Numerous chiropractors, articles and texts discuss the so-called proprioceptive insult hypothesis of subluxation. This erroneous hypothesis first assumes that proprioceptors exist in spine, and second, that proprioceptors are somehow insulted.
Proprioception was defined by Sherrington in 1906 as, "the perception of joint and body movement as well as position of the body, or body segments, in space (1,p.84)." It is commonly thought that proprioceptors in joints and muscles receive proprioceptive input. This, however, does not occur and current neurology texts clearly demonstrate this fact. Neither Principles of Neural Science (2,p342) or Guyton's Basic Neuroscience (3,p.103) use the word proprioceptor in their respective lists of sensory receptors. The two major sensory receptors described are mechanoreceptors and nociceptors, both of which are found in skin, muscles and joints. Clearly, if proprioceptors do not exist, then proprioceptive input cannot occur.
Recall that proprioception generally refers to the perception of body position. Generally speaking, in order to perceive anything, some type of input is required. In the case of proprioception, it occurs in response to afferent input that is initiated in mechanoreceptors. In Principles of Neural Science, we are told that proprioception occurs as a consequence of afferent input. We are also told that mechanoreceptors are subserved by large diameter axons (A-beta or larger), and nociceptors are subserved by small diameter axons (A-delta or C). Three main types of peripheral receptors allow for proprioception to occur including: 1) mechanoreceptors in joint capsules, 2) mechanoreceptors known as muscle spindles, and 3) cutaneous mechanoreceptors (2,p.347). Clearly, it is mechanoreceptive input that results in the perception referred to as proprioception.
It should also be clear that the term "proprioceptive input" should not be used because it suggests that proprioceptors or some time of specific proprioceptive end organ is at work. This implies that receptors, commonly referred to as proprioceptors, can sense proprioception, which they do not. Mechanoreceptors can only sense mechanical deformation of the tissues in which they reside, and such stimulation ultimately results in proprioception.
When we refer to the word proprioception in discussions or articles, we should make sure that the receiver of the information understands that proprioception is a perception that occurs as a result of activity within the proprioceptive system. Explain that the proprioceptive system includes tissue mechanoreceptors, afferent pathways, the spinal cord, brain stem, cerebellum and cerebral cortex. The integrated function of all of these components results in the complex phenomena that we refer to as proprioception.
Due to the fact that there is no such thing as a proprioceptor, it is impossible for a proprioceptive insult to occur. A proponent of the so-called proprioceptive insult hypothesis may respond by stating, "so what, so it is really mechanoreceptive insult that occurs." No, mechanoreceptive insult is also an inaccurate and vague term. Mechanoreceptive insult suggests that mechanoreceptors can be insulted, wounded or damaged. Although mechanoreceptors actually exist, it is known that mechanoreceptors do not respond to tissue damaging stimuli (4). However, every good neuroscience book will state that nociceptors do respond to tissue damaging stimuli. After examining such texts, it will become clear that nociceptors are probably the only variety of receptor that is capable of responding to tissue damaging stimuli. This is not to suggest that only nociceptors are involved in nociceptive processes. Mechanoreceptors may also be involved in nociceptive transmission; however, the mechanism involves spinal cord synaptic activity rather than a peripheral sensory process (4). This mechanism is still being investigated.
It should be understand that nociception, or the reception of tissue damaging stimuli by a nociceptor can have serious neurological consequences. For example, Dr. John Bonica, one of the worlds most respected authorities in the field of nociception and pain, states that nociceptive input can cause pain, vasoconstriction, muscle spasm and also alter visceral function (5). Dr. Bonica has recently passed away; however, his work lives on through an organization he founded known as the International Association of the Study of Pain (IASP). All chiropractors interested in the neurological responses of damaged tissues (such as damaged and dysfunctional vertebral joints, for example) should join the IASP. For more information and an application, please call the IASP at (206)547-6409. Unfortunately, at this point time, there are only about 10 chiropractors in America who are members.
A question that might come to mind is, "are mechanoreceptors involved in subluxation?" The answer to this question depends on how you view subluxation. The answer is no if you view subluxation as a bone out of place that is putting pressure on a nerve. If, however, you view subluxation as a complex pathological process that involves joint hypomobility, then the answer is YES. This is because joint hypomobility is known to result in reduced mechanoreceptor afferent stimulation (6).
This information suggests that subluxation involves reduced mechanorceptor input and increased nociceptor input. Such abnormal afferent input, or dysafferentation, could potentially influence numerous centers within the central nervous system and result in a wide variety of neurological symptoms, all of which could be reduced if dysafferention was normalized by restoring joint function. Our job is to understand which neurological centers are influenced by nociceptive and mechanoreceptive input.
- Fisher, A. et al., Sensory Integration: Theory and Practice, F.A. Davis, Philadelphia, 1991
- Kandel, Schwartz, Jessel, Principles of Neural Science (3rd ed), Elsevier, NY, 1991
- Guyton, A., Basic Neuroscience (2nd ed), W.B. Saunders, Philadelphia, 1991
- Price, D., Psychological and Neural Mechanisms of Pain, p.86-87, Raven Press, New York, 1988
- Bonica, J., The Management of Pain (2nd ed), p.52, Lea & Febiger, Philadelphia, 1990
- Hooshmand, H., Chronic Pain: Reflex Sympathetic Dystrophy, CRC Press, Boca Raton, FL, 1993