How does cerebrospinal fluid circulate in the central nervous system?

Question

Cerebrospinal fluid (CSF) is produced in the choroid plexus of the lateral ventricles and in the 4th ventricle of the brain. CSF then circulates through the ventricles of the brain and the subarachnoid space of the meninges. CSF is returned to the venous system via the arachnoid granulations connecting the subrachnoid space with the superior sagittal sinus at the superior portion of the neurocranium.

• What circulates the CSF such that it can return to the venous system against gravity?

• In other words, why does CSF not all just pool in the caudal cistern?

Answer 1

There are several points here.

Arachnoid granulations are not the only "sinks" for CSF.

Even though it is true that most of the CSF is eliminated from ventricular system and subarachnoid space through these granulations, there are also suggestions that there are also other potential mechanism of shunting CSF into the venous system:

1. Cranial nerves leaving CNS go through dura mater and these holes are not sealed, so it is suggested that some CSF can just go along the nerves and then accumulate in the lymphatic nodes in submucus.

2. Dura mater is mostly perforated at the area of lamina cribrosa, that basically separates the nasal holes and our brain. Leaking through multiple small holes here CSF is accumulated into the lymphatic submucusal nodes, found in abundance here. This way is considered to be primary CSF drainage way in newborns who don't have a well developed granularities in (sub)arachnoid space.

3. Spinal nerves, especially in the upper part of the spinal cord, also leave unsealed holes and the presence of lymphatic nodes adjacent to the places where these nerves leave the verterbral column also make them a potential place for CSF drainage.

Arachnoid granularitis do not only drain, they are capable of active CSF uptake.

CFS is not just "circulating" here going from the place of its origin in lateral ventricles to venous system. The granularities are here to actively resorb this fluid, meaning that they effectively take it up forming vacuoles and then excrete it to the venous system. Constant uptake leads to the pressure gradient (see also below!) that acts as a driving force for CSF.

Venous system has generally low pressure, thus sucking the CSF up to drainage points.

Speaking about drainage we also should consider the fact that the venous system has a pressure lower than the atmospheric pressure and much lower than pressure in any middle-sized arterial vessels. This is reached by having elastic walls and by transmitting the negative pressure from the mediastinum during the inhale, when the diaphragm and the thorax expand. This negative pressure propagates mostly to the connected vessels, including sinus system. Due to the elastic wall other parts of venous system can accommodate this negative pressure constrict their lumen, but not the sinus system which has a hard external framework formed by dura mater and bones. That is here we can measure the lowest pressure and this is the pressure that sucks CSF in into the venous system.

I hope that answers your question.

Answer 2

It is something of a misnomer to speak of CSF “circulation,” particularly in the spinal canal, as there is no continuous loop circulation of CSF as there is in the cardiovascular system.

For quite some time, it has been known that CSF movement results from the formation of new CSF and motion of cilia on the surface of the choroid plexus and ependyma lining the ventricles. - Fluids and Barriers of the CNS

The "circulation" of the CSF, as already mentioned, is something of a misnomer. CSF is not known to "circulate" in the manner of blood. It does get agitated by pressure differentials, and it is 'circulated' in terms of being reabsorbed and replaced every 6-7 hours. Other than that, no circulation occurs.

Blood circulation is not generated only by the heart. Pressure differentials throughout the body affect the circulation of blood as well. One that is easily demonstrated (first documented in 1733) is the effect of intrathoracic (chest) pressure on circulation. The blood pressure of healthy people falls during spontaneous inspiration. When someone takes a deep breath, the blood return to the heart via the vena cava decreases, and pressure is exerted on the right atrium. Both cause decreased filling, which will drop blood pressure. Although this is best demonstrated with a blood pressure cuff, it can be demonstrated without. An unrecommended method is exemplified in a childhood game of passing out. A Valsalva maneuver (deep breath and glottal closure) decreases blood flow to the heart. Squeezing the chest further decreases return, resulting in fainting.

The same pressure differentials agitate the CSF. Additionally, smaller movements were seen with pressure differentials caused by the beating of the heart.

By employing this respiration-induced spin labeling bSSFP cine method, we were able to visualize CSF movement induced by respiratory excursions. CSF moved cephalad (16.4 ± 7.7 mm) during deep inhalation and caudad (11.6 ± 3.0 mm) during deep exhalation in the prepontine cisternal area. Small but rapid cephalad (3.0 ± 0.4 mm) and caudad (3.0 ± 0.5 mm) movement was observed in the same region during breath holding and is thought to reflect cardiac pulsations.

This image from Wikipedia shows "circulation" that normally occurs with heartbeat.

There are other factors that cause movement of CSF, but they are intermittent and variable.

^{Influence of respiration on cerebrospinal fluid movement using magnetic resonance spin labeling, Yamada et. al., Fluids and Barriers of the CNS 2013, 10:36
MRI showing pulsation of CSF}

Answer 3

CSF is actively pumped in an ebb and flow manner by the pressure with our respiratory mechanism transmitted through the pelvic diaphram onto the sacral bone that pulls on the spinal dural membranes attached at S2. Think about it: the pelvic and thoracic diaphragm are linked in a respiratory phasic contraction (confirmed with MRI). The pulling on the sacral end of the dural membranes via the pelvic floor muscle will squeeze the CSF up the spinal canal into the brain with inspiration. This is a much stronger force than any ciliary or cardic induced influence of CSF flow. Recent MRI studies have confirmed this as well.

Answer 4

Circulation of cerebrospinal fluid (CSF) through the ventricular system is driven by motile cilia on ependymal cells of the brain.

Hagenlocher C1, Walentek P, M Ller C, Thumberger T, Feistel K.Ciliogenesis and cerebrospinal fluid flow in the developing Xenopus brain are regulated by foxj1.Cilia. 2013 Sep 24;2(1):12. doi: 10.1186/2046-2530-2-12.