But when I met the late Bob Elvey, he completely changed my way of thinking about these clinical problems. Bob’s mantra was that “muscles protect nerves.” He introduced me to the dynamics of the nervous system and I came to understand that peripheral nerves of the upper limb had evolved to be able to adapt to the various changes in limb position and length and that they were vulnerable at certain anatomical points along their course.
In brief, Geoff’s studies have had two major impacts on how we think about pain felt in muscles or other deep structures.
Firstly, he confirmed the presence of nociceptors with multiple receptive fields that branch within the nerve sheaths and extend to other deep tissues (nervi nervorum) . The implication of this finding is that activity in a receptor in one structure such as the nerve sheath, could be perceived in another, such as the muscle.
Secondly, he showed that inflammation of nerves has profound effects on these same axons, the nociceptors to deep structures. These effects include ongoing activity and abnormal mechanical sensitivity [8, and others]. The implication of this finding is that this activity will be perceived by the brain in the area of the receptive fields mapped for the deep structure nociceptors, not in the area of the problem.
Figure 1. Proposed hypothesis for the development of focal muscle sensitivity and possible alteration in muscle texture with a proximal neural cause. Inflammation affecting a peripheral nerve (red spot) results in spontaneous and mechanically evoked afferent and efferent action potentials in small caliber sensory neurons innervating non-cutaneous structures, and decreased sympathetic discharge (-). These processes may cause reflex motor discharge sufficient to cause a palpable contraction (?), which combined with clinical phenomena associated with neurogenic inflammation (+), could explain the clinical phenomenon that has become known as a “trigger point.”