain sensitivity was decreased, which, however, might indicate a partial block of group III and IV muscle afferents. In a third experiment, the tactile sensations elicited by electrical stimulation of the tibialis anterior muscle and skin at the lower leg were significantly decreased after 20 min of ischaemia, validating the blocking effects of group I and II nerve fibres.

The present data show a marginal contribution of cutaneous afferents to the pressure pain sensation that, however, is relatively more dependent on contributions from deep tissue group III and IV afferents. Moreover, a pressure sensation can be elicited from deep tissue probably mediated by group III and IV afferents involving low-threshold mechanoreceptors.

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