The myodural bridge of the American alligator (Alligator mississippiensis) alters CSF flow

Abstract

Disorders of the volume, pressure or circulation of the cerebrospinal fluid (CSF) lead to disease states in both newborns and adults; despite this significance, there is uncertainty regarding the basic mechanics of the CSF. The suboccipital muscles connect to the dura surrounding the spinal cord, forming a complex termed the ‘myodural bridge’. This study tests the hypothesis that the myodural bridge functions to alter the CSF circulation. The suboccipital muscles of American alligators were surgically exposed and electrically stimulated simultaneously with direct recordings of CSF pressure and flow. Contraction of the suboccipital muscles significantly changed both CSF flow and pressure. By demonstrating another influence on CSF circulation and pulsatility, the present study increases our understanding of the mechanics underlying the movement of the CSF.

The morphology of the suboccipital region in snakes, and the anatomical and functional diversity of the myodural bridge

“…These three components were also lacking in C. atrox and O. hannah, two snake species that are only distantly related to each other and to the colubrine species examined (e.g., Wiens et al, 2012). Though we could find no evidence of an MDB in any of the snake species examined, all three components were clearly evident in A. mississippiensis (Figure 6) as was previously reported from this species (Young, Adams, et al, 2021;Young et al, 2020) and a related crocodilian (Zhang et al, 2016). The approach used for this analysis-complete serial sections in multiple planes and micro-CT analyses-provides more than enough anatomical resolution to document the MDB if one were present.…”

“…The number of mammalian taxa examined led Zheng et al (2017) to argue that the MDB was a consistent feature among mammals. An MDB has also been reported in several bird species (Chen et al, 2021;Dou et al, 2019;Okoye et al, 2018), in Crocodylians (Young et al, 2020;Zhang et al, 2016), and in the turtle Trachemys scripta (Huangfu et al, 2019). The diversity of taxa studied to date, combined with the absence of accounts of taxa lacking the feature, suggests that the MDB may be a common attribute of amniotes.…”

“…Venne et al (2017) demonstrated that in the human cadaveric material force applied to the MDB was sufficient to displace the cervical dura. Electrical stimulation of the MDB was shown to alter CSF pressure in anesthetized alligators (Young, Adams, et al, 2021;Young et al, 2020) and dogs (Ma et al, 2021). Active movement of the human head (Xu et al, 2016), and passive movement of the dog's head (Ma et al, 2021) have been shown to induce CSF flow.…”

“…As a positive control, we also examined the suboccipital anatomy of the American alligator, Alligator mississippiensis (Daudin, 1802), a species in which the MDB has been previously described (Young, Adams, et al, 2021;Young et al, 2020). A sub-adult A. mississippiensis (total length of 188 cm) was anesthetized with isoflurane then imaged using a CT unit as described above.…”

“…Venne et al (2017) performed a biomechanical analysis of human cadaveric MDB and demonstrated that tensile force applied to the muscle resulted in significant displacement of the spinal dura. Subsequent experiments with live alligators (Young, Adams, et al, 2021;Young et al, 2020) and dogs (Ma et al, 2021) demonstrated that electrical stimulation of the musculature of the MDB resulted in significant changes to CSF pressure.…”