Citation

Reichel SM. Proc. Am. Assoc. Automot. Med. Annu. Conf. 1966; 10: 121-125.

Copyright

(Copyright © 1966, Association for the Advancement of Automotive Medicine)

DOI

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PMID

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Abstract

The stretch reflex has long been recognized as a conservator of cortical energy as it is an aid in maintaining posture without conscious effort. It is a simple monosynaptic spinal cord reflex which is initiated by a passive stretching force applied to a muscle, or a portion thereof, which stretches the stretch receptors known as muscle spindles in that muscle, or portion thereof. When stretched, the muscle spindles discharge sending stimuli to the spinal cord where there is synapse with anterior horn cells, resulting in a reflex contraction of the same muscle fibers which were stretched. While the knee jerk is perhaps the most familiar of the stretch reflexes, it is a fact that all of the voluntary muscles of the body possess muscle spindles and thus are equipped to respond to passive stretching forces with a reflex muscle contraction. However, there are wide variations in the response of different voluntary muscles. The stretch reflex has certain important characteristics which must be appreciated in relation to auto injuries. First, the muscle must be stretched passively or there will be no reflex contraction. When a person voluntarily causes a muscle to stretch, even violently, there is no reflex contraction, for, under volitional activity, the extra-pyramidal nervous system inhibits the stretch reflex. Second, the stretch reflex is most active in those muscles which counteract the force of gravity, the extensor muscles. Third, the more powerful and abrupt the stretching force, the stronger the reflex contraction of the muscle being stretched.

A spinal cord reflex that is similar to the stretch reflex in some respects, but is not initiated by receptors in the muscle tissues, is the extensor thrust reflex. This reflex is initiated by pressure exerted passively upon the pressure receptors on the bottom of the foot, the Pacinian corpuscles. Impulses from the pressure receptors pass upward to the spinal cord where they stimulate internuncial cells and then anterior horn cells, resulting in reflex contraction of the extensor muscles of the lower extremity. The reflex also normally acts as a conservator of cortical energy, for example in walking when, at the moment the bottom of the foot touches a surface the limb reflexively stiffens into extension, while, when the foot is lifted, the extensor muscles relax.

The stretch reflex, in addition to its well known role as a postural reflex, must now be recognized as a potent protective device which is in a state of instant readiness to protect vital organs as well as the skeletal system against injury due to unexpected, violent, passive movements of parts of the body. The reflex contraction of muscle fibers in opposition to the stretching force dissipates much of the inertial energy of the part and produces deceleration. Where powerful forces are involved, muscle fibers are selectively injured, with subsequent appearance of lesions of traumatic fibrositis.

The symptoms of moderate auto injuries are often noted to be bizarre and this is due to the unique characteristics of the muscle lesions, as well as reflex autonomic malfunction which may have its onset long after the accident. Impairment of sensation in a non-anatomic distribution may be of autonomic origin rather than hysteria. Through reflex autonomic involvement, organ systems distant to the site of injury may be impaired, with reduction in the capacity of the entire person to function.