Basic Neurology Underlying the Contracted Leg in Thompson Technique
John Minardi BHK, DC.
When a subluxation exists, the aberrant mechanics associated with the misalignment produces a distortion of the muscle, facet joint, intervertebral disc and tendon attached to the segment. This distortion/change is detected by the nuclear chain and nuclear bag fibers within the muscle spindle, the GTO within the tendon, and proprioceptive receptors located in the facet capsule and intervertebral disc.
All four of these structures are innervated by Type 1 and Type 2 nerve fibers. When the distortion produced by the subluxation is detected, these structures send this information via the Type 1 and Type 2 afferent fibers.
These nerve fibers send this information to the cerebellum through two primary ascending tracks; the ventral and dorsal spinocerebellar tracts. These tracts gather this proprioceptive information and send it to the cerebellum. The cerebellum now has the information concerning the distortion from the subluxation This cerebellar information is sent to the thalamus, which takes the information and sends it to the cortex. The cortex contains information about what should be happening within the body (ie. the distortion should not be occurring).
If the information of the cerebellum and motor cortex are the same, then everything is normal and no response is sent.
However, if the information differs, which is the case with a subluxation, then the cortex will send a response through the brainstem using the extrapyramidal tracts, the most important of which for the short leg phenomenon is the vestibulospinal tract.
The vestibulospinal tract is most important with regards to the short/contracted leg because unlike the other spinal tracts, the vestibulospinal tract is always excitatory and always facilitates postural muscles, primarily located in the lower limb and pelvic girdle. Thus, when this descending tract fires, it results in a physiological shortening/contraction of the extrafusal fibers of the postural muscles, thus causing a short/contracted leg.
This is why we are always looking for the short leg, and never the long leg, because we are looking for that facilitative shortening of those postural muscles because of the vestibulospinal tract.
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