Issue link: https://www.massageandbodyworkdigital.com/i/1476304
In the late 1980s, Jaap van der Wal reported about the rich presence and complex arrangement of sensory nerve endings in the muscular fascia of rats, 3 but this was largely ignored in mainstream medicine, at least for the subsequent two decades. What really changed the general medical perspective in a profound manner was the first international Fascia Research Congress, held at the Harvard Medical School Conference Center in Boston in 2007. During the congress, three teams from different universities and countries reported their independent findings of a rich presence of sensory nerves in fascial tissues. 4 Following that event, many studies were published about fascial innervation, suggesting that fascia plays an important role in the perception and internal representation of our own bodies. More recently, the German neurophysiologist Martin Grunwald estimated there were 100 million nerve endings in the body-wide fascial net. 5 This number related only to the mass of dense fibrous connective tissues, which was estimated at 5 kg for an average adult human body. In a subsequent calculation, a new estimate was tabulated using a more functional definition of "the fascial system," one that was based on recommendations from the Nomenclature Committee of the Fascia Research Society. That definition broadened to include all fibrous collagenous connective tissues, including intramuscular connective tissues, joint and organ capsules, ligaments, aponeuroses, tendons, loose connective tissues, epineural envelopes, as well as the intramuscular septi. 6 With this revised definition, the mass of fibrous connective tissue in humans increased to 12.5 kg or 17 percent of the total body weight. Subsequently, the estimated number of nerve endings in the body-wide fascial system increased to 250 million. 7 Compared with the estimated 200 million nerve endings in the skin 8 (which based on its richness and complexity is commonly described as "a sensory organ"), this new calculation suggests the human fascial network constitutes, in fact, our richest sensory organ. DIFFERENT TYPES OF FASCIAL SENSORY RECEPTORS Based on this recognition of the importance of the fascial system as a sensory organ, practitioners are increasingly interested in understanding which exact sensory qualities are served by this rich innervation. Image 1 demonstrates the most important components. The highest proportion is constituted by interstitial receptors (which includes all neurons terminating in free nerve endings, including C-fibers and A-delta fibers). These neurons are thin and sensitive to mechanical stimulation. If the extracellular matrix is chemically altered, or if the mechanical stimulation is too strong, many of these receptors could also become nociceptors (signaling potential tissue damage to the central nervous system). Interestingly, the mechanical activation threshold of the fascial free nerve endings is two times lower than in skin and muscle. 9 In addition, it was demonstrated that the free nerve endings in the human thoracolumbar fascia are significantly more sensitive to chemical irritation compared to the underlying muscles, and that they tend to maintain a long-lasting hypersensitivity. 10 For manual therapy practitioners, it is important to realize that not all interstitial receptors can be classified as nociceptive. For example, some of them are sensory 42 m a s s a g e & b o d y wo r k s e p te m b e r/o c to b e r 2 0 2 2 A detailed analysis of the individual neurons in the nerve supply of a lower leg portion in a cat reveals quantitative proportions shown above. While a small portion of the interstitial neurons may terminate inside bone, the remaining neurons can all be considered to terminate in fascial tissues. Even the sensory devices called muscle spindles are nestled within fibrous collagenous intramuscular tissues. Interstitial neurons terminate in free nerve endings. Some, but not all, of these interstitial nerves clearly have an interoceptive or nociceptive function. Note that a vast number of nerves are devoted to the fine-tuning of nutrient delivery via the vascular supply, which is regulated by the sympathetic nervous system (yellow portion). Data from J. H. Michell and R. F. Schmidt, Handbook of Physiology, 1977; illustration courtesy of fascialnet.com. 1