The Neurology of the Pettibon Weighting System
TECHNIQUE
Chris Cohen
DC, CCCN, DACNB
The Pettibon Weighting System is a powerful tool in the correction of spinal structure and dysfunction. Applied correctly, it will predictably and repeatedly make profound structural change that can be measured and appreciated by clinicians and patients. While the concept of the weighting system to correct posture is very simple, the application and mechanisms involved are multifaceted and complex. Posture and postural controls are largely motor-function based on a proper balance of flexor and extensor tone.
The functions of the brain can be divided into either sensory or motor, and the dominant numbers of neurons are sensory in function (approximately 80% sensory) versus motor (for the remaining 20%). In other words, we
are sensory-driven beings. Dr. Pettibon has pointed out that humans react in time and in need to the enviromuent (under the direction and control of the nervous system). There are two constant stimuli: gravity and movement. If we wish to change the motoric output, then it only makes sense to give the brain/nervous system different sensory information to work with, or in other words, alter the stimulus. The three most vital functions (neurologically) are joint position sense, vestibular sense, and visual target holding or tracking. The weighting system is a primary somatosensory stimulus that must involve both local and global responses. One mechanism involved that will be discussed in this article is the more local response but with full recognition that no one part of the body is separate from the ultimate connection the nervous system provides.
A muscle contains what may be called a sensory side or intrafusal system. It is the muscle spindle that monitors change in length and the rate of change in length of a muscle. It communicates through sensory fibers and directly excites alpha motor neurons that innervate the same muscle, as well as gamma motor neurons that innervate the polar ends of the spindle itself so that it maintains sensitivity (adjusts the gain) of the muscle spindle to continue to monitor change in the length of the muscle. The extrafusal system is referred to as the “work part” of the skeletal muscle, but it does contain sensory fibers too. My focus in the extrafusal portion will be on the Golgi tendon organ (GTO) and the inhibitory reflex response. The GTO monitors tension and change in tension of a muscle and inhibits alpha motor neurons by action of an inhibitory intemeuron in the spinal cord. The difference in an excitatory response during a myotatic (deep tendon) reflex and the inhibitory response during a chiropractic adjustment will be discussed first for clarity of normal function, and then the weighting application will be discussed. The author’s opinion is that the extraordinary corrective power of the Pettibon Weighting System is seated in the excitatory pathways of the
^ ^ The alpha motor neuron is the second half of the cord reflex that causes the muscle to contract, hence the spinal reflex response of the muscle.
dynamic (and static) stretch responses locally and the cerebellum globally. In truth, these systems are not so separate but would need to be addressed individually for clarity of discussion.
Let’s begin with the excitatory pathway. When a muscle, for example the biceps, is struck with a reflex hammer, the sudden change in length is registered by the intrafusal system of the muscle spindle. This is both the annulospiral endings of the nuclear bag and the nuclear chain of the muscle spindle. The mechanism of the dynamic stretch response is from the annulospiral endings of the nuclear bag, while the longer lasting static stretch response is via the nuclear chain. These afferents are group A, type 1 neurons with transmission speeds of 80 to 120 meters per second. The 1-A afferents enter the dorsal horn of the spinal cord and synapse in the ventral horn onto alpha and gamma motor neurons. Alpha motor neurons are also group A, type 1 neurons. The gamma motor neuron is a group A, type 2 neuron. The gamma motor neuron fires to the polar ends (contractile part) of the muscle spindle for maintaining the gain (sensitivity) of the sensory system. The alpha motor neuron is the second half of the cord reflex that causes the muscle to contract, hence the spinal reflex response of the muscle. This is a “dynamic stretch response.” This response happens when the muscle length is changed, whether slowly or quickly and intentionally or unintentionally. This happens at the spinal cord level, but it is important to note that the afferent fibers do send a branch through the dorsal spinocerebellar tract to the ipsilateral cerebellum.
When the muscle spindle detects change that is too great for the muscle to contract against (excessive load by either weight or speed of change), then the system “off loads” to the Golgi tendon organ (GTO). The GTO is the mechanism of the inhibitory pathway and is the sensory portion of the extrafusal system. It monitors tension and change in tension on the muscle. It fires via a 1-B afferent, also a group A, type 1 neuron and is inhibitory to alpha motor neurons. Its path is through the dorsal (sensory) root and synapses—not onto the alpha motor neuron directly but rather onto an inhibitory interneuron
called a Renshaw cell. The Renshaw cell then fires onto the alpha motor neuron for inhibition of the muscle. The inhibitory pathway is protected from forces that would otherwise overload the tensile capacity and cause tearing of the muscle. When the load on the muscle is too great, the GTO fires and inhibits the alpha motor neuron, shutting off the muscle. In specific regard to the adjustment, the load on the muscle caused by the speed as well as force of the adjustment must be enough to cause the GTO to inhibit the muscle. Most of us have experienced at least once when the adjustment lacked either the speed or force to cause the GTO to inhibit the muscle response, and what we end up with is a “stinger” of an adjustment. When the adjustment is sufficient to activate the inhibitory pathway, then the adjustment generally “feels” better and the muscles typically relax.
So to summarize what has been covered so far, the muscles respond to change in length and change in tension. Muscle spindles monitor length and change in length, and when activated, they cause activation of the alpha motor neuron and contraction of the same muscle from which the sensory signal began. Golgi tendon organs monitor tension and change in tension of the muscle, and when they are activated, they cause, through a Renshaw cell, inhibition of the alpha motor neuron and stop the contraction of the muscle. So this brings us to the weight-
ing system. I will keep the discussion to the head weights for simplicity, but realize that the same neurology applies to the shoulder and hip weights as well.
The head weights are a primary corrective tool for forward head posture (FHP) or anterior head carriage. Forward head posture is almost always found with a varying degree of cervical hypolordosis, straightening (aka military neck), or curve reversal. In some instances, it is possible that the cervical lordosis is preserved or even hyperlordotic due to thoracic hyperkyphosis. Head weights are applied to the front of the head for correction of FHP. Applying the head weights to the front of head might seem counterintuitive until we recognize the body is living and reactionary. It is the sensory-driven response we are counting on to make the change in tone and ultimately structure. The head weights cause a slow stretch and a subthreshold change in tension of the cervical extensor muscles. In other words, it is desirable to have the muscle spindle activation, but the head weight must not be so heavy that the GTO actively inhibits the extensor muscles. The slow stretch causes activation of the 1-A afferent neural fibers and then reflexive activation of the same muscle by way of the alpha motor neuron. Correctly applied weights will give a sensory stimulus (slow stretch of the muscle and subthreshold increase in muscle tension) and cause a new reflexive motoric output
(contraction of the extensor muscles) sufficient to change the position of the head. It should be noted that the weights cause a change in the pitch plane (“yes” motion) of the skull for a frontal head weight and can just as effectively be used to change roll (head tilt) and yaw (head turning) as a coupled motion with laterally placed weights. These motions are accomplished with the same muscle stretch responses on a local level.
References
1. Guyton and Had Medical Physiology, Chapter 54.
2. Principles of Neuroscience, Eric Kandel, Ch 35
3. Physiology of the Joints, Vol 3., Kapandji, I.A, pp. 214-219
"Chris Cohen, DC, CCCN, DACNB, is a 2001 Palmer graduate. He worked with Dr. Burl Pettibon for several years and has taught thousands of hours of spinal biomechanics and structural correction. He is board certified in chiropractic neurology and is in private practice in Gig Harbor, WA. "
email: drcohen@backbonerestored. com phone: 253-858-2474
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