The functional anatomy of the nervous system includes mechanisms to allow adaption to body movements. Injury or impairment of these mechanisms may lead to symptoms. Clinicians using tension tests as part of assessment and treatment have noted that altered nervous system movement and extensibility is a very frequent finding in many disorders. This paper describes a new model for assessment and treatment of mechanical disorders of the nervous system that is based on clinical observations and interpretations of anatomical, biomechanical and pathological literature. A broad approach is outlined which provides an insight into the possible mechanisms by which the nervous system can be responsible for symptom production. The concepts of intraneural and extraneural pathology are put forward and related to assessment and treatment.