Histol Histopath (1 988) 3: 103-113 and

Fine structure of the receptors at the myotendinous junction of human extraocular muscles

A. Sodii, M. Corsii, M.S. Faussone Pellegrini2and G. Salvii 'Eye Clinic, Chair of Physiopathological Optics and Departrnent of Hurnan Anatorny and Histology, Section of Histology, University of Florence, ltaly

Summary. The myotendinous junction of the human lntroduction extraocular muscles was studied by electron microscopy. Some peculiar receptorial structures have been found in The proprioceptors known as organs were first the majority of the samples examined. These structures identified by Golgi in 1880 in skeletal muscles. They were are very small and consist of 1) the terminal portion of first described at electron microscope level by Merrillees one muscle fibre, 2) the tendon into which it inserts and in 1962 and later by other authors (Schoultz and Swett, y), within the tendon, a rich nerve arborization, whose 1972, 1974; Barker, 1974: Zelena and Soukup, 1977; branches are always very close to the rnuscle component. Soukup and Zelena, 1985; Ovalle and Dow, 1983). In the Only one discontinuous layer, made up of tlat cells. extraocular muscles (EOM) the presence of tendon which lack a basa1 lamina and often show pinocytotic organs was first excluded by Golgi himself, but further vesicles, encapsules every musculo-tendinous complex. investigations (Dogiel. 1906: Loffredo-Sampaolo, 1952; The tendinous component consists of amorphous ground Bonavolonta, 1956, 1958) led to their identification and substance of different electron density. of collagen and description at light microscopy level in several animal elastic fibres and is divided in compartments by ramified species. More recently Ruskell (1979) described the cells, which make an inner capsular-like covering to the ultrastructural features of the EOM tendon organs in the nerve fibres. Three types of afferent nerve endings can rnonkey. These receptors showed some analogies and a be identified. One type is usually more frequent than the few differences when compared with the tendon organs others, possesses a large number of neurotubules and of skeletal muscles. Ttie main differences were found and few mitochondria and is always in the size of the receptors and in their relationships surrounded by a which forms finger-like with the muscle fibres. In fact, in monkey EOM these processes penetrating into the . The second receptors are significantly smaller and connected type is only partially enveloped by the Schwann cell. The only with one or two muscle fibres, and not with ten or axoplasm is devoid of neurotubules and contains few more as in the skeletal muscles. Moreover, in the neurofilarnents, several mitochondria and groups of monkey EOM, Ruskell (1978) also identified other small clear vesicles placed in the areas uncovered by the sensitive nerve endings, that he called ((myotendinous glial sheath. The third one is completely surrounded by cylindersn. the Schwann cell, but is devoid of neurotubules and Without doubt, these types of receptors play an neurofilaments and full of mitochondria. These important role in the oculomotor control. However. in rnorphological features correspond well with the humans there are no detailed investigations about this probable role of these receptorial structures, which is to subject. In fact, using light microscopy, only peculiar ensure very exact and precise ocular movements. structures. considered as receptors, the so called «palisade nerve endingsn, have been described at the Key words: Receptors, Extraocular niuscles. Myotendinous humans myotendinous junction of the extraocular junction. Man muscles, first by Dogiel (1906) and later confirmed by Cooper and Daniel (IY-IY), Richmond et al. (1984) and Steinbach (1986). However these structures have never been investigated using electron microscopy: whereas, Offprint requests to: Prof.ssa M.S. Faussone-Pellegrini, Department using electron microscopy only, Mukuno (1983) reports of Human Anatomy and Histology, Section of Histology, Viale G. the observation of a structure «similar to tendon organs,,, Pieraccini 18, 501 39, ~irenze,ltaly but does not describe it. 104 Receptors of human extraocular muscles

The aini of the present work was to coiifirm, ~ising Using electron microscopy, it is possible to identify electron microscopy. the presence of sensorial structures only one discontinous layer of flat cells very poor in at the human EOM myotendinous junction, anci to organelles, devoid of pinocytotic vesicles and basal decide if they could be related to Golgi tendon organs, to laniina (Figs. 4D, 6) around these complexes. These cells Dogiel's «palisade» structures, to monkey «myotendinous can be considered as constituting something similar to an cylindersn, or even others. outer capsule. The tendinous compartments are delimited by the ramified cells (Figs. 4A, 7-10), which Materials and rnethods overlap and interdigitate each other with their branches (Figs. 7-10), forming one to three layers, sometimes We considered the EOM of six patients who concentric and continuous (Figs. 7, 8), sometimes not underwent enucleation for different ocular pathologies. (Figs. 7, 10, 1 l), and determiningfor everycompartment without any disturbance of the ocular motor system. an inner capsule-like structure. These cells have a dark Three of the enucleated eves suffered from clioroidal nucleus (Fig. 4D) and in the cytoplasm there are some melanomas, two from penetrating injury and one from mitochondria, a small Golgi apparatus, a poorly absolute glaucoma. Age ranged from 36 to 62 years with developed rough (RER)and sinooth (SER)endoplasmic a mean of 52.5 years. reticulum and several filanients (Fig. 9). Sonietinies During surgery the muscle was completely excised pinocytotic vesicles are also present (Figs. 6. 8, 10, 13, about two centimeters from its scleral insertion. 15). Some long, thin ccll processes often envelop a small Immediately after the operation, the specimen was bundle of collagen or elastic fibres (Figs. 4D, Y, 15). Also separated from the sclera and divided into longitudinal the inncr capsular-like cells always lack a basal lamina strips. All samples were immersed in a fixative solution (Figs. 4D, 6-15). of 2% glutaraldehyde in 0. 1 M cncodylate buffer, pH 7.4, The terminal part of the muscle fibre is and remained in this solution for two to sis hours. The comprehensive of its tip (recognizable for its irregular specimens were then washed in the sanle buffer. diluted border, Figs. 1,2,5B)and of a tract of variable extension half and half in a sucrose solution 0.44 M. for twelve ensheathed by a thin collar of in hours or longer. The strips were then postfised in 1% continuity with the tendon. Also this collar can be osmium tetroxide in phosphate buffer 0.1 M, pH 7.4. divided in small compartments by the ramified cells of The samples were dehydrated in acctone at increasing the inner capsule and incompletely deliniited by the cells concentrations and embedded in epoxy resin, using flat of the outer capsule. In the compartments nearest to the capsules in order to allow their orientation. We placed terminal part of the muscle fibre, the nerve fibres and the the muscle strips in such a way to obtain both cross and nerve endings are located. One-three nerve endings can longitudinal sections. Seniithin anci ultrathin sections be contained in every compartment. Close to the niuscle were obtained using the Porter-Bluni MT 1 and LKB- fibre insertion into the tendon it is always possible to find Nova ultramicrotonies. The seniithin sections, 0.5 pm one blood vessel (Figs. 1, 5B). thick. were stained with a water solution of toluidine blue The connective tissue component located between the and sodium borate for light microscopy. Some of the nerve endings and the inner capsular-like cells may have ultrathin sections were stained with uranyl acetate and a peculiar aspect. The amorphous ground substance is lead citrate and the others with uranyl acetate and electron transparent mainly in the subcapsular space concentrated bismuth and finally exaniined and (Figs. 7. 8, 14), whereas it is electron dense close to the photographed under the Siemens Elmiskop 1 A and 102 nerve endings (Figs. 7, 8, 10, 12, 14). The collagen fibres electron microscopes. are grouped in bundles whicli are often few and located near the nerve endings (Figs. 8, 10, 15). These bundles Results are orientated parallel to the long axis of the niusculo- tendinous complex or slightly obliquely. Finally, the Some musculo-tendinous complexes were identified presence of elastic fibres should be noted. composed in the muscle component of the myotendinous junction both of inicrofibrils and amorphous substance.(Figs. 6, Y, of al1 the media1 recti and four lateral recti. 15). Thcse fibres usually run both on the outer and inner With light microscopy, these complexes look like sides of the inner capsular-like cells. cylindrical structures made up of the ending of onc muscle One myelinated nerve fibre can be found near the fibre and the beginning of the corresponding tendinous estremity of the muscle fibre inserting into the tendon collagcn bundle (Figs. 1, 2). The whole structurc sccms to (Figs. 5A. B): its is made up of one layer of be surrounded by one layer of thin capsular cclls (Figs. 1-3, cells ricli in pinocytotic vesicles and surrounded by a 4A,B and C). In cross-section, thc tendinous componcnt basal lamina (Fig. SB). appears to be divided into polygonal compartments by cclls Entering the tendinous compartments, the nerve fibre with deeply stained cytoplasm and well developed loses the covering and its arborizations maintain ramifications (Figs. 3. 4A.B.C and SA). For thesc rcasons the Schwann cell sheath through to their endings. At this these musculo-tendinous complexes can easily be lcvel tlie perineurium cannot be identified yet, and it is identified; in fact. the surrounding muscle fibres end in a partially substituted by other cells, the inner capsular- common tendinous component, without any capsular or like cells. Eacli Schwann cell sheath usually envelopsone tendinous compartment formations. nerve fibre (Figs. 7, 8, 10-13) and exceptionally two Receptors of human extraocular muscles

(Figs. 4D. 6,8,14, 15). A poorly developed RER, rather From one to three short, thin, finger-like cytoplasmic large mitochondria, severa1 lysosomes and some processes of the Schwann cells penetrating into the nerve microtubules are observed in the cytoplasm of the sheath endings were also noted (Figs. 11, 13). cells. The nucleus is spherical or oval, often with a The Schwann cell completely surrounds these nucleolus. The chromatin is distributed in coarse clumps, nerve endings and a basal lamina is always present. mainly in the peripheral area (Figs. 4D, 7,8, 10, 15). These nerve endings have a circular profile, and are Inside the preterminal nerve fibres the mitochondria very rich in neurofilaments and neurotubules. The are few, small and oval-shaped; scant neurotubules and latter are mostly near the , especially in the neurofilaments are also present (Fig. 6). area in correspondence with the Schwann cell At the level of the majority o€ the nerve endings we protrusions (Figs. 12, 13). The gap between the could identify (Figs. 7,8,10-13),the Schwann cell sheath nerve endings and the muscle fibre is sometimes shows a tortuous profile due to deep invaginations and reduced to 750 nm (Figs. 8, 12) and always contains thin evaginations, which partially delineate more thin processes of the inner capsular-like cells. Very electron dense connective tissue (Figs. 8, 10, 12, 14). rarely these nerve endings are found in the compartments

Fig. l. Longitudinal section of a musculo-tendinous complex. The compartment delineated by three layers of flat (inner) capsular-like insertion of the musclefibre into the tendon is clearly recognizable. At cells. x 20,000 this level the border of the muscle fibre has long, thin evaginations Fig. 9. Detail of one contact area between two capsular-like cells (arrows). In the tendinous component, near to the tip of the muscle delimiting a receptorial compartment (upper side). Elastic fibres (E) are fibre, numerous, but not identifiable, cells are located. BV=one blood present near one of these cells, on the sidefacing the inner part of the vessel. The whole complex seems to be delimited by a thin capsule. receptor compartment. The arrow indicates one collagen bundle Semi-thin section, toluidine blue. x 800 enveloped by one capsular-like cell process. x 25,000 Fig. 2. Oblique section of a musculo-tendinous complex. The irregular Fig. 10 Detail of the Figure 7. Two nerve endings (asterisks), partially extremity of the muscle fibre is clearly recognizable. Several cells, encapsulated, placed in the same compartment, but with very dividing the tendinous component in compartments, are located near different size. The Schwann cell surrounding the largest one has an the muscle fibre insertion and form a structure like a cup. The arrow irregular profile (arrows) in correspondence of the neme ending. x indicates one probable outer capsular cell. Semi-thin section, 20,000 toluidine blue. x 800 Fig. 11. One partially encapsulated nerve ending placed in the purely Fig. 3. Oblique section of a musculo-tendinous complex at its purely tendinous area, but not far from the muscle component. The arrow connective level. The asterisk indicates the tendinous component indicates afinger-likeprocess of the Schwann cell penetrating into the divided in compartments by deeply stained and extremely ramified neme ending. x 13,000 cells surrounded by a thin capsule (arrow). This tendinous component Fig. 12. One nerve ending placed very near to the muscle fibre. One continues (upper side) in a thinning tendinous structure, devoid of thin process of a capsular-like cell separates them. The Schwann cell capsule and not divided in compartments. Semithin section, toluidine shows two invaginations (arrows) delineating two moreelectron dense blue. x 800 connective tissue areas. Squares: groups of neurotubules located Fig. 4 A,B,C. and D. Serial cross sections of a musculo-tendinous close to the axolemma facing the Schwann cell invaginations. x complex. One musclefibre (upper side) and its tendon (right side). The 25,000 tendon is divided in large compartments by deeply stained and Fig. 13. One nerve ending of the more frequently found type, rich in ramified cells. With light microscopy (Figs. 4 A-C), this complex is mitochondria, smooth endoplasrnic reticulum cisternae (SER), apparently surrounded by a continuous capsule. By electron neurofilaments and neurotubules (squares). In this section, the microscopy (Fig. 4 D), only one incomplete layer of thin processes Schwann cell shows three finger-like processes (arrows) entering the (arrows) of capsular-like cells can be seen. In the compartments nerve ending. x 20,000 nearest to the striated muscle fibre (asterisk) the nerve fibres (NF) and Fig. 14. Two nerve endings (very rarely found), both completely neme endings (NE) are contained. A-C: semithin sections; toluidine enclosed in thesame Schwann cell.'rhesenerveendings are devoid of blue, x 800. D: x 6,250 neurotubules and neurofilaments and full of mitochondria. On the Fig. 5 A and B. Slightly oblique sections of two different musculo- lower right side the muscle fibre. x 20,000 tendinous complexes at the level where the myelinated nervefibre(NF) Fig. 15. Detail of the Figure4D. One nemeending (rarelyfound) looking enters the receptor body. BVkone blood vessel. The terminal border similar to those of typical Golgi tendon organs. In fact, this nerve of the musclefibre is tortuous on its insertion in the tendon (tendon). A: ending has a tortuous profile and is partially devoid of glial sheath semithin section, toluidine blue. x 800 B: x 7,500 (arrows). Several rnitochondria are present. Neurofilaments are not Fig. 6. On the lower leftside: one thin process of an outercapsular-like numerous. A peculiar feature of this nerve ending is the presence of cell surrounding a musculo-tendinous complex. One amyelinated groups of clear vesicles (about 50 nm in diameter) in the axoplasm nervefibre is present, embedded in aconnectivetissue rich in collagen beneath the areas devoid of glial sheath. E=elastic fibres enveloped and elastic fibres. This nervefibre is to be considered as a preterminal by thin processes of the capsular-like cells. x 20,000 one, since the neurotubules and neurofilaments are not numerous, the Fig. 16. One nerve ending full of mitochondria and roundclearvesicles mitochondria are small and few and the Schwann cell has a regular (50-80 nm in diameter) nearonestriated rnusclefibreat thelevel where outline (compare with figures 8,9-13). x 20,000 this fibre inserts in a capsulated tendinous cornponent, forming with Fig. 7. A musculo-tendinouscomplex sectioned at the level wheretwo this a musculo-tendinous complex (the same complex where the receptorial compartments are clearly distinguishable. The muscle nerve ending of the Figure 13 is located). The muscle fibre, in fibre (not visible in the micrograph) is not far from thern. One of these correspondence to this nerve ending, forms tubular invaginations compartments (upper side, asterisk) is completely encapsulated, the covered by one basal lamina identical to those of a typical subneural other is partially encapsulated. Both contain two nerve fibres of a very apparatus. The neme ending, in correspondence to this subneural different size. x 7,500 apparatus, is devoid of a glial sheath and is apposed to this apparatus, Fig. 8. Three nerve endings, each one completely surrounded by the like nerve endings of motor end plates do. This type of nerve ending, Schwann cell, placed near the muscle fibre (upper left side) inside a therefore, has been considered as an efferent one. x 25,000 Receptors of human extraocular muscles 107 Receptors of human extraocular muscles 108 Receptors of human extraocular muscles 109 Receptors of human extraocular muscles Receptors of human extraocular muscles

of the purely tendinous area. completely or incompletely In the myotendinous junction of the extraocular enveloped by the inner capsular-like cells, but never far muscles of mammals the motor innervation is usuallv from the muscle fibre. considered absent, whereas various types of sensory Very occasionally, it was possible to observe other nerve endings have been described: tendon organs, types of nerve endings. One type (Figs. 4D, 15) is never myotendinous cylinders, palisade nerve endings (Dogiel, located in the compartments nearest to the muscle fibre 1906: Cooper and Daniel. 1949; Loffredo-Sampaolo, and is only partially enveloped by the Schwann cell 1952; Bonavolonta, 1956. 1958; Ruskell, 1978, 1979: sheath, but a basal lamina is always present. The Mukuno, 1983; Richmondet al., 1984; Steinbach, 1986). axoplasm is devoid of neurotubules and contains only a One type of the nerve endings we observed looks few neurofilaments and a certain number of large, oval- identical to those of the end plates. However, we found shaped mitochondria. Groups of clear vesicles of about this type of nerve ending very rarely and, therefore, it 50 nm in diameter are seen in the opposite areas cannot be considered as typical of this area. Al1 other uncovered by the Schwann cell sheath (Fig. 15). The nerve endings we observed, on the contrary, are very shape of these nerve endings can be oval, or elongated or numerous and are not similar to commonly described tortuous. efferent nerve endings. Therefore, this seems to be a Another type of nerve ending (Fig. 14) has a circular valid reason to exclude the possibility that we observed profile and is completely surrounded by the Schwann cell only motor terminals (except those afore-mentioned) sheath, as those more frequently found, but it is devoid and. on the other hand, also valid to consider the of neurotubules and neurofilaments and full of large, musculo-tendinous complexes as receptorial structures. round mitochondria. This type of nerve ending is found However, as above mentioned. the nerve endings we only near the muscle fibre, separated from this by an described are different from those previously described inner capsular-like cell. Rarely, it is possible to find in this area in monkey EOM (the only ultrastructural another type of nerve ending (Fig. 16), located only in data we have for a comparison). corres~ondencewith the end of the muscle fibre. This The unmyelinated nerve structures we found inside nerve ending is very different from the previous ones. It every musculo-tendinous complex are very numerous is large. looks like a flat flask and is strictly apposed to the and share different features; these facts should indicate muscle fibre, with a gap of 50-100 nm only. At this that sorne (few) of them might be preterminals and come contact area, the Schwann cell sheath is interrupted and (many) terrninals. At least two types among these nerve the nerve ending is separated from the muscle fibre by structures have to be considered as nerve terminals (see only one basal lamina, which also penetrates in finger- Figs. 14 and 15) for their unequivocal features: one being like invaginations of the sarcolemma constituting a partially devoid of the Schwann cell sheath and typical subneural apparatus. The axoplasmic portion of containing mainly vesicles and mitochondria (the this nerve ending contains clusters of large, oval possibility that this type is efferent is excluded, see later mitochondria. dista1 to the contact area. anda lot of clear in the discussion); the other mostly containing vesicles of abbut 50-80 nm in diamete; in the proxirnal numerous, large mitochondria and lacking neurotubules one. We consider this nerve ending type as an efferent and neurofilaments (as commonly described for a large one because of its similarity to those of the end plates. number of sensitive nerve endings). All other nerve structures, containing neurotubules and neurofilaments Discussion and completely surrounded by the Schwann cell sheath, have to be examined more carefully in order to In the myotendinous junctions of the human EOM we distinguish between preterminals and terminals. Only were able to identify numerous small musculo-tendinous few nerve structures (see Fig. 6), having normal features complexes. These complexes possess an architecture and and looking identical to commonly considered ultrastructural features never described before. amyelinated nerve fibres, can be identified as These musculo-tendinous complexes are preterminals. The others which, on the contrary, are incompletely capsulated and are made up of three very numerous (as we can expect if these represent the components: 1) a muscular component, consisting in the terminal arborization of a receptorial nerve supply) terminal part of one striated muscle fibre ensheated by a share noticeable structural differences in respect to nerve capsulated tendinous collar; 2) a connective tissue fibres (preterminals included). In fact, the component, the tendon in which the muscle fibre inserts, and neurotubule content is enormous and the mitochondria divided in compartments by ramified cells; 3) a nerve are numerous, large, round and peripherally located. component, made up of a rich nerve supply, whose These features have been described also for other arborization is located inside the tendinous receptorial structures and considered characteristic of compartments. their terminal sensitive supply (Watanabe and Yamada. The nerve supply needs to be discussed in order to 1985). Moreover, the Schwann cell sheath at the leve1 of identify the nature of these musculo-tendinous the nerve structures we considered as nerve endin~sC complexes, since the nerve fibres share peculiar shows an irregular outline: on the outer side forms deep ultrastructural features which are somewhat different to invaginations and thin evaginations branching electron those already described in this area (Ruskell, 1978, dense connective tissue material; on the inner side 1979). penetrates into the axoplasm invaginating the axolemma Receptors of human extraocular muscles

by means of finger-like protrusions. These features have morphological features we considered them as efferent. not been described elsewhere (and will be discussed Moreover, this type of nerve ending was very rarely later) and are not characteristic for preterminal (neither found and, therefore. does not characterize the musculo- motor, nor sensitive) nerve fibres. tendinous complexes of humans. For al1 the above mentioned considerations we are Despite the afore-mentioned differences, the reasonably convinced of having found sensorial structures we found show, even if only a few, important structures with peculiar ultrastructural features. It needs similarities both with the tendon organs and the now to be seen if these musculo-tendinous complexes can myotendinous cylinders. As in the tendon organs, we can be identified as tendon organs (until now not definitely observe a tendinous component divided into severa1 confirmed in human EOM), as myotendinous cylinders compartments by cells which should correspond to those (never mentioned in humans), as palisade nerve endings of the inner capsule. But really. in the structures we (the only ones certainly present in human EOM,) or as identified, it is not possible to distinguish between two other rece~torialstructures unknown until now. different types of capsular cells, an outer and an inner Their ultrastructural morphology is in some respect one, because al1 the capsular cells we observed have different from that observed in monkey EOM Golgi intermediate features between those of the outer and tendon organs and myotendinous cylinders and from those of the inner capsule of the tendon organs. In fact. Golgi tendon organs described in skeletal muscles. In al1 the capsular cells we identified lack a basa1 lamina fact, in comparison with the Golgi tendon organs of both (like the inner capsule cells) (Schoultz and Swett, 1972; skeletal muscles (Schoultz and Swett, 1972, 1974; Zelena and Soukup, 1977; Ovalle and Dow, 1983), but Barker, 1974; Zelena and Soukup, 1977; Ovalle and very often show pinocytotic vesicles (like the outer Dow, 1983; Soukup and Zeleni, 1985) and EOM capsule cells) (Ovalle and Dow. 1983). The characteristics (Ruskell. 1979) the structures we described are smaller of the connective tissue interposed between the nerve (about 20pm in diameter), devoid of a typical outer ending and the inner capsule are also similar to tendon capsule (we ohserved only one discontinous layer of flat organs (Schoultz and Swett. 1972; Soukup and Zelena, cells), always found very near to the muscle component 1985). The similarities with the myotendinous cylinders (even more than the tendon organs of the monkey mostly consist in the small size, being both receptor types EOM), with a gap from the sarcolemma to the capsular usually made up of only one muscle fibre and its tendon, cell sometimes reduced to 750 nm. Moreover, only one and in the absence of a consistent outer capsule. ln both type of nerve ending (see Fig. 15), very rarely found, these types of receptors the nerve endings are mostly looks similar to those commonly described in tendon located near the muscle fibre, but at variance with the organs. This type of nerve ending possesses some areas myotendinous cylinders, we found them not only in uncovered by the Schwann cell sheath. contacts with the correspondence of the tip of the muscle fibre, but also connective tissue, has a tortuous profile and is poor in around its terminal part (the portion ensheathed in the neurofilaments and neurotubules. At variance with the capsulated tendinous collar). nerve endings of the tendon organs, but similarly to those In conclusion, the human EOM, in the area of the of Ruffini and Pacini corpuscles (Halata et al., 1985), this myotendinous junction. show some structures that seem type of nerve ending contains groups of clear vesicles more similar to Golgi tendon organs than to clustered in the areas uncovered by the Schwann cell myotendinous cylinders, but with important siructural sheath. These nerve endings, because of some similarities differences in comparison to both. with both those of tendon organs and Ruffini and Pacini In humans, Dogiel (1906) described in this region corpuscles, and since they were never found close to the peculiar sensorial structures, which he called (cpalisade muscle fibre. have been considered as sensorial ones. endingsn because of the typical nerve arborization. The The others, the most frequently observed, are very presence of this type of receptor has been confirmed different form those of tendon organs, since the glial recently by others, using light microscopy (Cooper and covering is always complete and, therefore. the nerve Daniel, 1949; Richmond et al.. 1984; Steinbach, 1986). ending never directly contacts the connective tissue Ruskell (1978. 1979) suggested that the «palisade» component. Moreover, most of these nerve endings are endings have an ~iltrastructural counterpart in the full of neurotubules and neurofilaments and show large ((myotendinouscylindersn he found in monkey EOM. mitochondria peripherally located. We must finally The nerve endings we described in human EOM show remember the peculiar behaviour of the Schwann cell an ultrastructure different in many respects to that of which has an irregular outline (both on the outer and myotendinous cylinders. We cannot, however, exclude inner side of these nerve endings). the possibility that the nerve endings we described at The structures we described also differ from the electron microscopy correspond to Dogiel's «palisade» myotendinous cylinders (Ruskell, 1978) of the monkey endings. EOM. In fact, the nerve endings we could observe closer It is difficult to relate the light microscopy to the muscle fibre and containing clear vesicles, at observations to our findings, obtained examining semi- variance with those described in the mvotendinous thin sections (where nerve endings cannot be identified) cylinders, are always in close contact with the muscle and ultrathin sections (where it is difficult to understand fibre, with a gap of 50-100 nm, and in presence of exactly the three dimensional organization of the whole a typical subneural apparatus. Because of these receptor). However. considering the similarities in the Receptors of human extraocular muscles

location (myotendinous junction) and size (one muscle ending has previously been reported also in the monkey fibre and its tendon) of the receptor and the same (Ruskell, 1978, 1979). This fact seems therefore to be relations the nerve supply seems to have with the striated related to the specific function of these receptors. Also muscle fibre (at the tip and along the terminal part of it), the smaller size of these receptors. compared to those of we think we can reasonably identify the Dogiel's the , as shown in both human and monkey <

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