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CNS Anatomy


The Anatomy of the Central Nervous System
and CranioSacral System


Here is a more in-depth description of the anatomy of the central nervous system and craniosacral system

The Pressurestat Model

The pressurestat model is a model for the craniosacral system developed by the Michigan State University in the early 1970s to explain cranial bone movement.

The pressurestat model is a semi-closed hydraulic system. It can be compared to a container (membranes, especially dura mater) with in and out flow (of cerebrospinal fluid). The dural membrane forms the functional boundary of the hydraulic system, whereas the cerebrospinal fluid is the hydraulic part.

The anatomical components of the craniosacral system are:
1. meningeal membranes
2. cerebrospinal fluid
3. structures related to production, resorption and containment of cerebrospinal fluid
4. osseous structures to which the meningeal membranes attach (including non-osseous connective tissue related to the membranes)

Let us explore each of these in more detail:

1. The membranes of the craniosacral system include the dura mater, the arachnoid membrane and the pia mater.
These three concentric membranes (known as meninges) surround the central nervous system (brain and spinal cord).
The dura mater is the outermost of the meninges and forms a dense impermeable connective tissue envelope around the central nervous system (= CNS).

The pia mater is the innermost meninge and adheres to the contour of the brain and spinal cord. It extends into the spinal cord as septa, forming compartments.

Between these membranes is a transparent sheath, the arachnoid membrane, stretched over a delicate layer of reticular fibres, the arachniod trabecula. This forms a weblike membrane, which extends medially to the pia mater.

The space between the arachnoid and pia, filled with trabeculae and cerebrospinal fluid (= CSF), is called subarachnoid space. In an extension of the subarachnoid space called perivascular space arterial vessels enter the substance of the brain, taking arachnoid and pia with them.

Thus the CSF circulation penetrates into the brain tissue (neurons and neuroglia)! This means that increased pressure of the CSF or inadequate drainage can compromise brain function.

Between the dural and arachnoid is a fluid filled space called subdural space. These membranes glide freely and are filled with a watery fluid.

The subdural space provides venous drainage of the brain, also draining the spent CSF via bulk flow through the arachnoid villi.

2. The cerebrospinal fluid circulates through the centre of the spinal cord in the central canal, which extends cranially within the brain forming ventricles (a system of specialised hollows).

The ventricles are laid out in a three-dimensional T – formation. There are two ram’s horn-shaped lateral ventricles (largest ventricles). Each lateral ventricle communicates with the narrow, middle third ventricle by the interventricular foramina of Munro. The forth ventricle, also midline, is broad and shallow, a rhomboid shaped cavity overlying the pons and medulla and extending superiorly from the central canal of the upper cervical spinal cord to the cerebral aqueduct of sylvius. The aqueduct of sylvius communicates between the third and forth ventricles.

Three small apertures exit at the caudal aspect of the forth ventricle: the midline foramen of magendie and two lateral foramina of luschka, which communicate with the subarachnoid space.

3. The formation of CSF occurs in the temporal horns of the lateral ventricles, the posterior portion of the third ventricle and the roof of the forth ventricle (95% in the lateral and third ventricle). It is exuded continuously by the surface of the choroids plexus (cauliflower-like growth of blood vessels covered with thin epithelial cells) and circulated throughout the CNS in an ordered, one-way pathway.

About 500ml of CSF is secreted daily (about four times the body’s contents). Reabsorption of CSF occurs at the arachnoid villi. Arachnoid villi (granulation) are specialised cauliflower-like structures that act as pressure dependent one-way valves for the bulk of the CSF out of the subarachnoid space into the superior sagittal sinus. This is a venous sinus created by the ‘splitting’ of the falx cerebri where it forms a junction with the cranial dura in the brain’s midline fissure. This represents the endpoint of CSF circulation as it drains into the blood circulation.

Production by the choroid plexuses is more rapid than the constant CSF resorption into the venous circulation. This intake/outflow mechanism of CSF qualifies the hydraulic system as semi-closed.

The mechanics involved

The regulation of the rate of fluid uptake or decrease depends on stretch and compression receptors in the sagittal suture. Dr. Upledger was able to isolate a single nerve axon from the sagittal suture through the meningeal membranes to the choroid processes of the third ventricle in monkeys (collagen/elastic fibres, vascular plexuses and nerve endings were also identified).

With stretch of the sagittal suture to capacity by increased dural membrane pressure, neural signals are sent to the choroid plexuses to reduce or stop fluid production (=fluid inflow). When the cerebrospinal fluid ebbs, the sagittal suture (parietal bones) as a result compresses, a nerve signal is generated by the pressure receptors to produce cerebrospinal fluid again (inflow exceeds outflow and the internal pressure of the dural membrane sac rises again).

Another mechanism to regulate pressure in the dural system is the ball-valve like mechanism of the arachnoid granulation body, located at the floor of the straight sinus/great cerebral vein junction.

When engorged they act as valves to decrease the outflow from the great cerebral vein, increasing back pressure and in turn decreasing the secretion of cerebrospinal fluid. These feedback loops keep the cerebrospinal fluid in homoeostasis (=self-correcting balance).

Under normal circumstances the system operates on about a six-second cycle.
Cerebrospinal fluid is produced for about three seconds and then production is shut down for three seconds. This creates a rhythmical rise and fall of fluid pressure within the boundaries of the semi-closed hydraulic system. This is called craniosacral rhythm. The normal rate is 6-12 cycles per minute.

The systemic effect on the body

The rise of cerebrospinal fluid (=flexion) can be felt throughout the body as a minute external rotation ("outrolling") and an increase of the transverse diameter in the skull (shorter in the anterior-posterior dimension).

The fall of craniosacral pressure can be felt as minute internal rotation ("inrolling") of the body (=extension) and an increase of sagittal diameter of the skull (the head narrows and elongates).

A complete cycle is composed of one flexion and one extension phase. Between the end of one phase and the beginning of the next phase is a pause called neutral zone. It is likened to an idling of a motor in neutral.

The palpable motion (internal/external rotation) of the body is probably related to the effect of the fluctuations of the CSF upon the motor cortex under the coronal sutures, which in turn influences the tonus of the body tissues.

Transmission of pressure occurs also according to the law of fluid mechanics – any force to the CSF surface is transmitted equally in all directions within the boundaries of the craniosacral system.

The craniosacral membrane system consists of falx cerebri, falx cerebelli, tentorium cerebelli (=intracranial membrane system) and the dural tube membranes, like a semi-compartmentalised balloon.

Each of the intracranial membranes is continuous with the meningeal dura and through direct or reciprocal relationship with each other. Dr. Sutherland (the father of osteopathy) therefore called it "reciprocal tension membrane system".

4. The brain meninges adhere to the brain (pia-arachnoid) and the internal skull(dura).

The cranial dura is bilaminal with outer and inner meningeal layer, closely adherent except in specific areas, where they separate to form the venous sinus.

The endosteal layer, which forms the periosteum of the skull, adheres specially to the sutures, the cranial base, and the foramen magnum.

The cranial dura ensheathes the cranial nerves within their osseous foramina and fuses with each cranial nerve epineurium. It ensheathes the optic and olfactory nerves and it provides the roof for the sella turcica of the sphenoid (housing the pituitary gland) as an extension of the dura called diaphragma sellae.

The meningeal layer of the cerebral dura invaginates into the matter of the brain, forming four partitions, which divide the cranial cavity.

The sickle-shaped falx cerebri bisects the dome of the skull and the cerebral hemispheres through the sagittal plane (arching over the corpus collosum). It is formed by an invagination of the cranial dura covering right and left brain hemispheres, into the sagittal sulcus of the brain under the superior sinus.

At its inferior border, it splits once again to create the inferior sagittal sinus.

The falx cerebi is narrow anteriorly and attaches to the floor of the cranial vault at the crista galli of the ethmoid bone and the ethmoid notch of the frontal bone. The attachment then follows the midline superiorly inside of the cranial vault along the internal aspect of the metopic suture, under bregma, underneath the sagittal suture and under lambda to the internal occipital protuberance. The inferior free border of the falx cerebri affords passage for the inferior sinus. Posteriorly it widens in a lateral direction to form the superior layer of the leaves of the tentorium cerebelli. The inferior layer of the leaves of the tentorium cerebelli come together medially to form the other vertical component of the reciprocal membrane system - the falx cerebelli.

Where these membranes join, is a roughly quadrangular space occupied by the straight sinus running anteriorly-posteriorly from the internal occipital protuberance to the union of the falx cerebri, falx cerebelli and two leaves of the tentorium cerebelli.

The horizontal tentorium cerebelli divides the occipital lobes of the cerebrum above from the cerebellum below. Its central free edge forms a cresent-shaped opening. The antertior points of this central edge of the falx attach on either side to the anterior clinoid process of the sphenoid bone(=superior leaves).

Posteriorly, the peripheral tentorium attaches to the transverse ridges of the occipital bone and posterior-inferior angle of the parietal bone (containing the transverse sinus), whereas it attaches laterally to the petrous ridge of the temporal bones and the mastoid portion of the temporal bones. The peripheral tentorium then continues anteriorly to cross beneath its free border and attaches bilaterally at the posterior clinoid process of the sphenoid (inferior leaves). Lateral to the clinoid attachments, both leaves attach to the petrous ridge of the temporal bones. Here the tentorium encloses the superior petrosal sinus.

Continuing beneath the straight sinus and tentorium cerebelli, the falx cerebelli shallowly divides the cerebellar hemispheres through the midline plane established by the falx cerebri.

Posteriorly it attaches to internal midline ridge of the occiput containing the occipital sinus, and inferiorly attaches around the foramen magnum as a dense fibrous ring.

The spinal dura mater forms the tube downwards through the vertebral canal between the foramen magnum and the sacro-coccygeal complex named the core link. Inside the vertebral canal the dural attachments to bone are to the posterior bodies of the second and third cervical vertebra and to the posterior body of the second sacral segment (the sacrum rotates at that level about its axis). In the sacral canal the dura blends with the terminal pia mater called filum terminale. It exits the vertebral canal through the sacral hiatus (at the level of the forth sacral segment) to merge with the periosteum of the coccyx (coccyx fractures therefore can distort the tension of the membrane system as far up as the intracranial dura).
Otherwise the dura mater glides relatively freely within the vertebral canal to accommodate spinal movements only bound gently to the posterior longitudinal ligament by fibrous clips called dentate ligaments.

The dural sheath extends into the foramina of the vertebrae in each segment covering the spinal nerve and is finally blending into the paravertebral fascia. These tubular prolongations of the dura are more transverse at the upper levels of the vertebral canal. More caudally the prolongations are more longitudinally oriented. This allows greater transmission of tension into the tube from caudal than cranial regions.

Using the cranial bones and sacro-coccygeal complex as levers certain parts of the craniosacral membranes can be influenced with specific craniosacral techniques:

Within the intracranial dura the vertical system can be mobilized in anterior-posterior direction by a frontal bone lift, whereas the superior-inferior part gets mobilised by a bilateral parietal bone lift (a preparatory parietal compression to free sutures is usually helpful).

The horizontal part of the intracranial membrane system can be mobilsed in a anterior-posterior direction by sphenoid bone compression-decompression technique (utilising the clinoid attachments), and the lateral aspects of the horizontal part responds to a bilateral lateral temporal bone decompression technique called "ear pull" (utilising the petrous ridge attachments).

The spinal dural tube can be mobilised in a superior-inferior direction by occipital traction, whereas the inferior-superior direction is addressed by sacro-coccygeal traction (traction from caudal).

The craniosacral system is ultimately related to, influences and is influenced by the nervous, the musculoskeletal, the respiratory, the lymphatic, the vascular and the endocrine system.

Abnormalities in the structure of any of these systems may influence the craniosacral system, and that in turn has effects on the development and/or function of the central and peripheral nervous system.

Tightness in the musculoskeletal system like a piriformis muscle spasm, for example, will immobilise the sacrum unilaterally and therefore impede free motion of the caudal end of the craniosacral system.

Chronically tight suboccipital muscles (due to poor posture or whiplash injury) can compress the occipital base, wedging the atlas further onto the occipital condyles. This not only impedes free craniosacral motion, but also compresses the jugular foramina causing vagal disturbances(CNX), tightness in accessory nerves supplying trapezius and sternocleido muscles(CNXI) and compromising the function of the glossopharyngeal nerve(CNIX) supplied structures.

A tight intracranial system or a sphenoid dysfunction can affect the pituitary gland in the turkish saddle and consequently cause hormonal imbalances.

Respiratory problems like asthma or cystic fibrosis can affect the thoracic inlet (tight accessory breathing muscles) and overuse the respiratory diaphragm, causing chronic restriction in these two transverse diaphragms and thus creating a constant drag on the craniosacral system.



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