Editor's note: Dr. Upledger has asked guest author John Rollinson, D. Eu, CST-D, to write this month's column.

The Pressurestat Model illustrates the mechanism behind the circulation of cerebrospinal fluid through the semi-closed, hydraulic craniosacral system. Originally defined by Dr. John Upledger and a team of researchers at Michigan State University in the 1970s, the model explains the palpable, rhythmic expansion and contraction of the craniosacral system.

The brain and spinal cord are surrounded by cerebrospinal fluid (CSF). This fluid transports nutrients, hormones and peptides. It removes metabolic waste and toxic substances. It serves as a shock absorber, floating the brain to counteract gravity. It even influences respiration and cerebral blood flow, among its many functions. Given all this, it's easy to see how essential it is for CSF to flow unimpaired. If an area of brain tissue is even partially deprived of optimal CSF motion and flow, that area will be forced into some degree of functional compromise.¹

Cerebrospinal fluid is held within the dural membrane that surrounds the brain and spinal cord. This tough, watertight sac takes the shape of the interior of the cranium and intervertebral canal. Though dura mater doesn't stretch much, this fluid container is flexible and allows for CSF pressure changes. When pressure increases, the dural membrane expands, and the bones of the cranium and sacrum move along with it. When pressure decreases, the reverse occurs.

This filling of the craniosacral system is known as flexion, and the emptying is known as extension. During flexion, the head becomes wider transversely and shorter in its anterior-posterior dimension. The whole body externally rotates and widens. After flexion, this motion passes through a neutral zone on its way into extension, during which the head narrows and elongates and the whole body internally rotates.

Under normal circumstances, the craniosacral system proceeds cyclically through flexion and extension at a rate of about six to 12 cycles per minute. We can feel this rhythm at various places on the body because "this whole-body response is probably due to the pumping effect of the cerebrospinal fluid upon the motor system ... which causes a rhythmical tonification and detonification of the myofascial system in response to rhythmically fluctuating nerve signals."²

Tracing the Flow of Cerebrospinal Fluid Through the Craniosacral System

So, we have a hydraulic system that surrounds the brain and spinal cord. To understand how it is semi-closed, we must first understand how CSF enters and leaves the system. Within the ventricles of the brain, you'll find a capillary network - the choroid plexus - that produces CSF. In essence, blood circulating through the choroid plexus is "turned into" CSF, which then enters the craniosacral system.

The choroid plexus has stretch- and compression-sensing receptors within the saggital suture of the cranium. As CSF is added to the craniosacral system and its volume increases, the dural container expands, spreading the bones of the head. The parietal bones then move apart and spread the saggital suture. When this happens, the whole neuromechanism signals the choroid plexus to stop or greatly reduce the production of CSF. As the fluid drains from the system, the dura and cranium shrink and the parietals come together, compressing the saggital suture. The pressure-sensing nerve endings connected to the choroid plexus then send a signal to resume CSF production and the cycle repeats.

Normally, the system seems to operate on a cycle of about six seconds; CSF is produced for about three seconds and then production ceases for about three seconds. This creates the rhythmical rise and fall of fluid pressure within the system.

From the lateral ventricles, CSF enters the third ventricle via the foramina of Monro, then the fourth ventricle via the cerebral aqueduct. The CSF then enters the subarachnoid space and the central canal of the spinal cord via the foramina of Luschka and of Magendie, where it joins the CSF that is already bathing the brain and spinal cord, and all neural tissue enclosed by the dura mater. The fluid then circulates down and around the spinal cord and up and around the brain.

Cerebrospinal fluid passes out of the semi-closed hydraulic system via folds - called arachnoid granulation bodies or arachnoid villae - of the arachnoid layer of the cranial meninges that project through the inner layer of dura mater into the venous sinuses of the brain.³ CSF is reabsorbed into the venous blood through these arachnoid villae, which are primarily in the saggital venous sinous.

Although the rate of reabsorption is fairly constant, it seems to be regulated (think of a car idling) by a cluster of arachnoid granulation bodies found at the anterior end of the straight sinus. From its position at the "crossroads" of the intracranial membranes, this cluster can become aware of any tension within the membrane system and may regulate the outflow of CSF accordingly. 

To summarize in a different way, the craniosacral system is like a leaking toilet with the tank cracked into pieces and lined with a giant exam glove (which is the dural membrane). The float-switch in the toilet tank is the saggital suture, which causes an inflow whenever enough water/CSF leaks away down the drain (sinuses).

Generating Whole-Body Effects

The craniosacral system is intimately related to the nervous, musculoskeletal, vascular, lymphatic, endocrine and respiratory systems. Just as abnormalities in the structure or function of any of these systems can influence the craniosacral system, abnormalities in or injuries to the structure or function of the craniosacral system can have profound and deleterious effects on the development or function of the nervous system, especially the brain.⁴

There are also ways in which the craniosacral system directly influences important, ongoing physiological processes. For instance, the continuing rhythmical movement of the system may serve to "milk" the pituitary gland and affect the neuroendocrine system. The rhythmic motion may also be an important stimulus for the development of the brain. Similarly, the motion around the skull sutures may pump the newly formed red blood cells out of the flat bones of the skull and into the general circulation.⁵

Of course, any abnormality of the craniosacral system could impact the body or any of its parts through the central nervous system. Any deficiency in circulation of CSF could affect brain and nerve functioning. Any restriction of nerves passing out of the craniosacral system due to restrictions in the cranial sutures or membranes may affect their end organs. Thanks to the Pressurestat Model, we can see why.

References

1. Upledger JE. Cerebrospinal Fluid: What It Is and Where To Find It. The Upledger Institute, Inc., 1988.
2. Ibid.
3. Tabor's Cyclopedic Medical Dictionary. FA Davis Co., 2001.
4. Upledger JE, Vredevoogd J. Craniosacral Therapy. Eastland Press, 1983.
5. Upledger JE. Craniosacral Therapy II: Beyond the Dura. Eastland Press, 1987.