Nociceptor Terminal Stimulation and Sensitization

Stimuli of afferent axons can be chemical and mechanical. Increased levels of glutamate in muscle correlate temporally with the appearance of pain after exercise or experimental injections of hypertonic saline. Glutamate injection into muscle in humans both produces pain and sensitizes muscle afferents to other stimuli. Some mechanosensitive unmyelinated afferent axons in muscle are stimulated by acromelic acid-A, a kainoid mushroom toxin that produces long-lasting allodynia and burning pain when ingested. The injection of acid (H⁺) into skeletal muscle elicits pain. Acid-sensing ion channel 3 (ASIC3) channels are expressed on sensory neurons innervating skeletal and cardiac muscle. H⁺ ions may produce muscle pain by activating ASIC3 channels on afferent axons. ASIC3 channels may initiate the anginal pain associated with myocardial ischemia. Lactate, an anaerobic metabolite, probably does not play a primary role in directly stimulating muscle pain. Patients with myophosphorylase deficiency do not produce lactate under ischemia yet experience pain. Lactate may potentiate the effects of H⁺ ions on ASIC3 channels in activating pain-related axons. Adenosine triphosphate (ATP), another metabolite, is present in increased levels in muscle interstitium during ischemic muscle contraction. Injection of ATP also elicits pain. Many peripheral nociceptors express ATP purinergic receptors, specifically P2X₃, P2Y₂, and P2Y₃. P2X₃/P2X_2/3 receptor antagonists can reverse the mechanical hyperalgesia that occurs with inflammation. Individual algogenic molecules only activate subsets of peripheral nociceptors. Combinations of protons, lactate, and other molecules synergistically activate the most sensory afferents in muscle tissue (Light et al., 2008).

Sensitization of nociceptive axon terminals is reduction of the threshold for their stimulation into the innocuous range. Sensitization of nociceptor terminals can have two effects on axons: (1) an increase in the frequency of action potentials in normally active nociceptors or (2) induction of new action potentials in a population of normally silent small axons that are especially prominent in viscera. Vanilloid receptors and tetrodotoxin-resistant sodium channels can play roles in the sensitization of nerve terminals. The heat and capsaicin receptor, transient receptor potential cation channel V1 (TRPV1) can be activated in strong acidic conditions. However, the main TRPV1 activator may be endogenous ligands such as oxidized metabolites of linoleic acid, which damaged myocytes potentially release. Factors released during damage or repetitive stimulation induce a reduction in the axon terminal thresholds in muscle. Substance P induces a low-frequency discharge in afferent axons that could contribute to spontaneous pain. Leukotriene D₄ may have a desensitizing effect on muscle nociceptors. The depression of muscle nociceptor activity by aspirin may reflect inhibition of the effects of prostaglandin E₂. Other endogenous substances proposed to play roles in activating or sensitizing peripheral nociceptive afferents include neurotransmitters (serotonin, histamine, glutamate, nitric oxide, adrenaline), neuropeptides (substance P, neurokinin 1, bradykinin, nerve growth factor [NGF], calcitonin gene-related peptide), inflammatory mediators (prostaglandins, cytokines), and potassium ions (Mizumura, 2009). GTP cyclohydrolase 1 (GCH1), a rate-limiting enzyme in the tetrahydrobiopterin synthetic pathway, may play a role in enhancing inflammatory and neuropathic pain sensitivity. Certain haplotypes of GCH1 are associated with reduced GCH1 activity and may be protective against pain in patients experiencing pain and normal healthy controls.

Nociceptive Axons

Many of the afferent nerve fibers that transmit painful stimuli from muscle (nociceptors) have small unmyelinated (free) axon terminals (Graven-Nielsen and Mense, 2001; Julius and Basbaum, 2001). These terminal axons (nerve endings) are mainly located near blood vessels and in connective tissue but do not contact muscle fibers. Free nerve endings have a small diameter (0.5 µm) with varicosities (expansions). They contain glutamate and neuropeptides. Noxious stimuli produce graded receptor potentials in nerve endings, with the amplitude dependent on strength of the stimulus. An action potential develops if the amplitude of the receptor potential is large enough to reach threshold. Action potentials arising in nociceptor terminals induce or potentiate pain by two mechanisms. Centripetal conduction to central branches of afferent axons brings nociceptive signals directly to the CNS. Centrifugal conduction of action potentials along peripheral axon branches causes indirect effects by invading other nerve terminals and causing release of glutamate and neuropeptides into the extracellular medium. These algesic substances can stimulate or sensitize terminals on other nociceptive axons.

Aδ-class (group III) and C-class (group IV) afferent axons play important roles in the conduction of pain-inducing stimuli from muscle to the CNS (Arendt-Nielsen and Graven-Nielsen, 2008). Blockade of both Aδ- and C-class axons eliminates the ability to detect acute noxious stimuli. Selective expression of sodium channel isoforms Na_v1.7, Na_v1.8, and Na_v1.9 occurs on axons and sensory ganglia in peripheral nerve pain pathways. Aδ-class nociceptive axons are thinly myelinated, conduct impulses at moderately slow velocities (3 to 13 m/sec), and have membrane sodium channels that are tetrodotoxin sensitive. Aδ axons are high-threshold mechanoceptors stimulated by strong local pressure and mediate rapid, acute, sharp (first-phase) muscle pain. Aδ-class axons probably mediate spontaneous pain and dysesthesias. C-class nociceptive axons are unmyelinated, conduct impulses at very slow velocities (0.6 to 1.2 m/sec), and have membrane sodium channels that are tetrodotoxin resistant. C-class axons in muscle are often polymodal, responding to a range of stimuli, but stimulus-specific axon terminals are also present. C fibers mediate somewhat delayed, diffuse, dull or burning (second-phase) pain evoked by noxious stimuli. Constituents of muscle nociceptor C axons include substance P, calcitonin gene-related peptide, and somatostatin. These constituents may place the nociceptive axons in a subgroup of C fibers that mediate hyperalgesia in response to inflammation. Aβ-class axons are large, myelinated, and conduct impulses at rapid velocities. They normally mediate innocuous stimuli, and stimulation may reduce the perception of pain. Inflammation or repetitive stimulation can sensitize Aβ axons, which then mediate mechanical allodynia in some tissues. Mediation of this “phenotypic switch” in Aβ axons may occur via up-regulation of neuropeptide Y and sprouting of terminals in the spinal cord from lamina III and IV into lamina II, with subsequent stimulation of ascending central pain pathways.

Central terminals of nociceptive axons from muscle end in lamina I in the dorsal horn of the spinal cord. Ascending central neurons with cell bodies in laminae I or II are stimulated by glutamate from the terminals of primary afferent axons and convey sensory pain modalities via a lateral nociceptive system that includes the contralateral spinothalamic tract, thalamic nuclei (Ren and Dubner, 2002), and somatosensory cortex. Some models suggest that tonic muscle pain may involve a medial set of central pathways including the ipsilateral insula and medial prefrontal regions (Thunberg et al., 2005). Transmission of affective features of pain may also involve medial pathways to the parabrachial nucleus, amygdala, thalamic intralaminar nucleus, and anterior cingulate gyrus. Interneurons and descending CNS pathways modulate afferent input, especially with chronic pain. Central sensitization to pain is associated with neurons containing substance P receptors. Glutamate acting at N-methyl-d-aspartate (NMDA) receptors is essential for the initiation of central sensitization and for the hyperexcitability of spinal cord neurons and persistent pain. Facilitation via descending CNS pathways may lead to allodynia and the maintenance of hyperalgesia. Decreased activation of inhibitory descending pathways is associated with increased opioid sensitivity and may provide a system of endogenous analgesia. There is enhanced net descending inhibition at sites of primary hyperalgesia associated with inflammation.

Pathological Conditions Producing Muscle Pain

Episodes of pain originating in muscle are commonly associated with exercise, inflammation, and trauma. Exercise can produce muscle pain via several pathways including exhaustion of fuel supply (with lack of training, vascular insufficiency [ischemia], or metabolic defects), cramps, or injury to muscle fibers or tendons. When muscle contracts while it is being stretched (eccentric contraction), damage and pain are especially likely. During exercise with eccentric contraction, the shearing forces on connective tissue may directly activate muscle nociceptors. Similar painful shearing forces occur during cramps, when the contracting segment stretches the remainder of the muscle. Delayed-onset muscle soreness (DOMS), a common syndrome after unaccustomed exercise, may be due to several factors including muscle fiber and connective tissue damage and inflammation. Pain with DOMS is associated with increased levels of glutamate in muscle. Muscle pain is especially prominent with fascial (connective tissue) damage.

In damaged muscles, tenderness, a decrease in pressure pain threshold, and pain with movement are due to sensitization of muscle nociceptors. The sensitized nociceptors have a lowered threshold of excitation and a greater response to noxious stimuli. With inflammation in muscle, pain at rest may be due to nociceptive axons that develop a raised level of background discharge. Mediators of this phenomenon could include algesic substances including substance P, bradykinin, and serotonin. The accumulation of algesic substances may relate to pain during muscle ischemia but probably not to lactate accumulation. Muscle pain may be associated with increased activity of group IV afferent axon activity. Eccentric exercise-induced mechanical hyperalgesia also involves centrally facilitated pain mechanisms. Muscle pain may produce increased discharges in motor neurons innervating agonist and antagonist muscles.

Fibromyalgia is a syndrome with diffuse chronic muscle pain and tenderness to palpation over trigger points. Central sensitization may underlie some of the symptoms of fibromyalgia and related syndromes of myofascial pain and stress-induced syndromes in which pain localizes to muscle, but without prominent physiological or morphological evidence of muscle damage (Bennett, 2005).

General Features of Muscle Pain

In the clinical setting, patients describe muscle discomfort using a variety of terms: pain, soreness, aching, fatigue, cramps, or spasms. The perception of pain originating from muscle is as emanating from deep tissues. Chronic muscle pain may localize poorly, referred to or from another (usually deep) location, and be associated with autonomic and affective symptoms. The properties of muscle pain may reflect convergence of afferent axons from various tissues that mask identification of a specific location by higher centers. Pain with muscle cramps has an acute onset and short duration. Cramp pain is associated with palpable muscle contraction, and stretching the muscle provides immediate relief. Pain originating from fascia and periosteum has relatively precise localization. Cutaneous pain differs from muscle or fascial pain by its distinct localization and sharp, pricking, stabbing, or burning nature. Pain with small-fiber neuropathies is often present outside length-dependent distributions and may be located in proximal as well as distal regions. In fibromyalgia syndromes, it is common for patients to complain that fatigue accompanies their muscle discomfort. Depression is more common in patients with chronic musculoskeletal pain (18%) than in a population without chronic pain (8%).

Evaluation of Muscle Discomfort

The basis for the classification of disorders underlying muscle discomfort can be anatomy, temporal relation to exercise, muscle pathology, and the presence or absence of active muscle contraction during the discomfort (Kincaid, 1997; Pestronk, 2010). Evaluation of muscle discomfort typically begins with a history that includes the type, localization, inducing factors, and evolution of the pain; drug use; and mood disorders. The physical examination requires special attention to the localization of any tenderness or weakness. Accurate assessment of strength may be difficult in the presence of pain. The sensory examination is important because small-fiber sensory neuropathies commonly cause discomfort with apparent localization in muscle. A general examination is important to evaluate the possibility that pain may be arising from other tissues such as joints. Blood studies may include a complete blood cell count, sedimentation rate, creatine kinase (CK), aldolase, potassium, calcium, phosphate, lactate, thyroid functions, and evaluation for systemic immune disorders. Evaluate urine myoglobin in patients with a high CK and severe myalgias, especially when they relate to exercise. Electromyography (EMG) is a sensitive test for myopathy. A normal EMG can suggest that muscle pain is arising from anatomical loci other than muscle. Nerve-conduction studies may detect an underlying neuropathy, but objective documentation of small-fiber axonopathies can require quantitative sensory testing or skin biopsy with staining of distal nerve fibers. Magnetic resonance imaging, ultrasound, or radionuclide scans may reveal focal or diffuse anomalies in muscle, joints, or fascia that can be useful to guide biopsy procedures. Phosphorous magnetic resonance spectroscopy may become useful in evaluating and monitoring some metabolic myopathies, but its utility remains to be determined. Muscle ultrasound can be a useful and noninvasive method of localizing and defining types of muscle pathology. Muscle biopsy is most often useful in the presence of another abnormal test result such as a high serum lactate, aldolase, CK, or an abnormal EMG. However, important clues to treatable disorders such as fasciitis or systemic immune disorders (connective tissue pathology, perivascular inflammation or granulomas) may be present in muscle in the absence of other positive testing. Examination of both muscle and connective tissue increases the yield of muscle biopsy in syndromes with muscle discomfort. There is increased diagnostic yield from muscle biopsies if in addition to routine morphological analysis and processing, histochemical analysis includes staining for acid phosphatase, alkaline phosphatase, esterase, mitochondrial enzymes, glycolytic enzymes, C_5b-9 complement, and MHC Class I. Measurement of oxidative enzyme activities can reveal causes of muscle discomfort or fatigue, even in disorders with no histopathological abnormalities. Ultrastructural examination of muscle rarely provides additional information in muscle pain syndromes.