| Objective:The structures in sub-occipital region are complicated. Previous studies showed that there were connective fibers between muscles in the deep layer of the sub-occipital region and the ligament and the spinal dura mater. These connective fibers were named “myodural bridge”. In the atlanto-occipital interspace, the rectus capitis posterior minor(RCPmi) was connected to the spinal dura mater by many fine connective tissue bands from the “myodural bridge” fibers of the RCPmi. In the atlanto-axial interspace, the rectus capitis major(RCPma), the oblique capitis inferior(OCI), and the nuchal ligament were connected to the spinal dura mater by many fine connective tissue bands of the “myodural bridge” fibers of these structures. Our group found that current speed and flow of the cerebrospinal fluid above the atlas level changed after head movement based MR research results. So we proposed that when the head is rotating and extending, the deep sub-occipital muscles contract and pull the dura mater. This pulling affects the volume of the subarachnoid space in the upper cervical portion via the “myodural bridge”. Thus, head and neck movements are thought to likely to contribute to the circulation of cerebral spinal fluid. Cerebrospinal fluid circulation is known as the human body third cycle. The cerebrospinal fluid which is produced by the choroid plexus of each ventricle flow into the ventricles and the subarachnoid space, and ultimately it return to the circulatory system through the venous system. There is no smooth muscle in the membranes of brain and spinal cord, therefore, it is a difficult problem about the dynamic mechanism of cerebrospinal fluid circulation in anatomy. The head and neck movements are likely to contribute to the circulation of cerebral spinal fluid via the “myodural bridge”. Firstly, the strength of the fibers of the “myodural bridge” will play a magnitude role. In the past, there were few studies on the fiber properties of the “myodural bridge”, and the results were all obtained by gross anatomical observation. Histological methods were used in this research to further analysis the fiber properties of the “myodural bridge”. This part of the results can be used to assess the size of the traction force produced by the contraction of the fiber of the “myodural bridge”. If the “myodural bridge” has an important physiological function, it should aslo be exist in other mammals. Useless it would gradually degenerate during the evolution of organisms. So far, there has been no studies focus on the existence of the “myodural bridge” in other mammals nor not. To determine the existence of the “myodural bridge” in other mammals, five different mammalian orders were examined in this study, represented by 4 Neophocaena phocaenoides, 6 dogs, 6 cat, 6 rabbit, 4 macaca mulatta, 6 rat and 6 cavy. Based on the above two aspects of researches, this paper will provide some morphological evidence for the function of the“myodural bridge” on the cerebrospinal fluid circulation.Materials and methods:35 gross human specimen of head and neck fixed in formalin. Twenty of the specimens were dissected. Ten specimens were sliced by P45 sheet plastination technique for the purpose of this study. The course and the connection form with the spinal dura mater were observed about the “myodural bridge” fibers orgin from the ligamentum nuchae. Five specimens were used for histological observation. The course and the connection form with the spinal dura mater were observed about the “myodural bridge” fibers orgin from the sub-occipital muscles. Special staining methods, polarized light microscopy and immunohistochemical staining were used to define the property of the fibers. Five different mammalian orders were examined in this study, represented by 4 Neophocaena phocaenoides, 6 dogs, 6 cat, 6 rabbit, 4 macaca mulatta, 6 rat and 6 cavy. To inversitgate the exsitence of the “myodural bridge” in other mammals with gross anatomy and the histological technique were used in this study.Results:1. The vertebrodural ligament:In each of the thirty-five head-neck specimens observed, the existence of a fibrous connection was found between the posterior aspect of the dura mater and the posterior wall of the spinal canal from the atlas to the axis. With further examinations, it was demonstrated that the complex of dense connective tissue completed these connections. According to its origin and termination, it was termed the vertebrodural ligament(VDL). The VDL is a dense connective tissue complex extending from the posterior arch of the atlas, the atlantoaxial interspace and the lamina of the axis to the dura matter. Because its origin was broad from the atlas to the axis, the VDL is mainly subdivided into three parts as follows: The superior portion was identified as the atlantal part(V1). It originated from the median area of inferior border of the anterior aspect of the posterior arch of the atlas.The middle portion was identified as the atlantoaxial interspace’s middle part(V2). This portion originated from the “myodural bridge” into the atlantoaxial interspace. The inferior portion we called the axial part(V3). It originated from the midline of the superior part of anterior aspect of the lamina of the axis and adhered to the lamina and became the connection between the axis and the dura mater2. The course and the connection form with the spinal dura mater were observed about the “myodural bridge” fibers orgin from the ligamentum nuchae.In each of the 29 NLs, a dense fibrous band was clearly identified arising from the tissue of the posterior border of NL projecting anteriorly and superiorly to enter the atlanto-axial interspace. It then traversed through the interspace and attached to the posterior aspect of the cervical dura mater between the first and second cervical vertebrae. This fibrous band within NL had never been named before, so we referred to it as to be named ligament(TBNL). As a dense part of NL, the TBNL was formed by either arcuate or radiated fibers. In 22 of 29 specimens the arcuate fibers of TBNL arose from the lower part of the posterior border of NL below the level of the spinal process of the vertebra C3. The arcuate fibers then ran anterosuperiorly crossing over the spinal process of axis to continue into the atlantoaxial interspace. Unlike the arcuate fibers, the radiated fibers of TBNL in 7 of 29 specimens arose from the upper part of the posterior border of NL just opposite to the spinal process of the vertebrae C2 and ran anteriorly and straight within NL inserting into the atlantoaxial interspace.3. The course and the connection form with the spinal dura mater were observed about the “myodural bridge” fibers orgin from the sub-occipital muscles.HE stained sections of 5 human sub-occipital specimens showed clearly the structures such as RCPmi, RCPma, OCI, atlas, axis, the spinal dural mater and so on. RCPmi located above the atlas, anteriorly to the RCPma. OCI located between the atlas and axis, anteriorly to the RCPma. The spinal dural mater located anteriorly to the atlas and axis.HE staining showed that, in the atlanto-occipital interspace, the “myodural bridge” fibers orgin from the ventral part of RCPmi go down and ventrally to integrate into or through the posterior atlanto-occipital membrane(PAOM). Then these fibers finally connect with the spinal dural mater indirectly or directly. The “myodural bridge” fibers orgin from the dorsal part of RCPmi go down and then fuse with the “myodural bridge” fibers orgin from RCPma. These fibers together connect with the VDL(V1) or continue with VDL(V2).In the atlanto-axis space, the “myodural bridge” fibers from of RCPma go down and connect with VDL(V1) or continue with VDL(V2). The “myodural bridge” fibers form of OCI go down and fuse with the “myodural bridge” fibers form RCPma. These fibers together continue with VDL(V2) finnally.4. The fiber property of the “myodural bridge”Four fibers stained sections of 5 human sub-occipital specimens could show reticular fiber, collagen fiber, elastic fiber and muscle fiber together. The color of collagen fibers is green. In the sections, the “myodural brige” fibers of RCPmi, RCPma, OCI showed green fiber, so the fiber properties of “myodural bridge” is collagen fiber.Sirius red stain sections of 5 human sub-occipital specimens were observation under polarized light microscope. The results showed that the collagen fibers of the “myodural bridge” is the red and yellow collagen I fiber. And this result was confirmed by immunoenzyme.5. The existence of the “myodural bridge” in other mammalsThe gross dissection results showed that the RCDmi(rectus capitis dorsal minor) located in the profound layer of sub-occipital region between the occipital bone and atlas in 4 dogs, 4 cats, 4 rabbits, 2 macaca mulattas, 4 rats and 4 cavies. PAOM cover the space between the foramen magnum and the atlas in these mammals. There was a dense connective tissue attached between the lower edge of RCDmi and PAOM. There was a dense fiber connecting PAOM and the spinal dural mater in the middle. In midline, the connection between PAOM and the spinal dural mater was more tightly. The epidural space still can be seen in other part. While, Neophocaena phocaenoides was different to other mammals, there was no PAOM between the occipital bone and the atlas. The RCDmi of Neophocaena phocaenoides connected with the spinal dural mater directly by penetrating into the atlanto-occipital space.The P45 plastinated sheets of 2 macaca mulattas and 2 dogs showed that a bundle of muscular fibers which protruded from the RCDmi, entering the atlanto-occipital interspaces, finally terminating at the posterior aspect of the PAOM. And the PAOM and the spinal dura mater were connected with each other tightly. Additionally, in 2 Neophocaena phocaenoides, a bundle of relatively strong muscular fibers was discovered protruding from the RCDmi and penetrating atlanto-axial interspace, connecting with the spinal dural mater directly.The HE stain results showed the “myodural bridge” fibers of RCDmi in 2 cats, 2 rabbits, 2 rats and 2 cavies go down and ventrally, and finally connect with PAOM. And the PAO membrane and the spinal dura mater were connected with each other tightly in the middle sections.Conclusion:1. Fibers origins of the “myodural bridge” are wideIn the atlanto-occipital interspace, the “myodural bridge” fibers orgin from RCDmi. In the atlanto-axial interspace, the “myodural bridge” fibers orgins from RCDmi, RCDma, OCI and ligamentum nuchae.2. The fiber property of the “myodural bridge”The fiber property of the “myodural bridge” is collagen I fiber. The tendon is mainly composed of type I collagen fibers. So it can be inferred that the histological properties of the “myodural bridge” is tendon, and its tensile strength, its tensile and ductility are all strong.3. The fiber course and connective form with the spinal dural mater were all complexIn the atlanto-occipital interspace, the”myodural bridge” fibers orgin from the ventral part of RCPmi go down and ventrally to integrate into or through PAOM. Then these fibers finally connect with the spinal dural mater indirectly or directly. In the atlanto-axial interspace, the “myodural bridge” fibers from RCDmi, RCDma, OCI and ligamentum nuchae converged together and finally connected with the spinal dural mater though the VDL.4. The “myodural bridge” is a universal, normal anatomical structure in mammalsThe presence of the “myodural bridge” in five mammals selected in this study could be a evidence that the possibility of the “myodural bridge” is a universal, normal anatomical structure in mammals. It also could be evidence that the “myodural bridge” is a highly conservative and important structure. 5. A hypothesis was firstly proposed by our team that the "myodural bridge" might be a source of power for the cerebrospinal fluid circulation.In summary, human specimens and other mammalian specimens were used in this study to reveal the existence of the “myodural bridge”, the fiber course, the fiber origin and the fiber property of the “myodural bridge”. The connective form with the spinal dura mater of the “myodural bridge” was also investigated. This study refined the structure of the “myodural bridge” in human, and verified the universal existence of the “myodural bridge” in mammals. These results can be a strong support for the functional study of the “myodural bridge”. The “myodural bridge” fibers were tendon fibers, the traction force produced by the contraction of the fiber of the“myodural bridge” was large. So it is a powerful morphological support for the hypothesis that the “myodural bridge” is involved in the regulation of cerebrospinal fluid circulation. The “myodural bridge” is a universal, normal anatomical structure in mammals. It also could be a evidence that the “myodural bridge” is a highly conservative and important structure. It is also an indirect morphological support for the hypothesis that the “myodural bridge” is involved in the regulation of cerebrospinal fluid circulation. |
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