Concept:Calcium and TrPs

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Calcium is the most abundant mineral in the human body and is essential to muscle contraction, nerve impulse transmission, and the release of acetylcholine (ACh) at the neuromuscular junction — making it directly relevant to the trigger point (TrP) mechanism. Calcium ions control the molecular machinery that initiates and terminates the actin-myosin interaction at the heart of every muscle twitch, and their dysregulation at the motor endplate is central to the hypothesis of TrP formation (see Chapter 2 of the source volume).

There is no study that has directly linked an abnormality of calcium metabolism to myofascial pain syndromes, and disturbances in serum calcium levels are extremely uncommon in patients with chronic MPS. Nevertheless, calcium is of great interest in MPS because of its role in the contraction of muscle, and also because of its role in modulating pain responses at the nociceptor cell level through voltage-gated calcium channels, at the triad where the sarcoplasmic reticulum communicates with the T tubule, and in the dorsal horn of the spinal cord.

Biochemical Functions

The Role of Calcium in Muscle Contraction

Calcium is essential to the transmission of an action potential across the myoneural junction and to normal excitation-contraction of the myofilaments.

The molecular sequence of skeletal muscle contraction depends on calcium at every step:

  1. An action potential travels along the motor nerve and arrives at the motor endplate
  2. The action potential triggers voltage-gated Ca²⁺ channels in the presynaptic terminal, causing Ca²⁺ influx — this Ca²⁺ influx is essential for the release of acetylcholine from synaptic vesicles into the synaptic cleft
  3. ACh crosses the cleft and binds nicotinic receptors on the muscle fibre membrane
  4. The resulting depolarisation propagates along the T-tubules deep into the muscle fibre
  5. Voltage sensors in the T-tubule membrane (dihydropyridine receptors) trigger opening of calcium-release channels (ryanodine receptors) in the sarcoplasmic reticulum
  6. Intracellular Ca²⁺ floods the cytoplasm — this is the calcium signal that initiates contraction
  7. Ca²⁺ binds to troponin C on the thin filament, causing a conformational change that moves tropomyosin away from the actin-binding sites
  8. Myosin heads can now bind actin and execute the power stroke — muscle shortening occurs
  9. Relaxation requires active re-sequestration of Ca²⁺ into the sarcoplasmic reticulum by the Ca²⁺-ATPase pump (SERCA)
  10. Removal of Ca²⁺ from the cytoplasm returns troponin C to its inhibitory conformation — the muscle relaxes

Muscle relaxation is controlled by the balance between fast and slow forms of calcium ATPase in the sarcoplasmic membrane — a process controlled by thyroid hormone T₃. In hypothyroidism, this balance is disturbed, causing the slow muscle relaxation characteristic of hypothyroid myopathy.

In excitation and contraction of skeletal muscle, depolarisation of the T-tubule membrane results in the opening of Ca²⁺ (ionised calcium) release channels in the sarcoplasmic reticulum. Intracellular Ca²⁺ plays a greater role than extracellular Ca²⁺ in this response to neural stimulation. Removal of Ca²⁺ depresses the twitch tension, and there is a dependence of muscle contraction on extracellular calcium concentration. Extracellular calcium concentration or blockade of Ca²⁺ entry can modulate contractile responses.

Calcium and the TrP Mechanism

In the integrated hypothesis of TrP formation (Travell and Simons), the sustained abnormal ACh release at the dysfunctional motor endplate creates a persistent localised calcium release from the sarcoplasmic reticulum. This results in:

  • Sustained sarcomere shortening — the taut band
  • Local energy depletion as SERCA pumps consume ATP attempting to restore Ca²⁺ to the SR
  • Local acidosis and accumulation of metabolic byproducts that sensitise nociceptors

The failure of the calcium pump under conditions of ATP depletion (from ischaemia or metabolic insufficiency) allows Ca²⁺ to remain elevated intracellularly — perpetuating the contraction and the energy crisis in a self-reinforcing loop. This is the molecular basis for the "energy crisis" hypothesis of TrP pathophysiology.

Calcium at the Nociceptor

Beyond the muscle itself, calcium plays roles in pain modulation:

  • Voltage-gated calcium channels in nociceptor cell membranes regulate the threshold for nociceptor firing
  • Ca²⁺ entry into dorsal horn neurons influences the release of excitatory neurotransmitters (glutamate, substance P) and contributes to the phenomenon of central sensitisation
  • Blockade of voltage-gated calcium channels (by gabapentinoids, for example) reduces nociceptor sensitivity — the pharmacological relevance of calcium to pain management

Calcium and Acetylcholine Release

Calcium is essential at the nerve terminal for release of acetylcholine — it is essential for release of acetylcholine at the nerve terminal and for the excitation-contraction mechanism of the actin and myosin filaments. The importance of this to TrP theory is that any state of abnormal calcium channel function at the motor endplate — whether from metabolic insufficiency, focal ischaemia, or neuromuscular junction abnormality — will dysregulate ACh release and potentially initiate the TrP cascade.

A careful search of the literature demonstrates reduced endplate activity that is lower in amplitude and harder to find in the context of low potassium. This indicates reduced release of excessive acetylcholine characteristic of active loci of TrPs — the role of calcium in ACh release makes calcium status relevant to the spontaneous electrical activity at TrP endplates.

Calcium in Bone

Calcium phosphate crystals form bone. Without vitamin C to provide the collagen needed for a firm vessel wall, and without calcium to deposit as the mineral phase, bone mineralisation fails. The roles of vitamin C (see Vitamin C and TrPs) and calcium are complementary in bone maintenance.

Calcium and the Magnesium Relationship

Hypocalcaemia that develops as the result of magnesium deficiency improves only with the administration of magnesium as well as calcium — low serum calcium from this cause will usually return to normal levels within a week after initiating magnesium repletion by oral supplements of antacid or laxative preparations containing magnesium.

The Ca/Mg ratio is important: the optimal Ca/Mg ratio of 2:1 — when not reached — may reduce the efficiency of Mg absorption, accentuate the effects of low oestrogen, and result in lowered Mg entry into bone, with consequent increased risk of osteoporosis. Many older individuals do not achieve the recommended dietary intake of Mg, yet take calcium supplements — in these individuals, the optimal Ca/Mg ratio is not reached.

Insufficiency, Deficiency and Hypocalcaemia

There is no study that has linked an abnormality of calcium metabolism to myofascial pain syndromes. Disturbances in serum calcium levels are extremely uncommon in patients with chronic MPS. Nonetheless, calcium insufficiency affects:

  • Neuromuscular transmission — hypocalcaemia produces increased neuromuscular excitability (tetany, positive Chvostek and Trousseau signs), which is the clinical manifestation of disordered Ca²⁺-dependent modulation of action potential generation
  • Note: Chvostek and Trousseau signs are also signs of magnesium deficiency; hypomagnesaemia often coexists with hypocalcaemia and may be the primary driver
  • Cardiac muscle function — the action potential duration and the plateau phase are calcium-dependent; hypocalcaemia can produce an abnormal electrocardiogram

A low serum total calcium on the blood chemistry profile suggests a calcium deficiency, but for determination of the adequacy of available calcium, a serum ionised calcium measurement is needed. A normal value of total serum calcium does not ensure adequate calcium nutrition — the physiologic effects of calcium depend on the free ionic calcium; the total calcium, much of which is bound to protein, has no direct correlation with the concentration of serum ionised calcium.

Requirements

Optimum calcium intake is estimated to be:

  • 1200–1500 mg/day for adolescents and young adults
  • 1000 mg/day for women between the ages of 25 and 50
  • 1500 mg/day for postmenopausal women taking oestrogen replacement therapy
  • 1500 mg/day for postmenopausal women not taking oestrogen replacement therapy
  • 1000 mg/day for adult men
  • 1500 mg for all persons over the age of 65

Vitamin D is essential for optimal absorption of calcium — it helps absorption of approximately 85% of calcium and 40% of phosphorus via the gut. Calcium intakes up to 2500 mg/day do not result in hypercalcaemia in normal persons.

Adequate absorption of calcium clearly requires sufficient vitamin D, with evidence that fluoride, phosphate, magnesium, and sometimes oestrogen are also important for its absorption and utilisation. Patients with gastrointestinal malabsorption disorders such as Crohn's disease may have low plasma calcium alongside deficiencies of vitamin C, copper, niacin, and zinc — calcium status should always be evaluated in the context of the full nutritional picture.

Sources

Dietary Sources

A simple way to meet dietary calcium needs is to eat at least 2 servings daily from the milk group:

  • 30 g (1.5 oz) brick cheese
  • A serving of yogurt
  • 2 cups of cottage cheese

For those who cannot drink milk because of allergy or lactose intolerance, calcium may be obtained from milk that is predigested by the enzyme lactase. One can avoid increased intake of saturated fat when eating dairy foods by using low-fat or no-fat dairy products.

Other food sources with meaningful calcium content:

  • Green leafy vegetables (broccoli, kale, bok choy)
  • Legumes
  • Canned salmon and sardines (with bones)
  • Oysters, clams
  • Dried figs and soybean curd (tofu)
  • Fortified non-dairy milks

Supplementation

If the patient cannot tolerate dietary sources, a supplement such as calcium phosphate or calcium carbonate should be prescribed — e.g., Os-Cal, from ground oyster shell, which has vitamin D added. Three 250-mg tablets provide 750 mg of elemental calcium and 375 units of vitamin D₂. However, the large 500-mg tablets contain no vitamin D. Calcium supplements have the same bioavailability as calcium supplied by drinking milk.

Important interactions:

  • Calcium supplements should not be taken together with iron supplements — calcium can decrease non-haem iron absorption by 50%, and can also significantly reduce absorption of haem iron (see Iron and TrPs)
  • Take calcium supplements at a different time of day from iron supplements
  • High dietary phosphate (from soft drinks and processed foods) impairs calcium absorption and may contribute to negative calcium balance

The Biology of Starvation: Calcium

Calcium's behaviour in starvation reveals a fundamental conflict between the body's short-term priorities and its long-term structural needs.

The conservation paradox: Unlike the water-soluble vitamins, calcium is not simply excreted in excess and depleted by restriction. Instead, the body maintains serum calcium within an extraordinarily narrow range (2.2–2.6 mmol/L) through a hormonal system — parathyroid hormone (PTH), calcitriol (1,25-dihydroxyvitamin D₃), and calcitonin — that will sacrifice bone to maintain serum calcium. In starvation:

  1. Dietary calcium restriction triggers PTH secretion
  2. PTH stimulates osteoclastic bone resorption — mobilising calcium from bone into blood
  3. Serum calcium remains normal until bone stores are substantially depleted
  4. Meanwhile, calcitriol increases intestinal calcium absorption efficiency to extract maximum calcium from reduced dietary intake

This means that serum calcium will appear normal throughout most of the starvation period despite progressive bone demineralisation. Standard blood tests for calcium are therefore poor indicators of calcium nutritional status in starvation.

Protein-energy starvation accelerates bone loss through additional mechanisms:

  • IGF-1 (which supports bone formation) falls in protein-energy malnutrition
  • Cortisol (elevated in starvation and chronic stress) directly inhibits osteoblast activity and reduces intestinal calcium absorption
  • The chronic inflammatory cytokines that accompany both starvation and chronic pain (TNF-α, IL-1, IL-6) stimulate osteoclast differentiation and bone resorption
  • Vitamin D deficiency — extremely common in populations with limited sunlight exposure and dietary restriction — impairs the intestinal absorption of calcium and the mineralisation of osteoid

The muscle relevance of starvation-related calcium dysregulation: In protein-energy malnutrition, muscle contractile protein is degraded for gluconeogenesis. As myosin and actin are consumed, the structural context for the Ca²⁺-dependent contractile cycle is disrupted. The sarcoplasmic reticulum, which sequesters and releases calcium in each contraction cycle, is itself a membrane-bound structure whose integrity depends on phospholipid synthesis (requiring B vitamins) and protein maintenance. Structural degradation of the SR in starvation impairs calcium handling at the most fundamental level — the SERCA pump cannot sequester Ca²⁺ efficiently when its membrane environment is damaged — creating the conditions for sustained intracellular Ca²⁺ elevation that characterises the TrP active locus.

In the context of myofascial pain, this means that patients with chronic pain who are simultaneously protein-malnourished (common in elderly patients, in patients with anorexia secondary to depression or chronic illness, and in patients on severely restricted diets) are at risk for impaired calcium handling in muscle independent of their serum calcium levels. The treatment implication is that nutritional rehabilitation — adequate protein, calcium, vitamin D, and magnesium — is a prerequisite for effective TrP management in nutritionally depleted patients.

References

  • Travell JG, Simons DG. Myofascial Pain and Dysfunction: The Trigger Point Manual, Volume 1. 2nd ed. Baltimore: Williams & Wilkins; 1999. Chapter 4, Section C.
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