Cerebrospinal Fluid Enters Peripheral Organs by Spinal Nerves Supporting Brain–Body Volume Transmission

Abstract

The cerebrospinal fluid (CSF) provides many vital functions to the central nervous system (CNS). The CSF irrigates the CNS from within (through the ventricular system and the central canal of the spinal cord) and from without (through the sub-arachnoid space of the cranium and spine). As the brain and spinal cord therefore float within the CSF, the CNS is hydromechanically protected by the CSF, and the Archimedean principle of buoyancy means that the mass of the suspended brain is reduced from ~1.5 kg to mere 50 g. The CSF also acts as a hydraulic shock absorber that prevents the brain from hitting the skull and thus provides us the freedom of movement and acceleration. Simply put, the hydraulic shock absorption is the reason none of us incur debilitating concussions each time we take a step! Moreover, the CSF nourishes the CNS by providing a highway for nutrients and signaling molecules that are transported from the blood in the cerebral circulation to the CSF, and then to the interstitial fluid within the extracellular space of the nervous tissue. The CSF also provides haulage to a variety of waste products. In addition to these housekeeping functions, the CSF is a medium for long-range signaling within the CNS and between the CNS and the body, by carrying hormones, neurotransmitters, neuromodulators, signal-competent molecules, or extracellular vesicles over long distances.

The CSF production and flow are intimately associated with the ventricular system of the brain, the central canal of the spinal cord, and the sub-arachnoid space. The mammalian brain contains four ventricles: two lateral, localized quasi-symmetrically in each of the hemispheres, the third ventricle at the midline of the diencephalon, as well as the fourth ventricle of the hindbrain. The lateral ventricles are connected to the third ventricle through the foramen of Monro, while the third and fourth ventricles are linked with the cerebral aqueduct of Sylvius. The fourth ventricle is continuous with the central canal of the spinal cord and connected to the subarachnoid space through exits at the foraminae Magendie and Luschka [1]. The CSF is produced mainly by four choroid plexi (one per ventricle) although extrachoroid sites may also contribute [2]. Choroid plexi are lined with a monolayer of specialized “choroid” epithelium. This name is, however, incorrect: cells building the choroid plexi are bona fide ependymoglia: committed precursors of choroid cells are scions of a subpopulation of neuroepithelial precursors that emerge around embryonic Day 11 in mice, prior to the start of neurogenesis [3]. The cuboid choroid ependymocytes have several cilia and form a CSF-blood barrier reinforced by intercellular junctional complexes made of tight junctions, adherens junctions, and desmosomes [4].

The production of the CSF is supported by multiple plasmalemmal transporters that are selectively recruited at apical vs. basal membranes of choroid ependymocytes. The apical membranes, in particular, underpin the secretion of Na⁺, Cl⁻, and HCO₃⁻, as well as the reabsorption of K⁺. Main transporters involved in CSF production are Na⁺-K⁺ ATPase (NKA), Na⁺-K⁺-Cl⁻ co-transporter (NKCC1), K⁺-Cl⁻ co-transporters (KCC1/3/4), Na⁺-HCO₃⁻ co-transporter (NBCe1/n1), and Na⁺-H⁺ exchanger (NHE1). Besides, ions and water diffuse through plasmalemmal channels such as aquaporins (AQP1/4), inward rectifier anion channel and volume-regulated anion (VRAC) channel, voltage-dependent K⁺ channels (K_v1.1/1.3/1.6), and inward rectifier K⁺ channels (K_ir7.1) [5]. The apical membrane of the choroid plexus ependymocytes is enriched in NKA, NKCC1, and anion channels. NKA extrudes Na⁺ from the cytosol towards the CSF, thus lowering the cytosolic Na⁺ concentration. This in turn triggers the influx of HCO₃⁻ through Na⁺-dependent Cl⁻-HCO₃⁻ exchangers operating in the basolateral membrane. This is associated with an increase in cytosolic concentration of Cl⁻ and HCO₃⁻, which are subsequently secreted into the CSF [2]. Thus, the cellular localization of the above channels ensures an unidirectional extrusion of fluid from choroid ependymocytes, thus defining the initial direction of flow for the CSF.

The daily production of CSF is as high as 500–600 mL, into a total volume of CSF that ranges between 125 and 250 mL. The CSF is therefore fully replaced at least twice every day [6], and requires continuous drainage into the lymphatic system. At the exit from the ventricles, a substantial part of the CSF enters the glymphatic system through the cisterna magna and pontine cistern [7]. Any remaining CSF distributes through the subarachnoid space and exits into the blood circulation through gates that include the arachnoid villi extending into the venous sinuses of the dura mater, dural lymphatics, cranial nerves, cribriform plate, parasagittal spaces, and granulations and adventitia of large cerebral vessels [6].

There is however another exit route for the CSF once in the central canal of the spinal cord. This is through the spinal nerves and the periaxonal space, which is formed between axons and their ensheathing Schwann cells, through which CSF can dissipate into peripheral organs, such as liver, pancreas, and even skeletal muscles (Figure 1 [8]). This route was considered previously [9] but functionally visualized only recently by following fluorescent tracers injected into the cisterna magna [8]. These tracers, distributed through the spinal cord, ultimately appeared in peripheral organs. The transection of thoracic spinal nerves occluded the flow of tracers to the liver by ~70%; whereas ligation of the vena cava reduced peripheral tracer accumulation only by ~30%, confirming a role for drainage along spinal nerves for CSF outflow [8]. When traveling along the nerves, CSF can be mixed with endoneurial fluid flowing between individual axons or eventually be transported through periaxonal spaces delineated by the basement membrane and Schwann cells. The CSF enters the peripheral nerves either from the spinal subarachnoid space or directly through the spinal cord parenchyma where CSF enters either with the glymphatic flow or from the central canal. The latter entry can be dynamically regulated by the ependymoglia lining the central canal. The ependymocytes that line the central canal of the spinal cord receive catecholaminergic, mainly serotonergic, innervation from dorsal raphe neurones for the dynamic control of their size and interactions. Stimulation of 5-HT_2B receptors located on ependymal glia triggers Ca²⁺ signals and instigates F-actin polymerization leading to a transient shrinkage of ependymogliocytes. This widens the water gate connecting the central canal with the spinal cord parenchyma and increases CSF flow to and through the latter towards peripheral organs [8].

CSF outflow through the spinal nerves extends its outreach beyond the CNS to the periphery. The amount of CSF in the spinal cord is relatively small and contributes relatively little to the daily drainage of the CSF, but suffices to deliver waves of signaling molecules and metabolites-rich fluid to visceral organs and to the skeleto-muscular system, satisfying the principle of long-range volume transmission, qualifying the CSF as a signaling medium. Hormones, neurotransmitters, and extracellular vesicles with signaling cargo could be directly secreted into the CSF from ependymal glia at all levels [10]. These signaling substances transported with the CSF find relevant targets in the peripheral organs and thus increase the complexity and versatility of the brain–body connectivity.

The authors declare no conflicts of interest.