theorganization of movement theorganizationof movement ......16 amsatjournal /fall 2014 /issueno.6...

AmSAT Journal / Fall 2014 / Issue No. 6 www.AmSATonline.org 15

The Organization of Movement

Our Segmented Design In Part 3 of this series (AmSAT Journal #5, Spring 2014), we saw that the most basic type of movement is limited segmental movement. For example, a worm can react with one or a few adjacent segments of its body. Each segment has a sensitive patch for sensing tactile contact, to which it can respond in a local way. The worm does not see things at a distance, has not evolved a spine, and cannot coordinate all its segments as a

whole to move through space.

In vertebrates, the body is also organized in segments, each of which has a corresponding tactile patch that is responsive to stimuli that activate the same or adjacent segments (Fig. 1). In Part 3, for instance, we saw that a knee-jerk response is quite localized, involving a stimulus to the tendon and muscle, which

triggers a neural impulse to the spine that is reflected back to the same and related muscles. This example is proprioceptive, but such localized responses also happen when, for instance, someone puts a flame to our hand, in which case we respond by withdrawing the limb, not by moving the body as a whole.

What makes vertebrates unique, however, is the way they are designed as a whole for movement in space. First, vertebrates have a front end, or head, which gathers information about the environment, and a central spine with muscles arranged around it that are designed to produce locomotion in space. The head houses an enlarged ganglion of nerves, or brain, for processing incoming sensory information about the environment, and for organizing the segments to act synergistically. Just as in the more primitive worm, each segment of the human body carries sensory information from the skin surface, but because land animals are designed for movement on the ground, this sensory information also includes proprioceptive information about the state of our muscles, limb position, tendons, and so on. All of this information is carried to the spinal cord and up to the brain so that the body as a whole can be coordinated to produce movement in space.

The Organization of Movement Four Talks on the Primary Control

Part 4. Neck Reflexes: How the Neck Reflexes Play a Key Role in Organizing Tone Throughout the Body

by Theodore Dimon, Jr. In the first three parts of this series on primary control, we have looked at: (1) our tensegrity design with its remarkable muscle/

bone architecture, which depends on lengthening and elastic muscles;; (2) stretch reflexes and their role in stabilizing this structure;; and (3) the head/trunk relationship and its role in organizing the entire system for movement in space. The neck reflexes are the final piece of the puzzle, explaining the role played by the head/trunk relationship in organizing the postural system as a whole.

Theodore Dimon

Figure 1. Dermatomes and segments.

Figure 2. The 31 peripheral nerves exiting the spine.

16 www.AmSATonline.org AmSAT Journal / Fall 2014 / Issue No. 6


Seen in this way, the nervous system has a very logical anatomical arrangement. At the top, or front, end we have the head, which contains the brain and the main sensory organs––nose, eyes, ears. Extending from the brain, the spinal cord carries information up and down the trunk and to the limbs. The trunk is divided into segments, and at each segment, peripheral nerves extend from the spinal cord, carrying afferent or incoming tactile and proprioceptive information from the periphery to the spinal cord and up to the brain, and efferent or outgoing motor impulses from the spinal cord and brain to the periphery (Fig. 2).

The head is also served by nerves that take in incoming sensory information and send outgoing motor information. These cranial nerves, which are sequenced roughly in parallel with the peripheral nerves below them, are not involved in movement but in managing the functions associated with the front end of the body: eating, breathing, and taking in sensory information through the nose, eyes, and ears (Fig. 3). So the peripheral nerves in the trunk take in tactile and proprioceptive information and send motor signals to the muscles;; the cranial nerves take in information through the eyes, ears, and nose and manage the eating, swallowing, and breathing functions;; and the brain processes incoming sensory information and organizes movement of the body as a whole.

The neck and trunk are comprised of 31 segments: eight cervical, twelve thoracic, five lumbar, five sacral, and the coccyx, each served by peripheral nerves that exit laterally from each side of the spine (Figs. 1 & 2). These nerves receive tactile and proprioceptive information and send motor signals to the muscles. In addition, each segment has a corresponding sensory patch, or dermatome, on the skin surface. Taken together, the dermatomes are arranged in overlapping patches that cover the back of the head, the entire neck, trunk, and limbs. There are twelve cranial nerves in the head, which serve the front-end functions of eating, breathing, and taking in sensory information. The front of the head, or face and jaw, is obviously

very sensitive to touch;; this tactile input has been taken over by the trigeminal or 5th cranial nerve, which has three branches covering the forehead, the middle region of the face, and the jaw, including the teeth and tongue (Fig. 4). So the dermatomes really include not just the 31 spinal nerves that serve all the patches on the neck and body and limbs, but also the region of the mouth, jaw, face, and teeth, which is served by the trigeminal nerve. All these segments carry the incoming sensory and outgoing motor information designed for producing movement in space.

The Sub-Occipital Segment

There is one exception to this pattern, and that is the first cervical segment, which serves two sets of neck muscles on each side of the body––the semispinalis and sub-occipital muscles (Fig. 5). Since this segment is associated with the first spinal nerve, there are two things we would expect: (1) that like all the segments below it, this segment would have a corresponding dermatome with sensory nerves carrying tactile information;; and (2) this segment of muscles would be served by afferent nerves that send signals from the muscle spindles to the spinal cord and then synapse with efferent motor nerves in the spinal cord that travel back to the muscles to form a basic reflex arc. But the first cervical segment does not work that way.

First, there is no dermatome corresponding to the first cervical segment. The skin surface of the face, as we saw, is served by the trigeminal nerve (Fig. 4);; and the surface of the back of the head is served by the second and third cervical nerves. There simply is no dermatome for the first cervical nerve anywhere to be found (Fig. 1).

Second, the neck muscle spindles send sensory information to the spinal cord, but there are no motor nerves sending

Figure 3. The 12 cranial nerves in humans.

Figure 4. The tactile regions of the face served by the trigeminal nerve, which complete the dermatome coverage of the body’s skin surface. The back of the head and the neck are served by the

second and third cervical nerves.


impulses back––that is, the basic reflex arc pathways found in all the other spinal segments, which mediate posture in these segments, are not present in this first cervical segment. Their function has been taken over by specific interneurons in the spine––that is, neurons in the spine connected to adjacent segments.

Yet there are muscle spindles, or stretch receptors, in the neck muscles––in fact, lots of them. We have seen that muscle spindles can be found everywhere in the body––shoulder girdle, leg muscles, back muscles, even the muscles of the eyes and larynx. But whereas spindle density in most muscles is around 1 to 20 spindles per gram, in the muscles at the nape of the neck it is around 30 spindles per gram;; in the deep muscles of the neck––that is, in the muscles served by the first cervical segment—it increases to over 40 spindles per gram! No one knows exactly why, because it hasn’t been possible to trace these neural pathways in a living subject. So, if they do not function as part of the stretch reflex arc, what is the role of these spindles, and why is this segment unlike all the others?

The answer can be found in the unique function of these muscles, which occupy a key role at the leading end of the motor system. The head, like other parts of the body, needs to be supported above the ground––that is to say, it requires postural support just as other parts of the body do. But the relation of the head to trunk sets the tone for what the body as a whole needs to do in space, and this means that it is not just another local segment but has a larger role to play in the grand scheme of bodily movement. When a vertebrate, humans included, moves or supports itself as a whole in space, the neck muscles––particularly the all-important sub-occipital muscles attaching at the first two vertebrae and connecting to the occipital bone at the base of the skull––sense the forward balance of the head in relation to the spine. These muscles form the key neuromuscular segment that is charged with the duty of

organizing muscular tone in the neck, trunk, and limbs required for total body movement.

We saw earlier that tonic postural activity is maintained by the brain stem, and that the main source of this tonus is proprioceptive outflow from the muscle spindles in muscles throughout the body, which sets off stretch reflexes designed to maintain stability in body parts. The neck-muscle spindles play a special role in organizing this activity because sensory pathways from these spindles connect to the entire postural system via the brain stem and convey vital information concerning tonic support of the neck, trunk, and limbs. This is why input from neck-muscle spindles to the brain stem is so important: The information from these spindles affects neuronal discharge along the entire length of the spinal cord, modulating extensor and flexor activity and coordinating posture as a whole. And the condition that triggers this spindle output is the antagonistic action of the neck muscles, which sets off the spindle afferents.

The neck reflexes also affect the balance of extensor and flexor tone in the trunk and limbs. Generally, stretch reflexes “reflect” incoming sensory impulses back to the same muscle on which the spindles are located;; other reflexes, such as withdrawal or cross-extensor reflexes, link spindle input to related muscles in fairly limited interneuronal networks. The vestibular and neck reflexes, however, have much more extensive connections with the axial musculature and the limbs, relaying down the vestibulo-spinal tract to facilitate extensor motor neurons and inhibit flexor neurons of both the upper and lower extremities. Neck segments, then, not only organize the whole;; they also mediate pathways that balance flexor/extensor activity. This may explain why, when we are “going up,” we have the sense not only that the system is working automatically, but also that extensor and flexor tone is being regulated, usually in the form of decreasing flexor tone and increasing extensor tone. In short, the neck muscle spindles convey information to the entire postural system about tonic support of the neck, trunk, and limbs, and function as an automatic system regulating muscle tone throughout the body.

Antagonistic Action

As we’ve seen, the human postural system is much more sophisticated and more easily disturbed than that of a four-legged animal, for example, a dog. Remember that muscles contract in the context of a design that is based on elasticity and stretch––and sensing stretch is what the spindles are for. In a dog, this system normally works undisturbed because the head drops down and lengthens forward out of the body, naturally stretching the neck muscles and stimulating proprioceptive input from the spindles.

In humans, however, the head drops forward at the atlanto-occipital joint, which stretches the extensors of the neck and back;; this, in turn, is connected with the head going up and the trunk lengthening. In the human, the poise of the head must counteract the direct downward pull of gravity, and lots of things can interfere with this balance. If we habitually pull back the head, narrow and shorten back muscles, and sink into the legs, the entire stretch system cannot operate and the neck-muscle spindles do not register stretch. To rectify this problem, we must restore the elastic system of antagonistic pulls throughout the body. When we restore this system, the head


Figure 5. The suboccipital muscles served by the first cervical nerve.


will again balance properly and the deeper postural muscles as well as the larger skeletal muscles will again register stretch, enabling the spindles to operate.

When the spindles operate properly, their proprioceptive input exerts an integrative effect on the entire system because this input is not linked to local stretch reflexes but instead integrates the trunk and limbs as a whole. In short, the system works properly when the head is balanced forward on the spine, which can happen only when the system is coordinated and the antagonistic pulls are working to allow the head to balance forward and go up in space as the spine lengthens, the shoulders widen, the trunk lengthens and expands, and the knees go apart.

The neck and vestibular reflexes, then, are hard-wired and will keep you upright and functioning whether you are pulled down or not. But the entire system depends on the antagonistic working of muscles that maintain the lengthened support of the spine, and the dynamic working of this system is the key to the balance of the head in relation to the trunk. If the system is in compensatory mode and there is inappropriate shortening of the muscles, then the main signaling system of the neck muscle spindles that organizes the whole thing cannot work, disrupting the overall distribution of muscle tone and allowing the body to remain in a state of imbalance. To regulate muscle tone and re-establish a balanced state, we must restore the elasticity of the muscle system that enables these delicate nervous structures to do their work, so that the neck spindles can be naturally stretched and send their afferent signals to the central nervous system, where muscle tone throughout the body can be automatically organized at a reflex level.

Neck-Muscle Spindle Sensitivity and the

Coordination of the Whole Remember that we cannot achieve this

condition directly. When we speak about neck reflexes, it is tempting to think that, if the key reflex connections that integrate the total pattern of postural support are located in the neck, we can directly access these pathways of activity by manipulating or directly altering the balance of the head. But we have to remember that, for the deeper neck muscles to register stretch, they need the length and support of the trunk, and this requires the various elements that contribute to upright posture to be working well––in particular that the larger skeletal muscles do not overwork or interfere with the body’s ability to lengthen. If this system is interfered with, as it almost always is in adults, removing the interference is essential for the underlying neck reflexes to work properly.

If, for instance, there is pulling and shortening in back, collapse of the body in front, or tension and fixing in the throat,

then these systems, instead of being suspended from the base of the skull, will drag upon it so that the head cannot balance properly. These systems must be released so that the spine and its deeper supporting muscles can work properly, which in turn allows the neck to release and the head to balance forward. This

is why experienced teachers, when working with a student, will often leave the neck alone and trust that as the system comes into balance the neck will be able to let go on its own. Our job as teachers is to find out what is interfering with these conditions and in so doing to help re-establish balance, not to try to directly elicit neck reflexes. Sometimes it is more important to sort out someone’s shoulders or to get the knees going away as part of restoring length in the trunk than to try to mess about directly with the head. The working of the neck reflexes is dependent on the integrated, coordinated working of the various parts of the body. The incorrect working of any one of these systems can impair head balance, the lengthened support of the spinal column, and the functioning of the deeper muscles that support the spine. This means that although the balance of the head affecting the muscle spindles in the neck plays a central neurological role, we still have to restore the working of the entire system in order to establish the conditions that allow the head to balance forward and, in turn, to activate the neck reflexes. In short, all the different parts have to be working properly for the primary control to work (Fig. 6).

Local Reflexes v. Neck Reflexes One other aspect of the postural system is sometimes confusing. It is well known that local reflexes in the feet and legs are a critical part of bodily support. Cutaneous and pressure receptors in the feet, for instance, elicit supporting reflexes in the leg, suggesting that postural support does not come entirely from neck reflexes and brain stem activity but from the bottom up.1 However, the main organizing stimulus of postural support is not produced primarily in the legs. Neural wiring at the spinal cord level

is the most basic and primitive part of the nervous system––like a good manager who does not try to control everything from the top because he knows that people work best when you leave them alone, the nervous system utilizes these lower tiers of management whenever possible. Stretch reflexes, which as we have seen are wired at the spinal cord level, are part of this multi-tiered background system, and they function as a basic support for the leading levels of control.

All of these simpler circuits are ultimately coordinated as a whole by the neck reflexes because: (1) the limbs operate secondarily to the trunk;; (2) there are many segments that have


Figure 6. When all the parts are working, the conditions are established to

let the neck reflexes work.


to work together;; and (3) all movement is organized in relation to the head.

However, there are aspects of postural control that are triggered from the bottom up. In squatting and standing, for instance, the dominant tactile skin input relating to head balance comes from the soles of the feet, especially the pads and heels.2 We have evolved sensitive supporting reflexes that are stimulated directly by contact with the ground. But there is nothing in the existence of such supporting reactions in the legs that is inconsistent with the fact that head balance in relation to a lengthening trunk is a central, coordinating element in posture. Without it, our complex standing posture, organized for directive movement in space, could not have evolved as it did.

To review, we have a segmented structure with a central spinal cord running down the center and peripheral nerves extending to the segments on each side of the body. The head, with 12 cranial nerves, houses the mouth, the specialized sense organs and the brain, which processes information and coordinates the movement of all the segments.

The body segments are served by nerves running to and from the muscles that trigger stretch reflexes that maintain postural support––all, that is, except for the first of these body segments, which is buried beneath the surface because it has no outside skin patch, and no reflex arcs to its own muscles. Instead, its neural inputs modulate the activity of neurons projecting down the spine because their function is to regulate postural tone in the trunk and limbs. These nerves are sensitive to the forward balance of the head, which exerts stretch on the muscles and activates the muscle spindles in the neck. And this relationship, which in turn depends on the coordinated working of the entire muscular system, is the mechanism that we call the primary control.

Neck Reflexes and Modern Neuroscience

Let’s conclude this series with a few observations about neck reflexes from a widely-respected textbook on neuroscience, Principles of Neural Science by Kandel et. al.3

1. We saw earlier that simple reflexes involve specific

muscle groups in fairly limited interneuronal networks. In contrast, “the vestibular and neck reflexes produce complex patterns of facilitation and inhibition in motor neurons innervating widely distributed muscles.... Both the vestibulospinal and reticulospinal tracts excite interneurons and long propriospinal neurons responsible for distributing the patterns of excitation and inhibition widely to many groups of motor neurons.”4 Some of these pathways “facilitate extensor motor neurons and inhibit flexor motor neurons of both the upper and lower extremities through interneurons and propriospinal neurons.”5 In short, sensory information from the neck muscle spindles is relayed to the entire postural system and conveys basic information about tonic support of the neck, trunk, and limbs.

2. “Even though muscle spindles are abundant in neck muscles, direct monosynaptic connections between these muscle afferents and the homonymous muscle motor neurons

are very weak. Instead, excitation of homonymous motor neurons produced by stretch of these muscles is mediated by local interneurons. Vestibular and neck afferent signals are integrated at several levels, beginning in the vestibular nuclei, where input from neck muscle spindles strongly modulates the

discharge of neurons projecting down both the medial and lateral vestibulospinal tracts.”6

In other words, the afferent nerves from muscle spindles in the neck do not synapse with motor nerves serving the same muscle to form a simple reflex arc. Instead, they tend to modulate many different spinal segments affecting posture. In short, analysis of neck-spindle input suggests that neck-muscle spindles play a special role in organizing tonic activity throughout the body.

The above observations suggest that there is a great deal of evidence in modern neuroscience that neck reflexes, in contrast to other spinal reflexes, play a primary role in influencing muscle tone and posture. Tonic postural activity is maintained by the brain stem, and the main source of this tonic activity is proprioceptive outflow from the muscle spindles in the muscles themselves. The neck muscle spindles play a special role in organizing this activity because, unlike normal spindles which form simple stretch reflex arcs and operate more locally at the spinal level, spindles in this region of the body make general connections with spinal tracts that modulate discharge of neurons projecting to the entire trunk and limbs, organizing the various bodily segments as a whole. And what elicits the afferent input from neck muscle spindles that organizes posture as a whole? The condition that triggers this activity is the natural balance of the head in relation to a lengthened and supported spine, or what we would call the primary control.

Endnotes

1. Dart, Raymond, “An Anatomists Tribute to F. Matthias Alexander,” in Skill and Poise (London: STAT Books, 1996), 38. Dart writes: “In the human squatting or standing (or orthograde) positions, the dominant segmental skin information concerned in human head balance is probably that coming from the sacral or hindmost body segments supplying the soles of the feet, especially the pads of the toes and heels.”

2. Ibid. 3. Principles of Neural Science, edited by Eric R. Kandel, James

Schwartz, Thomas Jessell, and A.J. Hudspeth (New York: McGraw-Hill, 2010).

4. Ibid, 602. 5. Ibid. 6. Ibid.

References: Alexander, F. Matthias. Articles and Lectures. London: Mouritz,

1995. Bernstein, Nicholai A. Dexterity and Its Development. Edited by

Mark L. Latash and Michael T. Turvey. Mahwah, NJ:


“...there is a great deal of evidence in modern neuroscience that neck reflexes, in contrast to other spinal reflexes, play a primary role in influencing muscle

tone and posture.”


Lawrence Erlbaum Associates, 1996. Brink, E. E., I. Suzuki, S. J. Timerick, and V. J. Wilson. “Tonic

Neck Reflex of the Decerebrate Cat: A Role for Propriospinal Neurons.” J Neurophysiol 54 (1985): 978–987.

Cacciatore, Tim. “Science and Alexander: Towards a Common Understanding.” Direction—A Journal on the Alexander Technique, Vol. 2, no. 10.

Dart, Raymond. Skill and Poise. London: STAT Books, 1996. Fuller, Buckminster. Synergetics. New York: Macmillan

Publishing Co., 1996. Kardong, Kenneth V. Vertebrates: Comparative Anatomy,

Function, Evolution. New York: WCB/McGraw-Hill, 1998. Magnus, Rudolph. “Animal Posture.” Proceedings of the Royal

Society of London 98 (Ser. B). ----. Body Posture (Korperstellung). Berlin: Verlag Von Julius

Spring, 1924. ----. “Physiology of Posture.” Lancet (1926): 211. Miller, Kenneth E., Vickie D. Douglas, A. Brent Richards,

Margaret J. Chandler, and Robert D. Foreman. “Propriospinal Neurons in the C1-C2 Spinal Segments Project to the L5-S1 Segments of the Rat Spinal Cord.” Brain Research Bulletin 47, no. 1 (1998): 43–47.

Nashner, L.M. “Adapting Reflexes Controlling the Human Posture.” Exp. Brain Res. 26 (1976): 59–72.

Principles of Neural Science. Edited by Eric R. Kandel, James Schwartz, Thomas Jessell, and A.J. Hudspeth. New York: McGraw-Hill, 2010.

Roberts, Tristan D. M. Neurophysiology of Postural Mechanisms. New York: Plenum Press, 1967.

----. “Reflexes, Habits and Skills.” Direction—A Journal on the Alexander Technique 2, no. 10.

----. Understanding Balance: The Mechanics of Posture and Locomotion. London: Chapman & Hall, 1995.

Romer, Alfred Sherwood and Thomaso Parsons. The Vertebrate Body. Philadelphia: Saunders College Publishing, 1977.

Sherrington, Sir Charles. The Integrative Action of the Nervous System. New Haven: Yale University Press, 1961.

Young, J. Z. The Life of Vertebrates. Oxford: Clarendon Press, 1981.

Drawings by Helen Leshinsky.

Dr. Theodore (Ted) Dimon received MA and EdD 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.

© 2014 Theodore Dimon, Jr. All rights reserved.

Photograph of Ted Dimon by Marie-France Drouet.


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