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The Organization of Movement The Organization of Movement Four Talks on the Primary Control Part 2: Stretch Reflexes and the Musculoskeletal Framework: How Stretch Reflexes Convert the Musculoskeletal System into a Spring-like Framework by Theodore Dimon In Part 1 (AmSAT Journal #3, Spring 2013) we saw that muscles form a complex webbed system that, working in conjunction with the latticework of bones, produces a network of support based on an architectural structure of elastic stretch. This background system is crucial to how we move; no specific movement can take place without this larger network of support. For the system to work, Theodore Dimon however, it is not enough for muscles to be elastic, important though this may be. They must also maintain tone in response to the stretch exerted by the bony members of the skeleton in order to support the skeleton and also to shorten to produce movement. How do they do this? If I lift my arm, the movement is produced by the contraction of muscles that are operated by motor nerves. A nerve impulse beginning in the higher cortical centers of the brain carries a motor message to the muscles that combines with other nervous impulses to produce coordinated movement. But movement is more complicated than that. We saw in Part 1 that specific movement takes place in the context of a larger system of muscular support. For instance, if I simply raise my arm, many muscles are involved in the support of the shoulder girdle and in the postural support of the body as a whole. This process is far too complex to be directed piece by piece. We never just contract one muscle; the entire support system must constantly adjust itself in relation to whatever we are doing as the background against which the specific contraction takes place. This overall support, which produces what we all know as posture, is the work of stretch reflexes. Muscle Sensors and the Stretch Reflex The stretch reflex is the automatic contraction of a muscle in response to being stretched. The most familiar example of a stretch reflex is the knee-jerk test: the doctor uses a rubber mallet to tap the patellar tendon just below the kneecap, stretching the quadriceps muscle, which contains sensors that register the change in length due to the stretch. These sensors send an impulse to the spinal cord reporting the change in length, which in turn excites the motor nerve innervating the quadriceps and causes the muscle to contract, eliciting the kneejerk response (Fig. 1). In this example, the doctor artificially produces a stretch reflex for the purpose of testing muscles and reflex responses, but the real function of the stretch reflex is to maintain stability of body parts. When you are standing, gravity is acting on your body and causing many of your joints to buckle, including the knees. This buckling of the knees causes the quadriceps muscle 16 to stretch––just as the doctor’s mallet did. The muscle, registering this change, sends an impulse back to the spinal cord, which in turn sends an impulse to the muscle telling it to contract. This contraction of the quadriceps keeps the knee from buckling, thus maintaining stability in the leg. In its simplest form, then, the stretch reflex is a basic reflex arc designed to respond to stretch—not just in the knees but throughout the body—so that parts of the body that are buckling can be stabilized, thus maintaining posture. In this way, the body’s elegant elastic system maintains constant support and tone. Fig. 1. The knee-tendon reflex There is a great deal more to say about this process, but first I want to address the basic nature of a reflex arc. Figure 1 shows the knee-tendon reflex. The quadriceps muscle on the thigh is charged with the task of keeping the leg straight at the knee. When the knee buckles, the quadriceps is stretched and, in order to maintain the support of the leg, contracts in response. Why should it be necessary for a nerve in the muscle to send a sensory signal to the spine, and for another nerve in the spine to receive this impulse and send a motor signal back to the muscle, when the muscle could just respond directly and avoid all that unnecessary and apparently redundant signaling? To understand this, we must remember that muscles are served by motor nerves with cell bodies located in the spinal cord and axons that project out from the spinal cord as peripheral nerves to the muscles. Signals for muscles to contract originate either in the spinal cord and travel to the muscle through the peripheral nerves or originate higher up in the brain and travel down to these peripheral nerves. In any www.AmSATonline.org AmSAT Journal / Fall 2013 / Issue No. 4 The Organization of Movement case, the muscle receives the impulse to contract from a motor nerve located in the central nervous system. The muscle does not know when and how much to contract; that information can only come from sensors in the muscle that register stretch in the muscle. So the spinal cord must first receive an impulse from the muscle sensor telling it when contraction is required, and then send the motor signal to the muscle (Fig. 2) telling it how much to contract. hands, which do not seem to need postural support, have background tone that is maintained by stretch reflexes. In this sense, every part of the body participates in postural support–– the arms, hands, and face no less than the legs and back. Sherrington found that stretching a muscle not only caused the muscle to resist the buckling of the limb by contracting, but also caused related muscles to contract and support the activity of the extended limb. At the same time, activity in the antagonistic or opposing muscles was inhibited or prevented––a phenomenon he called reciprocal innervation.4 Although the stretch reflex forms a simple reflex arc, it is wired up to similar and opposing groups of muscles so that, even at the spinal cord level, the coordination of the reflex begins to get rather complex (Fig. 3). Fig. 2. The reflex arc Sherrington and the Study of Posture C.S. Sherrington, a major founding figure in the study of neuroscience, was the first to identify and describe the stretch reflex. In trying to determine what parts of the brain were responsible for movement, Sherrington conducted experiments on rabbits and cats by operating on the brain. When he transected the brain of a cat just above the level of the midbrain, so that only the brain stem remained, the animal was incapable of spontaneous action; yet it extended its limbs in an exaggerated way and would stand indefinitely in forced extension until it was knocked over––a condition he called decerebrate rigidity. Furthermore, trying to flex the cat’s limbs heightened the contraction of the muscles so that the leg forcibly resisted his efforts. 1 His experiments demonstrated three things: First, the exaggerated extension of the limbs was not caused by the voluntary part of the brain, but by automatic, constant signals from the brain stem that maintained tone in the limbs––what Sherrington called tonic activity, and which he distinguished from more active contraction of muscles.2 This concept of constant, low-level tone in muscles is still widely accepted today. Second, when he tried to flex the decerebrate animal’s limb, the resistance to flexion indicated that receptors in the muscles themselves must be activating the muscles that extended the limb, which meant that the main source of this activity must be proprioceptive outflow from stretch receptors in the muscles.3 Third, these stretch reflexes clearly played a role in maintaining posture, since the extended limbs were resistant to buckling at the joints and therefore helped to maintain the animal’s upright support against gravity. It is important to mention that the stretch reflexes are not confined to the extensor muscles that keep our joints from buckling. In fact, stretch reflexes act on virtually all the different parts of the body, helping to maintain its stability. For instance, the shoulder girdle is supported by muscles acting upon it from various directions. Even freely hanging arms and AmSAT Journal / Fall 2013 / Issue No. 4 Fig. 3. The spinal circuits in the stretch reflex system at the elbow: a. motor neuron to same (homonymous) muscle; b. motor neuron to related (synergistic) muscles; c. inhibition of motor neuron to opposing (antagonistic) muscle Since all this was discovered many years ago, we have learned much more about posture and reflexes. T.D.M. Roberts, for instance, points out that balance is maintained not only by automatic stretch reflexes, but also by anticipatory and learned responses,5 which play an important role in posture and support. But underlying all of these responses are stretch reflexes, which are operating constantly throughout the body as the basic functional unit of the postural system. As we have seen, these reflexes are wired at the spinal cord level so that even the www.AmSATonline.org 17 The Organization of Movement higher functions are built upon and work in conjunction with stretch reflexes, adjusting some and turning off others to produce a wide variety of voluntary movements in the context of postural support. Basically our complex neural wiring involving muscle spindles, afferent nerves, spinal cord, and motor nerves keeps the whole body taut and the limbs engaged but flexible and fluid when necessary, like the strings of a marionette. The higher centers of the nervous system––the brain stem, cortex, and cerebellum––work with and build upon these circuits. The entire muscle system, then, is invested with sensors that play a crucial part in our postural support by enabling muscles to sense changes in length and send messages that, in turn, trigger muscle activity designed to counteract these changes in length and maintain postural tone. Coupled with the elastic and energy-storing potential of muscle tissue, the stretch reflex system converts the musculoskeletal structure into a spring-like framework capable of automatically supporting the body against gravity and stabilizing all the parts of the body in movement. Wrapping around the muscle spindle is an afferent nerve that carries sensory information to the spinal cord. The part that wraps around the spindle is called an “annulospiral” receptor because of its shape (Fig. 4b.). When the larger muscle fibers are stretched this also stretches the intrafusal fibers that make up the muscle spindle and elongates the annulospiral receptor endings wrapped around it. This activates the nerve, which sends an impulse to the spinal cord, where it synapses with the motor nerve innervating the same muscle, telling the muscle to contract. In this way, the muscle spindle functions as a very sensitive organ for detecting changes in the length of a muscle and sending an impulse to the spine, which in turn “reflects back” a motor impulse to the same muscle. Muscles Spindles and the Reflex Arc How do stretch reflexes work? We saw a moment ago that stretch reflexes operate as a reflex arc that begins with a sensory signal sent by a stretch receptor in the muscle. The stretch receptor, called a muscle spindle, is an amazing little organ (Fig. 4a.). It is called a muscle spindle because of its resemblance to a textile spindle; that is, it is shaped like a cylinder that bulges at the middle and tapers toward the ends. Fig. 4b. Annulospiral receptor wrapping around the intrafusal fibers that make up the muscle spindle Fig. 4a. Muscle spindles located on the main muscle fibers The spindle is made up of a bundle of very tiny contractile muscle fibers––about three to ten in number––that attach where they taper at each end to the connective tissue that binds together the fibers of the main muscle. These tiny fibers, sometimes called intrafusal fibers to distinguish them from the extrafusal fibers that make up the main muscle, are much smaller in diameter and length than the extrafusal fibers––they are only four to ten mm in length. 18 This reflex arc operates as a negative feedback loop. When, for instance, we are standing and our knees buckle from the force of gravity, this buckling stretches the quadriceps muscle, which stretches the muscle spindle, activating the annulospiral receptor wrapped around the spindle, which sends impulses to the spine and synapses with motor nerves that, in turn, tell the quadriceps muscle to contract. This muscular contraction reduces stretch on the spindle so that the annulospiral receptor stops firing––a negative loop that is turned on by stretch and stopped when the muscle contracts and the spindle is no longer being stretched. This feedback loop thus operates continuously to maintain stability in the joints (Fig. 5 ). www.AmSATonline.org AmSAT Journal / Fall 2013 / Issue No. 4 The Organization of Movement Fig. 5. Negative feedback loop of the stretch reflex arc: a. flexor muscles keep the arm flexed at the elbow; b. muscle is stretched, muscle spindle fires and activates reflex arc; c. motor neuron fires, muscle contracts, spindle is no longer stretched and stops firing Antagonistic Action and Stretch Reflexes But this stretch reflex system cannot work properly if the muscular system is not working efficiently. In Part 1, we saw that the musculoskeletal structure can be described as a system of tensile components and struts in which the tensile components support the struts, but the struts maintain length on the tensile components. If this system is interfered with––in other words, if we tighten the neck and interfere with head balance so that the back muscles are shortened––muscles throughout the body must compensate to maintain upright support. When this happens, the muscles can no longer lengthen and the spindles no longer register stretch. Parts of the stretch reflex system, quite literally, shut down. When this situation occurs, the remedy is to restore length to the system. This can be achieved by placing the skeleton in supportive positions in order to restore length to particular muscle groups that have forgotten how to maintain length and perform their supportive function. This restores natural tensegrity support and increases the stored potential energy in muscles, which imparts more spring and elasticity to the framework. But because the tensegrity system is invested with sensors that register stretch, restoring muscle length also has an effect on the stretch reflex system. Stretch reflexes that previously were not operative begin to work, the body begins to feel lighter, the spine regains a kind of inward buoyancy, and muscles that were flaccid and tight spontaneously tone up and release. These changes in the musculoskeletal system can be observed and felt from head to toe. We begin, as we say, to “go up.” What has happened is that the condition of elasticity and release in muscles, which is part of the tensegrity design, has stimulated stretch reflexes. We might assume, because muscle spindles are capable of adjusting to the shortened condition of muscles, that the stretch reflex is always operative. However, when muscles become too shortened, the spindles cannot adjust and the stretch reflexes shut down. To work properly, the AmSAT Journal / Fall 2013 / Issue No. 4 stretch reflexes require both the elasticity of muscles and the dynamic relationships between muscles and bones that are established when the system works as a whole. Without these conditions, the reflexes are inoperative; when these conditions are restored, the stretch reflexes are stimulated, and the muscular system regains its automatic, buoyant support. Stretch reflexes, then, do not work independently of the condition of muscles, but only function properly when muscles are elastically supporting the skeleton. If this network is not operating elastically, the muscular support system is not properly activated, and effortless support is replaced with chronic contraction of muscles, which are now needed to maintain upright posture; some stretch reflexes end up operating in a compensatory mode and many do not operate at all. When the system is restored, the stretch reflexes are activated, muscles no longer need to forcibly contract, and muscles perform their naturally supportive function with more length and tone. In short, elasticity in the context of the architectural whole is the key to the stretch reflexes operating effectively to provide muscular support of the whole with a minimum of effort. Stretch Reflexes and Muscle Tone Knowing how stretch reflexes work is essential to understanding what makes muscles healthy. In Part 1, we saw that we cannot describe a muscle as being healthy in the absence of elasticity. A muscle is certainly not healthy simply because it is built up and can perform work or because it is relaxed and can be mechanically stretched. For muscle tissue to be healthy, there must be an active relationship between length and tension. For the stretch reflex system to operate fully, and for muscles to contract most effectively, they must function antagonistically within the context of a skeletal framework that imposes stretch so that the muscle spindles can sensitively register changes in muscle length and respond with a lively, spring-like activity. This is a key part of what makes muscles healthy and toned. www.AmSATonline.org 19 The Organization of Movement An Alexander Technique teacher can detect the presence of this lively tone and the life in the muscle. This is not just fanciful or metaphorical. It is an observable condition in which muscles are lengthening, the spindles are firing, and the contractile components of the muscles maintain a healthy amount of resistance against lengthening—in other words, underlying electrical and chemical activity combines with an elastic component to produce an overall lively, flexible quality in the muscle that can be felt. And it cannot be achieved through any sort of treatment, bodywork, or exercise, but only in the context of active support and movement in the gravitational field. Because the system works as a whole, it cannot be put together piecemeal by trying to correct one part or another; all the parts must be readjusted in relation to gravity as an interrelated whole. Reciprocity of Nerves and Body Although neuroscientists have explained a great deal about how muscles spindles work, their work has not yet taken into account the functioning of the body as a whole. When we consider the reflex arc, it is easy to assume that it is basically a two-way street—a sensory response to stretch in muscle followed by a motor impulse that is reflected back to the muscle. But in order to work properly, muscle spindles must operate in the context of a system of muscle pulls and stretches The Alexander Technique Understanding this principle of muscle function was one of that maintain the proper organic conditions under which the Alexander’s great accomplishments. Fully one half of the muscle spindles are at their most sensitive and the muscles at Alexander work is about restoring the postural/stretch system to their most healthy and responsive. Stretch reflexes are designed its normal working; the other half is about gaining conscious to respond automatically to stretch, but they function as part of control over this system so that we the dynamic interplay between the can apply this knowledge in our daily signaling to and from muscles and “The idea is to restore the stretch reflex activities. When we are born, skeletal the state of the musculoskeletal system, including the ability of muscle support imparts a natural length to system as a whole and cannot be to sensitively register changes in length, our muscles so that our inborn, expected to work normally if this natural reflexes can respond to system is imbalanced. which is a big part of their function––and stretch all over the body, making it One of the things this shows is a big part of what the Alexander possible for the system to provide that awareness alone cannot improve Technique is all about.” support in an efficient and effortless muscular and motor functioning. We way. Habitual tightening of muscles often hear Alexander Technique however––as well as other influences in life––begin to interfere spoken of as a kinesthetic method, which of course it is. Other with this system, and the body has no other alternative but to methods, such as Feldenkrais and relaxation techniques, also maintain support by replacing stretch with muscle tension. At utilize kinesthetic awareness to reduce unnecessary muscle first, this may not be a problem; but over time, the tension. We receive proprioceptive information from muscle compensations become chronic and the muscles do not have the spindles, but if muscles are chronically tight, this information is length they need for the stretch reflexes to be activated. We are drastically reduced and less accurate. This means that unless we then faced with the dual disaster of losing tone in many of the establish lengthening of muscles as a precondition for healthful muscles needed to support the body against gravity, and other function, trying to be aware of muscles is virtually useless, not muscles working far too much and becoming chronically tight. to mention misleading. Before we can rely on kinesthetic input, The body cannot find a way to recover and the postural system we must first restore length to muscles and, as much as cannot work correctly. New activities, meanwhile, are possible, remove the interferences with normal and natural undertaken in an increasingly harmful way until, with time, muscle tone. Awareness is not the first step in the process––it dysfunction and collapse result. This disarrangement is most depends on basic organic conditions of elasticity in muscles, easily seen in the pulling back of the head, the shortening of the which is why awareness must be based on re-education. spine, the overworking of the lower part of the back, and Summary collapse of other parts of the body. The antagonistic stretch on To summarize, the body is organized as a tensegrity the muscles is compromised, the stretch reflex system cannot structure; stretch reflexes convert this structure into a springwork properly, and the system, unable to work as it is designed like framework. Essential to this system is the elastic condition to work, goes into collapse and fixation. of the muscles that erect the tensegrity structure, so that all the It is at this point that re-educational work is needed, muscles, tendons, and ligaments––the tensile parts of the beginning with some form of mechanical support (such as system, the guy wires––perform work appropriately and the sitting with a supported back or lying in a semi-supine workload is distributed across the whole network. This position). The idea is to restore the stretch reflex system, elasticity imparts potential energy to the muscles, since muscles including the ability of muscle to sensitively register changes in store energy when they are lengthened; the resulting rebound length, which is a big part of their function––and a big part of effect helps to maintain the integrity of the system. what the Alexander Technique is all about. One option is to Loss of length in muscle means that the structure cannot address this by stretching and strengthening muscles, but this support itself properly, with the result that various guy wires will not work, because the system is designed to work as a take on too much load; it also means loss of spring-like support whole, with muscles functioning antagonistically in the context and potential energy in muscle. We experience this in the of postural support. No amount of strengthening, exercising, heaviness and stiffness in our legs as we age, in contrast to the relaxing, or balancing muscles can restore this condition when spring-like legs of children. the system is so maladjusted and sensory input so distorted. Continued on page 22. 20 www.AmSATonline.org AmSAT Journal / Fall 2013 / Issue No. 4 The Organization of Movement, continued from page 20. At the same time, muscles must maintain tone to stabilize the tensegrity structure, which is made possible by the stretch reflex system. This system can only work efficiently when length is restored to muscles, which stimulates their reflex activity. The antagonistic action of muscles, then, is the condition under which the stretch reflex system operates to best advantage. In the next installment in this four part series, we will look at the role of the head and trunk in organizing the working of this system as a whole. Endnotes 1. See Sir Charles Sherrington, The Integrative Action of the Nervous System (New Haven, CT: Yale University Press, 1961), 299–302. 2. Speaking of extensor activity in the decerebrate animal, Sherrington writes: “These muscles counteract a force (gravity) that continually threatens to upset the natural posture. The force acts continuously and the muscles exhibit continued action, tonus” (Integrative Action, 302). He later continues: “Two separable systems of motor innervation appear thus controlling two sets of musculature: one system exhibits those transient phases of heightened reaction which constitute reflex movements; the other maintains that steady tonic response which supplies the muscular contractions necessary to attitude” (Ibid.). 3. Sherrington, Integrative Action, 337: “[I]n the decerebrate dog the tonic extensor rigidity of the leg appears reflexly maintained by afferent neurones reaching the cord from the 22 deep structures of the leg itself. Similarly, if the knee-jerk be accepted as evidence in the spinal animal of a spinal tonus in the extensor muscle, this tonus seems maintained by afferent fibres from the extensor muscle itself, since the knee-jerk is extinguished by severance of those fibres.” 4. Ibid., 86–100. 5. See Tristan D. M. Roberts, Understanding Balance: The Mechanics of Posture and Locomotion (London: Chapman & Hall, 1995); see also Tristan D. M. Roberts, “Reflexes, Habits and Skills.” Direction–A Journal on the Alexander Technique, no. 10, 23–28. Drawings by Helen Leshinsky. Dr. Theodore (Ted) Dimon received M.A. and Ed.D degrees in Education from Harvard University and Alexander Technique teacher certification from Walter Carrington. Dimon is the author of five books: Anatomy of the Moving Body; The Body in Motion; Your Body, Your Voice; The Elements of Skill; and The Undivided Self. He is the founder and director of The Dimon Institute in New York City and an adjunct professor of Education and Psychology at Teachers College, Columbia University. More information about Dimon’s work and The Dimon Institute can be found at: www.dimoninstitute.org. © 2013 Theodore Dimon. All rights reserved. Photograph of Ted Dimon by Marie-France Drouet. www.AmSATonline.org AmSAT Journal / Fall 2013 / Issue No. 4