The pressurestat model of the CranioSacral System

by John Rollinson, D.Eu, CST

The Pressurestat Model describes the inflow, circulation and outflow of cerebrospinal fluid in the semi-closed hydraulic system of the craniosacral system. It is an explanation for the palpable, rhythmic expansion and contraction of the boundary of the craniosacral system which we know as the craniosacral rhythm.

The brain and spinal cord are surrounded by cerebrospinal fluid, which transports nutrients, hormones and peptides of all kinds; removes metabolic waste products and toxic substances from brain tissue; serves as a shock absorber in jolts to the head; floats the brain, thus counteracting the forces of gravity; and influences respiration and cerebral blood flow through its pH, to name some of its known functions. In fact, "because of its singular and continuous fluid system, in order to bathe the neurons and glial cells of the brain, it is essential that cerebrospinal fluid flow not be impaired. If an area of brain tissue is even partially deprived of optimally effective cerebrospinal fluid motion and flow, that brain area will be forced into some degree of functional compromise." ^i.

The cerebrospinal fluid is held within the dural membrane, a tough (hence "dura"-ble) and watertight sack which takes, for the most part, the shape of the interior of the cranium and intervertebral canal. Though the dura mater does not stretch much, it is flexible and allows for changes in pressure of the cerebrospinal fluid-container-system. If pressure increases, the container-membrane expands and the bones of the cranium plus the sacrum move along with it. The reverse occurs when pressure decreases. It is thus that the craniosacral rhythm can be readily palpated at any of these bones; the alternation between increase and decrease in pressure occurs rhythmically.

The filling of the craniosacral system is known a flexion, and the emptying of the same as extension. (Sutherland’s terms) The craniosacral system proceeds through cyclical flexion and extension at a rate of approximately 6 – 12 cycles per minute under normal circumstances. Flexion is an extreme range of motion during which the head becomes wider transversely and shorter in its anterior-posterior dimension. During flexion, 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, the whole body internally rotating.

It is believed that the reason we can feel the rhythm elsewhere on the body, in fact anywhere, is that "this whole-body response is probably due to the pumping effect of the cerebrospinal fluid upon the motor system, (increase or decrease of cerebrospinal fluid pressure upon the brain) which causes a rhythmical tonification and detonification of the myofascial system in response to rhythmically fluctuating nerve signals." ^ii.

So, we have a hydraulic system. In order for it to be "semi closed", we must now describe how cerebrospinal fluid enters and leaves the system. In the ventricles of the brain, but chiefly in the lateral cerebral ventricles is found a capillary network called the choroid plexus which produces cerebrospinal fluid by filtration and secretion. The choroid plexus is a projection of the arachnoid mater into the cerebrospinal fluid filled ventricles. Blood circulating through the choroid plexus is "turned into" cerebrospinal fluid and enters the craniosacral system.

The choroid plexus has stretch and compression sensing receptors within the Saggital suture of the cranium, connected to it by nerve tracts running through the falx cerebri. As cerebrospinal fluid is added to the system and its volume increases, the dural container expands, spreading the bones of the head. The parietal bones move apart, spreading the saggital suture, from whence the neuromechanism signals the choroid plexus to stop or greatly reduce the production of cerebrospinal fluid. As the cerebrospinal fluid drains from the system (see below), the dura and the cranium along with it shrink again and the parietals come together, compressing the saggital suture. The pressure sensing nerve endings connected to the choroid plexus send a signal to resume production of cerebrospinal fluid, and the cycle repeats. Normally, the system seems to operate on a cycle of about six seconds – cerebrospinal fluid 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 the cerebrospinal fluid enters the 3rd ventricle via the foramina of Monro, then the 4th via the cerebral aquaduct. It then enters the subarachnoid space and the central canal of the spinal cord via the foramina of Luschka and of Magendie and joins the cerebrospinal fluid already bathing the brain and spinal cord; bathing all neural tissue enclosed by the dura mater. 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 which project through the inner layer of dura mater into the venous sinuses of the brain. ^iii. Cerebrospinal fluid is reabsorbed into the venous blood through these arachnoid villae, which are found mostly in the saggital venous sinous. The rate of reabsorption is fairly constant, but seems nevertheless to be regulated (as the idle of your car) by a cluster of arachnoid granulation bodies found at the anterior end of the straight sinus. From this position at the crossroads, so to speak, of the intracranial membranes this cluster is aware of any tension within this membrane system and may regulate the outflow of cerebrospinal fluid accordingly.

To summarize, the craniosacral system is like a leaking toilet with the tank cracked into pieces and lined with a giant exam glove (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).

The craniosacral system is intimately related to, influences and is influenced by:

1. The nervous system
2. The musculoskeletal system
3. The vascular system
4. The lymphatic system
5. The endocrine system
6. The respiratory system

Abnormalities in the structure or function of any of these systems may influence the craniosacral system. Abnormalities in the structure or function of the cranio sacral system will necessarily have profound, and frequently deleterious effects upon the development or function of the nervous system, especially the brain. ^iv.

There are also ways in which the craniosacral system may have a direct influence on important, ongoing physiological processes. For example, the continuing rhythmical movement of the craniosacral system may serve to "milk" the pituitary gland, with all the implications this would hold for the neuroendocrine system. It is also possible that this rhythmic motion is also 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. ^v.

Of course, any abnormality of the craniosacral rhythm, whatever its cause, may have an effect upon the body or any part of it via the central nervous system; any deficiency in the circulation of cerebrospinal fluid may affect brain and nervous functioning, any restriction of nerves passing out of the craniosacral system due to restrictions in cranial sutures or membranes may affect their end organs. The same sutural restrictions may affect blood flow into the cranial vault, in turn impinging on brain function. Malfunction within the pessurestat model itself, such as hydrocephaly, where the fluid cannot escape quickly enough, obviously has drastic effects upon the body.

The anatomical components of the craniosacral system are:

1. The meningeal membranes
2. The osseous structures to which the meningeal membranes attach
3. The other non-osseous connective tissue structures which are intimately related to meningeal membranes
4. The cerebrospinal fluid
5. All structures related to production, resorption and containment of the cerebrospinal fluid

The meningeal membrane and its contents form the hydraulic system. This membrane is composed of: The pia mater, which faithfully follows the contours of the brain and spinal cord and contains a vascular network. External to that, the arachnoid mater, internal to which is the subarachnoid space, in which circulates the cerebrospinal fluid and external to which is the subdural space. The two fluid filled spaces allow some independent movement of the three membranes. External to that, the dura mater, which is fused with the internal aspect of the skull. Its inner layer forms vertical sheets, the falx cerebri and cerebelli, which separate the hemispheres of the cerebrum and cerebellum respectively. It also forms the horizontal tentorium cerebelli, bilaterally, which separate the cerebellum from the cerebrum. At the foramen magnum the outer dura becomes the endosteum of the vertebral canal and the inner dura becomes the dural tube and extends from its attachment at the foramen magnum (it is also said to attach to the posterior bodies of the 2nd and 3rd cervical vertebra, however we were unable to find these attachments in our CS Dissection class and it is postulated that they could be a result of the embalming process) into the sacrum. At the level of the 2nd sacral segment it again attaches firmly and then, blending with the other layers, exits at the sacral hiatus at S4 to become the periosteum of the coccyx. It is via these bony attachments that tensions can be transmitted from the extraCSS connective tissues into the dural membrane system and vice versa. As the spinal nerves exit the vertebral canal, they are covered by extensions of the dural sheath (as are the optic nerves) which blends into the paravertebral fascia. All that enjoys enclosure within the dura mater belongs to the cranio sacral system.

i. John Upledger, "Cerebrospinal fluid: what it is and where to find it", 1998 by The Upledger Institute
ii. Ibid
iii. Tabor's Cyclopedic Medical Dictionary, 2001 by F.A. Davis Co
iv. John Upledger, Craniosacral Therapy, 1983 by Eastland Press
v. John Upledger, Craniosacral Therapy-2, 1987 by Eastland Press