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Principles of Manual Medicine

Facilitated Segment

The relationship of the thoracic spine to the sympathetic division of the autonomic nervous system is important because visceral afferent pathways usually follow the sympathetic pathways. Visceral disturbances often result in increased musculoskeletal tension in structures innervated from the corresponding spinal level through viscero-somatic reflexes.

For example, abnormal levels of afferent activity occuring at one region of the body (for example, the intestines) can maintain a pool of neurons in another region of the body (for example, muscles of the rib cage) in an elevated state of sensitivity. In this state, small additional increases in afferent activity may result in an exaggerated response at the sensitized site. Once a sensitized state is established, the spinal segment is said to be facilitated. Facilitation may lead to alterations in muscle tone that may result in stiffness, spasm, and pain. It is essential for the osteopathic physician to understand and have committed to memory the somatic regions that are likely to be effected by viscero-somatic reflexes resulting from visceral pathology (Click here to open the sympathetic drill exercise). Manipulative treatment can be used to reduce afferent input from associated facilitated structures, which can help to restore normal reflex activity by reducing somato-sympathetic activity to the affected viscus.

In order to better understand the concept of the facilitated spinal segment, it will be helpful to review some fundimentals of spinal reflex activity.
  1. The somatic (monosynaptic) reflex arc (Click here to review a simple somatic reflex arc).
  2. The anatomy of the sympathetic ganglionic trunk (Click here to review the anatomy of the sympathetic ganglionic trunk).
  3. The sympathetic reflex arc (Click here to review a simple sympathetic reflex arc).

Dysfunction in visceral structures and somatic structures can adversely influence the functionality of visceral and somatic structures that are segmentally related. Some common examples of this are listed below:

Somato-somatic reflex - occurs when somatic stimuli produce patterns of reflex activity in segmentally related somatic structures. For example, neurons in the spinal cord may receive abnormal levels of afferent activity from muscle spindles as a result of injury. Segmentally related somatic fibers may then stimulate muscles fibers resulting in tightness and pain.

Viscero-cutaneous reflex --- occurs when localized visceral stimuli produce patterns of reflex activity in segmentally related visceral structures. For example, preganglionic autonomic neurons in the spinal cord may receive afferent activity from the intestines as a result of intestinal gastritis. Segmentally related postganglionic fibers in the sympathetic trunk may then stimulate cutaneous receptors resulting in dilation of blood vessels and reddening of the skin.

Viscero-somatic reflex - occurs when localized visceral stimuli produce patterns of reflex activity in segmentally related somatic structures. For example, preganglionic autonomic neurons in the spinal cord may receive afferent activity from the intestines as a result of intestinal gastritis. Segmentally related somatic fibers may then stimulate abdominal muscles fibers resulting in tightness and pain.

Cutaneo-visceral reflex -- occurs when localized cutaneous stimuli produce patterns of reflex activity in segmentally related visceral structures. For example, preganglionic autonomic neurons in the spinal cord may receive afferent activity from cutaneous thermoreceptors. Segmentally related postganglionic fibers in abdominal ganglia may then inhibit intestinal motion caused by pathology such as gastritis, reducing pain caused by convulsive contractions.

Viscero-visceral reflex -- occurs when localized visceral stimuli produce patterns of reflex activity in segmentally related visceral structures. For example, preganglionic autonomic neurons in the spinal cord may receive afferent activity from the viscera. They may excite postganglionic autonomic neurons (sympathetic efferents) which may result in an undesirable immobilization of the intestines following surgery.

 

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