While Golgi’s laboratory was emerging internationally in neuroanatomical and neurohistological studies and Golgi had the black reaction in his hands, he did not neglect to turn his attention to other fields of investigation as well. His intuition made him imagine that they could obtain important results.

The material basis of tendon sensitivity that underlay motor and proprioceptive mechanisms was a topic which generated conflicting views. Various physiological experiments had shown that anatomical knowledge of the relationships between nerves and tendons was incomplete.

Before Golgi, two researchers had tried to identify the material substrate of tendon sensitivity. Alexander Rollet (1834–1903), an Austrian physiologist, had studied the tendons of amphibians and found a plexus of myelinated fibers resulting from the division of the nerve in charge of it. From this plexus other fibers would originate and would further be divided in single “point” or with a final expansion in the end (Rollet 1876). Carl Sachs (1853–1878), who worked in the physiological laboratory of the University of Heidelberg, investigated the presence of nerve endings in the tendons of reptiles, amphibians, birds and mammals. He found constant endings in the tendons of amphibians and lizards where the nerve fiber eventually resolved into a brush of pale arborizations. In frogs he also observed a club-like termination. On the other hand, according to Sachs, such figures were missing in mammals where nerves would merge in the substance of the tendon, without specific morphological characteristics (Sachs 1875).

In Golgi’s research program was the idea that since there were nerve fibers within the tendons, following their course would lead to the discovery of their termination patterns.

Golgi’s originality was evidenced by his applying established histological techniques in a new context for this type of investigation.

The focus of his research was on human tendons. However, he also extended his “observations to several other mammals (rabbit, dog, cat, mouse), to some birds (sparrow, finch, swallow) and to amphibians (frog) and reptiles (lizard)” (Golgi 1880). Golgi declared that he was indeed guided - somewhat phylogenetically - to his results obtained in human anatomy by the observations he had made first in lizards, then in birds and finally in rabbits. The observation of nerve endings in reptiles, constituted almost a kind of cognitive map that made him clearly perceive the fundamental structure that he would later characterize also in mammals and in humans (Golgi 1880) (Fig. 2).

Pencil sketches (around 1878–1879) by Camillo Golgi of observations on sensory corpuscles in tendons. University of Pavia, Museum for the History of the University, Veratti Archives

Golgi used several methods of histological procedures. After he carefully succeeded in separating the tendon from the muscle, he subjected the tendon to chemical treatment with solutions of certain acids (e.g., acetic, hydrochloric, or nitric acid) that had the effect of removing the natural opacity of the tendon that could hinder the observation of nerve fibers. To make the treatment more obvious, the tendon was also treated with osmic acid solutions that caused blackening of the nerve fibers themselves. These procedures also allowed the preparations to be preserved in glycerin. This first phase was followed by Golgi’s immersion in arsenic acid followed by a passage in a solution of gold chloride and potassium.

At the end of this procedure, microscopic observation made to highlight spindle formations possible that at one end appeared to stand near the tendon–muscle junction, almost in relation to muscle fibers, while at the other end they gradually blended with tendon bundles. Golgi discovered that each of these formations received from one to four myelinated nerve fibers, which lost their myelin sheath as they entered the bundle, within which they divided dichotically, terminating in a large number of ending arborizations that had the appearance of reticular intertwines (Fig. 3).

Golgi’s muscle-tendon organs. Originally published in Golgi in 1880 and reprinted in the Opera Omnia (Golgi 1903). University of Pavia, Camillo Golgi Museum

He also observed, already at the first stage of the procedure, a second category of formations in the superficial thickness of the tendon, near the muscle (body like clubs and Pacini body-like formations) (Fig. 4).

Pencil sketch (around 1878) of Pacini body-like tendon corpuscles (Golgi Mazzoni corpuscles). University of Pavia, Museum for the History of the University, Veratti Archives

Considering the position of the first corpuscles, which he called muscle-tendinous organs, Golgi concluded that they - given their position - had a “harmonizing function with that of the muscles” and, precisely, that they could “be organs of a special muscular sensitivity,” the “gauges of muscle tension” (Golgi 1880).

“As for the second type of terminal nervous apparatuses”, Golgi observed, “their more superficial situation” and their “analogy with other terminal organs of known function” led to the idea that they were “tactile bodies” (Golgi 1880), that is, those responsible for coding sensitivity to pressure (baresthesia).

Thus Golgi, on the basis of shape, position and characteristics alone, had precisely and correctly identified the function of the sensory corpuscles had discovered. Golgi found it astonishing that “despite of the diligence with which anatomical research were conducted in the modern epoch” this discovery “had hitherto remained unnoticed by anatomists”: peculiarities of organization “so pronounced, so easy to prove and […] of such relevant physiological signification,” such as those underlying “the relation of nerves to tendons” (Golgi 1880) (Fig. 5a and b).

(a, b) Golgi muscle-tendon organs. Photomicrograph (20x) from original preparations from the histotheca of Golgi’s laboratory (presumably dated around 1878–1880). University of Pavia, Camillo Golgi Museum

Golgi presented his research in 1878 with a preliminary note (Golgi 1878a), and soon afterwards with another brief communication to the seventh extraordinary meeting of the Italian Society of Natural Sciences (Golgi 1878b). To conclude this type of research he published a detailed work in the proceedings of the Royal Academy of Sciences of Turin (Golgi 1880).

Preliminary notes were immediately reviewed in the influential German medical journal Centralblatt für die medicinischen Wissenschaften (1876) and taken up by the histologist Ludwig Thanhoffer (1843–1909) - professor in Budapest - in a communication to the Hungarian Academy of Sciences (Thanhoffer 1882). The discovery was soon described even in elementary histology textbooks (Klein 1885, pp. 209–210).

Golgi’s name, as the eponym for the muscle-tendon, organs entered the scientific literature almost immediately thanks to the work by Vittorio Marchi, a young student who practiced in the Ophthalmology clinic of the University of Pavia, and who later became internationally famous for developing a method of staining central nerve pathways. Marchi succeeded in demonstrating the existence of Golgi muscle-tendon organs in the tendons of the striated muscles of the eye (where Golgi had failed to find them), calling them Golgi bodies (Marchi 1882). Alfonso Cattaneo (second half of the 19th century) also studied these corpuscles in a very detailed investigation under pathological conditions. After cutting spinal nerves he observed a degeneration of peripheral nerve fibers and some wrinkling of the fusiform structure of the organ (Cattaneo 1888a, b).

The naturalist Sergio Pansini named the muscle-tendon organ with the term Golgi corpuscles in a work of his (Pansini 1888).

In 1889, the authoritative Handbuch der Gewebelehre des Menschen by Albert von Kölliker (1817–1905) called the muscle-tendon organs as Golgi’sche Sehnenspindeln (Golgi’s tendon spindles), in honor of their discoverer (Kölliker 1889) but also Golgi’s organs, a term also taken up in an important Italian treatise on histology (Fusari and Monti 1891); the eponym Golgi’s muscle-tendon organs was also used (Ciaccio 1889; Ruffini 1894). The name Golgi tendon organ(s) came into common use during the twentieth century (Moore 1984; Jami 1992).

A real advance in the analysis of nerve endings in tendons was achieved with the study by Vittorio Mazzoni (1864–1897) devoted to the formations already observed by Golgi who had described them as “balls, clubs and different forms of Pacinian corpuscles” (Golgi 1880; Mazzoni 1890; Arieti 2008). This study, which defined more precisely the morphology of this second category of tendon nerve endings, was taken up again by Angelo Ruffini (1864–1929) who proposed to call these formations Golgi-Mazzoni corpuscles, from the name of their discoverer and of the person who had described their morphological aspect with precision (Ruffini 1894).

During the twentieth century, studies on the two sensitive corpuscles discovered by Golgi flourished with the publication of hundreds of articles, especially on the Golgi tendon organ. Several researches have studied Golgi tendon organ structure (Nitatori 1988; Rein et al. 2023), its physiology (Houk and Simon 1967; Kistemaker et al. 2013; Oliver et al. 2021) and functions in various conditions of mechanical stress (Gregory et al. 2003; Qiu and Kang 2017), its role in sports activity (Moore 2007) and its involvement in pathological conditions (Burne and Lippold 1996). More limited research has been done on the Golgi-Mazzoni corpuscles which are slowly adapting receptors sensitive to particular types of compression in the joint capsule (Grigg et al. 1982; Chouchkov and Maslarski 2019).