Existence Of The Myodural Bridge In Selected Mammalian And Avian Species

Introduction: The four suboccipital muscles are rectus capitis posterior major,rectus capitis posterior minor,obliquus capitis superior and the obliquus capitis inferior.They have relatively high muscle spindle density.Muscle spindles sense muscle length and the rate of length change,and communicate this information to the central nervous system which utilizes the information to maintain muscle tone,body posture and balance.Though the suboccipital muscles are involved in head extension and head rotation,other larger muscles are involved in these head movements and some researchers believe that the rectus capitis posterior muscles play a little role in head movement.Little flexion at the upper cervical joints results in major firing of the muscle spindle afferents in the peri-vertebral muscles.Thus,considering the proprioceptive endowment of the suboccipital muscles,they may be involved in co-ordinating posture,head movement,sensing joint movement and position.In 1995,a dense connective tissue connecting the rectus capitis posterior minor muscles with the dorsal cervical spinal dura mater via the atlanto-occipital space was discovered.The dense connective tissue connection was dubbed ‘the Myodural Bridge'.The myodural bridge connection was later verified by numerous researchers via gross anatomical dissection,histologically and plastination techniques.The myodural bridge passing through the atlanto-axial interspace and formed by the rectus capitis posterior muscle,the oblique capitis inferior muscles and the ligamentum nuchae are welldocumented.There exists a relationship between the suboccipital muscles and chronic headache,chronic neck pain(not in chronic,insidious-onset neck pain)and tension type headache,but the link has remained obscure.Studies have demonstrated fatty infiltration and a significant reduction in the cross-sectional area of suboccipital muscles in chronic neck and tension type headache and also the whiplash associated disorder.Conversely,in chronic headache,the cross-sectional area of the rectus capitis minor was increased.Occiput-atlas-axis joint manipulation and suboccipital inhibition technique have been proven to be effective in the treatment of tension type headache.The myodural bridge represents at least in part the underlying effectiveness of manipulative treatment by decreasing muscle tension and reducing/eliminating pain.Recently,studies have demonstrated the presence of the myodural bridge in mammals and a reptile.The mammals identified range from sea mammals to terrestrial mammals.Except for the finless porpoise,the rectus capitis dorsalis minor muscle of the studied mammals are connected to the dorsal atlantooccipital membrane via dense connective tissue fibres.The dorsal atlantooccipital membrane is in turn tightly adherent to the cervical spinal dura mater and forms the dorsal atlantooccipital membrane–dura complex.In the finless porpoise,the dorsal atlantooccipital membrane is entirely absent and the belly of the rectus capitis dorsalis minor muscle passes through the atlantooccipital interspace and inserts on the dorsal cervical spinal dura mater.Researchers have suggested that the myodural bridge may assist in resisting dura infolding during head extension and may prevent hyper-translation or hyperflexion,may monitor the stress on the dura mater and provide proprioceptive feedback to the brain and spinal cord,may also keep the cerebellomedullary cistern patent.A new hypothesis proposed that when the myodural bridge contracts,it pulls the dura mater posteriorly and this causes a negative/decrease in pressure in the subarachnoid space.The effectual negative pressure can be liken to a pump which can cause propulsion of the cerebrospinal fluid.A recent study has shown that head movement affects cerebrospinal fluid circulation.Since the only anatomical link between the muscles of head movement and the spinal cord meninges is the myodural bridge,this throws an indirect weight on the hypothesis that the myodural bridge affects the cerebrospinal fluid circulation.The available data on the myodural bridge in mammalian species are limited and noninclusive.Moreover,the existence of the myodural bridge in birds has not been explored.Animal studies on the myodural bridge will provide insight into the evolution and the function of the myodural bridge.In this study,we included two mammals and a bird specie.For the mammalian species,bats and Tupaia belangeri(Tree shrew)were studied.Bats are nocturnal and the only mammal that can perform a true and sustained flight.Tupaia belangeri are arboreal mammals that often climb to a high altitude of about 10,000 feet.The Columba livia(Common Rock Pigeon)was selected as a representative of birds.Methods: Gross anatomical dissection was carried out with the aid of a stereomicroscope.The specimens' upper dorsal cervical region were exposed by dissection.The relationship between the suboccipital muscles and the dorsal atlantoocciptal membrane(DAOM);and the relationship between the DAOM and the dura mater were explored.Tissue samples having the occiput and the cervical region were processed for histological analysis by fixing the tissues,decalcifying,dehydrating and embedding them in a paraffin wax.In order to explore the properties of the fibers that constitutes the myodural bridge,the tissue sections obtained after microtome were subjected to analysis using conventional and special histological stains.The stains used are haematoxylin and eosin,Masson trichrome and Sirius red in saturated carbazotic acid.The Sirius red in saturated acid stained sections were also studied using a polarized microscope.In the Columba livia,the fiber orientation of the bridge connecting the DAOM and the spinal dura mater was additionally studied using the scanning electron microscope.Results: The gross anatomical studies of the bats and tree shrews show the presence of the four subooccipital muscles which are the rectus capitis dorsalis major,rectus capitis dorsalis minor,obliquus capitis superior and obliquus capitis inferior muscles.The rectus capitis dorsalis major,obliquus capitis inferior and obliquus capitis superior form the suboccipital triangle.The ventral surface of the rectus capitis dorsal minor was connected to the DAOM by dense connective tissues.Furthermore,the internal surface of the DAOM is adherent to the spinal dura mater by dense connective tissues.The myodural bridge in these mammals share similaries with that existing in humans.In the Columba livia,the ventral side of the M.rectus capitis dorsalis minor has dense connective tissues linking it to the dorsal atlanto-occipital membrane.The dorsal atlanto-occipital membrane by means of several fibrous dense connective cords emanating from its internal surface inserts on the cervical dorsal spinal dura mater.These cords pass through the cervical sinus.Histological sections of bats show that the atlantooccipital membrane covers the atlantooccipital space.The membrane extends from the inferior margin of the occipital bone to the atlas,whereas in the tree shrew,the posterior atlantooccipital membrane does not completely span the atlantooccipital space,it falls short of the distal third of the atlantoocciptal interspace.The one third part is covered by the connective tissue of the rectus capitis dorsalis minor mucle.Dense connective tissue fibers from the rectus capitis dorsalis minor muscle inserts into the DAOM.The internal surface of the DAOM is in turn adherent to the spinal dura mater by dense connective tissue.The fibrous connection between the rectus capitis dorsalis minor muscle and the DAOM as well as that between the latter and the spinal dura mater constitutes the myodural bridge.The Masson trichrome stained sections reveal that the fibers of the myodural bridge are dense connective collagen fibers.In the Sirius red stained sections,analyses of the fibres of the myodural bridge using the polarized microscope confirm that the collagenous fibres of the myodural bridge are composed primarily of type I collagen fibers in addition to the few type III collagen fibres.Thus,the histological analysis of the bridge in these species demonstrates that the myodural bridge is composed primarily of type I collagen fibers.In the Columba livia,the hematoxylin & eosin stained sections confirmed that dense fibrous bundles arising from the M.rectus capitis dorsalis minor insert on the dorsal atlanto-occipital membrane and that cords of dense connective tissue link the internal surface of the dorsal atlanto-occipital membrane to the cervical dorsal spinal dura mater.These connecting dense connective tissues liniking muscles to the DAOM and the DAOM to the dura mater constitute the myodural bridge.The Masson trichrome stained sections indicated that the myodural bridge is made up of collagenous connective tissue.Using the polarized microscope for examination of the Sirius red(in saturated carbazotic acid)sections,the myodural bridge fibres were revealed to be primarily type I collagen fibers.The ultra-structural study of the Columba livia's myodural bridge demonstrates that the dorsal atlanto-occipital membrane is made up of several laminae.The lamellae are curly and evenly arranged;the fibers are made up of collagen fibres that are arranged in longitudinal direction.The myodural bridge connecting the internal aspect of the dorsal atlanto-occipital membrane to the dorsal cervical dura mater are cords of connective tissue.Each cord contains parallel running curly collagen fibres.On reaching the dura mater,the fibres of the myodural bridge cord interlace with the fibres of the dura mater,and thus they have a strong anchorage to the dura mater.Discussion: There exists a dense tissue connection(myodural bridge)between the rectus capitis dorsalis muscles and the cervical spinal dura mater in bats and tree shrews.This supports the hypothesis that the myodural bridge is a universal existing structure in mammals.Because the myodural bridge is rich in type I collagen fibres,it is strong and may have properties similar to tendons.In the same vein,this study establishes the presence of the myodural bridge in the Columba livia,the bridging structure is made up of cord-like collagen type I fiber bundles,this confers a high tensile resistance property to the myodural bridge and this indicates that the myodural bridge is well suited structurally to pull the dura in order to prevent hyperflexion or hyper-translation.Additionally,the pull can exert a substantial effect on the cerebrospinal fluid flow by causing cerebrospinal fluid propulsion and may generate pain in cases of pathology of this structure or the associated muscles.Comparison of the myodural bridge of the Columba livia and that of the studied mammals reveal that the myodural bridge linking the DAOM and the spinal dura mater in the Columba livia are cord-like dense connective tissues;the myodural bridge cords pass through the epidural space at the level of the atlantoccipital interspace,where it is abuted by venous blood of the venous sinus.Meanwhile in mammals,the dorsal atlantooccipital membrane is intimately adherent to the dura mater and so here,the bridge linking the spinal dura mater consists of dense connective tissue occupying the epidural space at the level of the atlantoccipital interspace.It is conceivable to say that the myodural bridge evolved from a cord-like dense connective tissue structure into a dense connective tissue which is initimately adherent to the dura mater and thus occupies the epidural space.In birds,the occipital venous sinus drains into the cervical venous sinus and the sinus drains caudally into the single voluminous spinal vein which is just dorsal to the spinal cord.In contrast,the internal veterbral venous plexus of the studied mammals consists of two longitudinal veins located ventrolaterally in the epidural space of the vertebral canal,hence they possess a pair of anterior IVVP,and lack the posterior IVVP.This alteration of venous blood flow from the dorsal to the ventrolateral side of the epidural space of the vertebral canal may have occurred due to the evolution of the myodural bridge,i.e from a cord-like dense connective tissue structure into a dense connective tissue which is initimately adherent to the dura mater and thus occupies the cervical dorsomedian epidural space.Conclusion: Myodural bridge exists in bats,tree shrews and common rock pigeons.The fibre of the myodural bridge is made up of parallel running collagen type 1 fiber bundles,which indicates that the structure has a high tensile resistance property.This research proves that the myodural is a common normal anatomical structure in mammals and indicates its presence in birds.It also supports the hypothesis that the myodural bridge functions as a source of power for the circulation of the myodural bridge.