Occipital-Dural Muscle:A Specialized Myodural Bridge In Neophocaena Asiaeorientalis Sunameri

Objective:A dense bridge-like tissue named the myodural bridge connecting the suboccipital muscles and the spinal dura mater was originally discovered in humans.Using gross anatomical and P45 sectional plastination technique,our team found fibers from the rectus capitis posterior minor,rectus capitis posterior major,the oblique capitis inferior muscle,and the nuchae ligaments connect to the spinal dura mater,via the posterior atlanto-occipital and posterior atlanto-axial interspaces.These structures all above formed a functional unit named ‘Myodural Bridge Complex’.Researchers speculated that the myodural bridge may play essential roles,they predicted the function and clinical significance of the myodural bridge including transmission of proprioception,preventing in-folding of the spinal dura mater,and might be related to cervicogenic headache.According to the evidence we have investigated,our team believe when the head moves,the myodural bridge cloud transmit the contraction pull of the suboccipital muscles to the spinal dura mater.It might affect the circulation of the cerebrospinal fluid by altering the volume of subarachnoid space of spine.Furthermore,our group investigated that the MDB existed in more mammalian taxa including Macaca mulatta,Canis familiaris,Felis catus,Oryctolagus cuniculus,Ratus norvegicus,Cavia porcellus,and Indoasian finless porpoise.In addition,it was confirmed that this structure is also present in reptiles(Crocodylus siamensis),and avifauna(Columba livia and Gallus domesticus).This universal existence inferred that the MDB could be physiologically significant in both humans and other species.A previous study has confirmed the existence of the MDB in Indoasian finless porpoise,yet the posterior Atlanta-occipital(PAO)membrane was not found.Upon that,we initiated this research to investigate the occipital-dural muscle which insert the atlanto-occipital interspace in Neophocaena asiaeorientalis sunameri,figure out the relationship between this muscle and the MDB in humans,and infer the physiological function of it.Materials and methods:Five collected dead bodies of Neophocaena asiaeorientalis sunameri were conducted arterial perfusion through the aorta with 10% formalin solution.One specimen was sliced in sagittal section for p45 sheet plastination.Four specimens’ heads and necks were continuously scanned by GE 128-row VCT,and dual phase serial computer tomography(CT)images were obtained.The images were analyzed for modeling and reconstruction in MIMICS software(MIMICS 18.0.0.525,Materialise,Leuven,Belgium).After that,these four specimens were dissected layer-by-layer at the posterior occipital region.Then two of the specimens were used for histological analysis,the other two specimens were used for scanning electron microscope study.Results:The reconstructed 3D model of cranium and cervical vertebrae of the Neophocaena asiaeorientalis sunameri demonstrated that the atlanto-occipital interspace was broader compared with humans and some other terrestrial mammals,to the cervical dura mater.The first three cervical vertebrae merged into one unit.During the gross anatomy,rectus capitis dorsalis was found in the deep post-occipital region.The cranial attachment of this rectus muscle was at occiput,while the caudal attachment was at the transverse process of the fused cervical vertebrae.Another muscle was found underneath the rectus capitis dorsalis,which originated from the occiput,and ended at spinal dura mater.We named it‘the occipital-dural muscle’.Yet the dorsal atlanto-occipital membrane was not found in finless porpoise during the dissection.Median sagittal sections of the P45 plastination sheets showed that all the fibers of the occipital-dural muscle extended into the atlanto-occipital interspace,attached to the cervical spinal dura mater ultimately.The sections of VG staining showed that the occipital-dural muscle’s muscular fibers were stained in yellow,while entering the atlanto-occipital interspace,the extending fibers turned to red,which revealed that the extended parts were collagenous fibers as muscle tendon.All of the muscular fibers of the occipital-dural muscle inserted into the atlanto-occipital interspace,terminated and merged with the spinal cervical dura mater directly.Conclusion:We found the MDB in Neophocaena asiaeorientalis sunameri was isolated as an independent muscle that originated from the occiput,extended through the atlanto-occipital interspace,and terminated at the spinal dura mater.Unlike humans,this muscle directly connects with the spinal dura mater in Neophocaena asiaeorientalis sunameri,transmits the strong traction force to the spinal dura mater,by muscular contraction and relaxation.Furthermore,the mechanism is also highly related to sustainable relative movement between occiput and the fusion of the first three cervical vertebrae.This mechanism is obviously more powerful and representative than that in humans and most of other species we have investigated before.Meanwhile,it enhances the credibility of the significant physiological function of MDB.