The Primary Respiratory Mechanism (PRM) is a foundational concept in craniosacral therapy. It was developed by Dr. William Garner Sutherland, based on the idea that there is a subtle, rhythmic motion within the body, specifically around the brain and spinal cord, which plays a crucial role in health and healing. He viewed the Central Nervous System, Cerebrospinal Fluid (a clear, colorless liquid that surrounds the brain and spinal cord), and the meninges (three layers of protective tissue that surround the brain and spinal cord) as a complete functional unit. He coined the term “Primary respiratory mechanism” (PRM) to describe the involuntary motion and physiological function of this functional unit. This mechanism is referred to as "primary" as it is fundamental to our function. It is ‘respiratory’ as it physiologically facilitates nutrition and metabolic effects on the body tissues.
The PRM is central to the practice of craniosacral therapy. Practitioners use their hands to feel these subtle movements and work to correct any restrictions or imbalances in the craniosacral system. The goal is to restore the body’s natural rhythms, improving function, reducing pain, and promoting overall health.
The 5 key components of Primary Respiratory Mechanism
Five components contribute to the primary respiratory mechanism. They are distinct but interrelated functions of the whole body system.
The fluctuation of the cerebrospinal fluid
The cerebrospinal fluid (CSF) is a clear and colorless fluid that fills the spaces in and around the central nervous system (CNS). This fluid cushions the brain and spinal cord, helps remove waste, and provides nutrients.
Around the CNS, the CSF is within the subarachnoid space (a space between two layers of tissue that surround the brain and spinal cord: the arachnoid mater and the pia mater). Within the CNS, the CSF is in the ventricles (a series of interconnected cavities in the brain that produce and contain CSF), cerebral aqueduct (a narrow channel in the brain that connects two of its ventricles), and central canal of the spinal cord.
CSF is produced by the choroid plexus (a network of cells and blood vessels) of the ventricles - majorly by the choroid plexus of the lateral ventricles but 3rd and 4th ventricles also participate. Production of CSF continues until pressure sensors in the cranium are stimulated, indicating that the maximum volume has been reached. This signal triggers the choroid plexus to stop producing CSF. The pressure sensors are believed to be located in the sagittal suture (a seam or joint that runs along the top of the skull, connecting the two large bones called the parietal bones).
Then CSF circulates around the craniosacral system mostly in the subarachnoid space. As it circulates around, there is a general leakage of CSF somewhere in the system. For this reason, the craniosacral system is semi-closed hydraulic system. The CSF also exits from the subarachnoid space into arachnoid granulations (a mushroom-shaped projection of arachnoid mater) to join the venous drainage (the process by which blood is carried away from the tissues and organs and returned to the heart). Most of the arachnoid granulations are found nearby the superior sagittal sinus which sits beneath the sagittal suture (where the pressure sensors are located). For this reason, drainage of the CSF stimulates the pressure sensor, indicating that the minimal volume has been reached. This triggers the choroid plexus to start the production of CSF.
The CSF in the sagittal sinus drains into other sinuses in the brain (namely, confluence of sinuses, transverse sinus, and sigmoid sinus) to eventually become the internal jugular vein. Internal jugular vein is an important vein that collectively drains the blood from head, face, and neck into the terminal station before the heart.
Due to the general leakage and drainage through the venous drainage system, the CSF is drained fast. For this reason, the production of CSF is generally twice as fast as the drainage of it.
The mobility of the meninges
Meninges are the coverings of the central nervous system (CNS - Brain and spinal cord) that protects and supports the CNS within the bony framework of the skull and vertebral column.
The meninges fuse with the internal surface of the cranial bones and form important membranes that protect and compartmentalize the brain. The tough and relatively inelastic layer of the meninges is dura mater. The dura mater forms the vertical membrane (the falx cerebri and the falx cerebelli) that seperates the left and right hemispheres of the brain. It also forms the horizontal membrane (the tentorium cerebelli) that separates the cerebrum from the cerebellum.
More importantly, the meninges form the pathways for the CSF to circulate in and around the craniosacral system while providing the exiting route for the drainage of it. For this reason, healthy fluctuation of the CSF and mobility of the meninges are associated. There are three layers in the meninges. The first layer is dura mater that was explained above. The second layer is arachnoid mater. The third layer is pia mater. Pia mater is a delicate tissue that follows the convolutions of the brain and spinal cord. However, the arachnoid mater does not follow the convolutions of the brain. The space between the arachnoid mater and pia mater is important as it is the pathway for the CSF circulation. This space is called the subarachnoid space. The independent mobility of all three layers of the meninges is crucial for the subarachnoid space to accommodate the healthy circulation of the CSF.
The mobility of cranial bones
As mentioned above, the meninges are fused with the internal aspect of cranial bones. Therefore, the mobility of cranial bones and that of the meninges are mutually related, influencing the circulation and drainage of the CSF.
Most of the connection between cranial bones are sutural articulations except for temporomandibular joint (TMJ) and sphenobasilar synchondrosis (SBS). Sutures are fibrous joints where two bones in the skull meet. These bones interlock along jagged or saw-tooth-like edges, creating a firm connection.
With age, the fibrous tissue in the suture is gradually replaced by bone. For this reason, it was traditionally believed that the cranial bones have no movement. However, after many years of study and research by Dr. Sutherland and Dr. Upledger, it is now established that each of the cranial bones has an inherent and involuntary motion that is permitted by sutures. The shape of the sutures allows the articulating cranial bones have a slight coupled motion like the cogwheels of a clock.
Restriction of any of the inherent cranial motion can affect not only local structures but also elsewhere in the body. For example, the restriction of the parietal bones can affect the mobility of the sagittal suture in which an important pressure sensor for CSF regulation is located. It can eventually disrupt the overall rhythm of the CSF fluctuation.
As the rhythmic fluctuation of CSF is crucial, rhythmic motion of the cranium bones is important for the whole body function. Crainum bones on either side of the body are generally paired, while the bones in the center are single or unpaired bones. The unpaired bones move into flexion and extension as the CSF fills in and drains out. Due to the cogwheel-like connection through the sutures, these movements are accompanied by external rotation and internal rotation of the paired bones.
The involuntary movement of the sacrum between the ilia
The sacrum is a large, triangular-shaped bone located at the base of the spine, just above the tailbone. The ilia are the large, wing-shaped bones that form the uppermost part of the pelvis. The sacrum and ilia form joints on each side of the body.
Part of cranial meninges exits cranium to cover the spinal cords. This is called the dural tube. This continuation of the cranial meninges attaches to the 2nd level of the sacrum and the tailbone. This makes the cranium and sacrum move like a unit via the tension of the meninges. For this reason, the dural tube is also called "reciprocal tension membrane."
As the CSF is filled in, the dural tube is pulled towards the top of the cranium, making the front part of sacrum pulled up (counternutation). When the CSF drains out, the opposite occurs. This creates an involuntary motion of sacrum that is independent from the ilia via the fluctuation of the CSF. Therefore, the involuntary motion of sacrum and CSF fluctuation are also associated.
The inherent motility of the central nervous system (Brain and spinal cord)
Dr. Sutherland proposed that there is an inherent movement within the central nervous system (CNS) that has a rhythmic sinusoidal cycle. This inherent movement was termed motility.
Later on, the inherent and rhythmic movement of the CNS was observed by Dr. Upledger during a spinal surgery.
The sinusoidal cycle is composed of the two phases where the entire CNS goes into shortening and widening, followed by lengthening and narrowing. They are similar to how the whole connection between the cranial meninges and sacrum behaves as well as the CSF fluctuation and involuntary movement of the cranium. These phases were named inhalation and exhalation, respectively.
Dr. Sutherland believed that the origin of the CNS motility was in the floor of the 4th ventricle of the brain. The 4th ventricle is where the centers that control breathing, circulation, digestion, and elimination are found. The 4th ventricle is located within the pons or in the upper part of the medulla oblongata of the brain stem.
Conclusion
The Primary Respiratory Mechanism is a foundational concept in craniosacral therapy, describing the subtle rhythmic movements of the cranial bones, sacrum, cerebrospinal fluid, and membranes that surround the brain and spinal cord. By working with these rhythms, manual osteopathic therapists aim to restore balance and promote the body’s natural healing processes.
References
Diminutto, D. (2022). Involuntary Mechanism Module. Manual Osteopathic College of Canada.
Parsons, J., & Marcer, N. (2005). Osteopathy: Models for Diagnosis, Treatment and Practice. Elsevier Health Sciences.
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