Primary respiratory mechanism (PRM)

• The cranial rhythmic impulse (CRI) refers to the palpatory sensation of the widening and narrowing of the skull.

  • Normal rate: 8–14 times per minute

  • A result of the PRM

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• The PRM is composed of five elements :

  1. Inherent motility of the brain and spinal cord

     • Brain and spinal cord have an inherent wave-like motion.

     • Described as coiling and uncoiling of the central nervous system (CNS).

  2. Fluctuations of the cerebrospinal fluid (CSF): volume of CSF changes in relation to the cranial rhythmic impulse (CRI).

  3. Mobility of the intracranial and intraspinal membranes

     • The meninges surround the CNS and are made up of the dura, arachnoid, and pia mater.

     • A continuous connection is present from the foramen magnum to the cervical vertebrae and second sacral segment.

     • These membranes move due to the inherent motility of the brain and spinal cord and fluctuations of the CSF.

     • Motion of these membranes causes the cranial bones and sacrum

        to move in relation to one another.

  4. Articular mobility of cranial bones

     • Although cranial sutures fuse shortly after birth, they contain small motions that cannot be felt individually.

     • An amalgamation of the cranial bones and multiple sutures allow for palpable motion.

  5. Involuntary mobility of the sacrum between the ilia

     • The dural connection between the cranium and S2 of the vertebral column causes sacral movement.

     • This dural connection is termed the reciprocal tension membrane (RTM).

     • The bones move in rhythm with the motion of the shifting tensions of the RTM.

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• The primary physiologic motion is between the articulation of the sphenoid and the occiput called the sphenobasilar synchondrosis.

  • Moves in a biphasic cycle (flexion and extension) in response to the pull of the reciprocal tension membrane and fluctuations of the CSF.​

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