An understanding of the upright posture in man a requires knowing how man senses the vertical and then how he controls his body to avoid falling over. There are some useful concepts and reason-able explanations that scientists have developed to explain this ability. This essay will first consider the contributions from a number of sensory input stop the detection of the vertical, or knowing “which way is up”. Second, it will examine the properties of muscle stretch receptors, their control, and their contributions to muscle tone and to reflex stimulation of antigravity muscles.
Finally, some aspects of sensory processing that are typical of the nervous)system will be considered. These ideas will demonstrate the importance of understanding the functions of the new ours system in considering human alignment, posture, and movement.
Importance of vision and high heads
The most important way of knowing which way is up is vision. That is true both of us and of other animals. If you throw a cat into the air, it always land son four feet. What it does, if you take stop frame moving pictures of it, is to turn its head until its eyes find horizontal. Then it operates through potent neck reflexes to bring its shoulders under its head and get its body aligned with its shoulders, so it lands on four feet. The head comes first.
One purpose of the head is to be high. When man came out of the trees in evolution and down onto the plains, his head go thigher. This was an advantage, since he could see farther, either his enemies or friends or prey or predators. On the plains of East Africa, he knew printed from ROLF LINES, October1984.where the horizontal was, and he had his smell and mouth and hearing all up high too. So he had this platform on top of the body that took in all the special sensations. The body spends a lot of energy in keeping the head up and seeing the horizon as horizontal. So, the primary way to know which way is up is from the visual system.
Contact with the ground
If your eyes are closed, what is the most important way of knowing which way is up? Traditionally, one expects the vestibular organs next to the inner ear to give information about balance. In fact, the feet are more important. That has been demonsizated in a few ways. Twenty years ago in England patients with neurological problems were studied. When patients with no sensation from the vestibular organs were blind folded, they still knew which way was up, and they would not stumble when walking and turning. Such patients might have had tumors or some sort of damage to the nerves from both sides, or to the vestibular organs themselves, so input was blocked. On the other hand, when patients with Tabes Dorsalis or any disease that interfered with sensory input from the legs and feet were blind folded and asked to walk, particularly to turn, they would fall down. So, pressure of the feet with the ground is also important for detection of vertical.
Another way to demonstrate the importance of contact with the ground comes from people who go scuba diving in caves in Missouri. Scuba diving is dangerous in the dark. Divers cannot see, and they cannot make contact with the ground, so they depend on their vestibular system. On both sides of the head there are three semi ciatular canals that detect angular acceleration, plus a couple of sacks called the saccule and utricle. These latter two have surfaces lined with hair cells that have granules on them. They detect which way is down from where the granules bend the hair when attracted by gravity. Water pressure on the ear drums of scuba divers confuses the information coming fam the saccule and utricle in the same nerve bundle. Unfortunately, sometimes people go down instead of up, and they down. That is why scuba divers, before they go diving in caves, learn the emergency trick of pulling off their face masks and feeling the bubbles rising. With no vision, no touch, and confused vestibular input, there is inadequate information about which way is up.
A third example of the importance of contact with the ground is elderly people getting out of bed at night. They wake up and have to go to the bath womin the dark. All sensations tend to beat tenuated in the elderly-touch, hearing, and vision may be limited. They often stumble and break their hips get-ting out of bed, because they are not sure which way is up. The dark limits vision,and they do not have good contact with the ground, even though the vestibular organs may be intact.
Interaction of vestibular and visual input
In man there is an interaction between the semicircular canals and the eyes that helps to produce a unified sense of vertical. Nystagmus is a medical phenomenon in which the eyes tract slowly in one direction and then flick back. You can generate it with your eyes open by moving the visual field-when the field of vision moves, you tend to fixate on paint until you lose it and then quickly recover to find another fixation point. Another way to generate nystagmus is to twirl. Imagine sitting in a barber’s chair and being twirled with your eyes closed. When you stop, you would be dizzy and stagger when trying to stand, because the sensory input is confusing. You will also have the tracking and flicking in your eyes called nystagmus. This demonstrates that information from these micircular canals, the specialized sense organ for balance, can be expressed as changes in the visual system.
When people have certain types of dizziness or unsteadiness, it may be due to incorrect information from the vestibular organs. We use information fam the balancing organs in the head during some muting activities. So if people get information from the balancing organs that contradicts other sensations, then they are confused in their sense of vertical.
Neck reflexes are an important step between knowing which direction is vertical and the stable, vertical alignment of the body. Recent studies show how man stabilizes his head, then his neck and shoulders, and then the rest of his body. You could see how crucial the neck is by holding the body of a now in your hands while it looks at something of interest. You could move its body at all angles, and its head would stay motionless. If you blindfolded it and did the same thing, it would be almost as stable because of the vestibular system. The head becomes a stabilized platform. The neck reflexes and the vestibular system stabilize the head so the visual field will not change.
You can experience the interaction of neck reflexes with vision for your self. Put a finger out in front of you and look at it. First, move your finger slowly sideways, keeping your head and neck fixed and just following it with your eyes. See what that feels like. You are tracking it; see how easily you are able to keep it in focus. Second, instead of moving you re yes in your head, turn your head to follow the moving finger. You almost have to subconsciously turn your neck and eyes in order to follow it. Last, leave your finger in the same place and turn your head from side to side while you are still watching it. Consider which of these thee allowed you to see your finger most clearly and with the least effort. Moving the eyes? Moving the head? Or keeping the finger fixed? Most people find that the last one is best, when your eyes turn in one direction and your neck in the opposite direction. This demonstrates how yoked together are eye movements with head movements. In order to see the finger with every turn of the neck and head, your eyes compensate by moving. The muscles of head and neck work together with the visual control as an integrated system.
On factor in the fine control of head position is probably a high density of stretch receptors in neck muscles. Stretch receptors are small sense organs that monitor changes of length in muscles. There are also small receptors in the tendons that monitor tension on muscles. Recently scientists examined the density of stretch receptors in muscles so closely attached to vertebrae that they could not be peeled off. The density of stretch receptors in these muscles on neck vertebrae was about a hundred times greater than in limb muscles. So the very deepest muscles, those closest to the bones of the neck, have the highest density of length receptors of any muscles in the body.
The stretch receptor or length receptor is a specialized organ called a muscle spindle. If you tap a tendon to elicit a knee jerk reflex, it is initiated by stretch of the muscle stretch receptors and not from receptors in the tendon. The whole muscle itself, the muscle mass, as well as these small stretch receptors within the muscle, get stretched. Activation of stretch receptors sends signals into the central nervous system that make that muscle contract, to oppose the stretch. This is a negative feedback or control system. When you lengthen a muscle briskly, it responds by contracting to oppose the lengthening. The stretch reflex, or knee jerk, is a mono synaptic reflex, the fastest response in the body. The largest diameter, fastest conducting sensory fibers carry signals into the spinal cord. This immediately activates the largest motor neurons, and they send signals to the muscle fibers that cause contraction.
The stretch reflex is also the basis for our upright posture. The reason that you do not collapse when you are standing up is because of tonic stretch reflexes. They occur all the time in the extensor, antigravity muscles of your body. Gravity puts a mild stretch on muscle spindles and the muscles in your legs and trunk. The muscles respond with tonic continuous stretch reflexes that cause them to contract and hold you erect.
Muscle tone is a reflex. If you take a muscle out of the body and lay it on a table, it does not have any tone. It has only some viscosity. An intact nervous system is required for muscle tone. The stretch reflex is the basis of muscle tone. The continuous, mild stretch reflex makes the upright pasture possible in man.
When you lie down to sleep, you lose the pull of gravity an stretch receptors ,and consequently, you lose muscle tone. As you relax your mind, you also sup-press the stimulating influences from the brain onto these spinal stretch re-flexes. When you wake up in the morning, you have almost no muscle tone. It may take a few seconds for the brain to activate motor neurons to be ready to stand. You feel stiff in the morning; because inactive muscles have marginal blood supply and also because relaxed muscles have not been getting stretched as much as they do during the day. So, before you get out of bed in the morning, you wake up enough to stretch, which increases circulation and tone of the muscles. This gives time for your new ous system to be activated, so that when your feet hit the floor, you do not collapse. You adjust the muscle tone, those muscle stretch reflexes, to support you once you are out of bed.
Muscle cramps occur when there is a lack of circulation to muscles. In addition a sensory irritation in the muscle may cause a reflex contraction of the muscle. A strong muscle contraction is sufficient to cut off its own blood supply.
The cramped muscle must be massaged to restore the blood supply and stop excessive firing of sense organs in the muscle. Restoring the blood flow into the muscle breaks the feedback cycle, decreasing the sensory activity to allow the muscle to be stretched again.
Muscle fibers within muscle spindles
Within the muscle spindle there are little muscle fibers called “intra fusal” muscle fibers, named for the fusi form shape of the spindle. These intra fusal muscle fibers are innervated by small motor fibers, providing another way for the nervous system to content muscle movements.
When these intra fusal muscle fibers contract, they activate the stretch receptors, which fire as though there were an external stretch on the muscle. Patients with spastic states and other neurological problems have too much muscle tone, because descending activity from the brain especially activates the small motor fibers going to these intra fusal muscle fibers.
Contraction of intra fusal muscle fibers can initiate a stretch reflex. The nervous system activates small motar, neurons, their fine motor fibers are fired, intra fusal muscle fibers contract, stretch. receptors are pulled, and their sensory, fibers increase activity. More sensory] signals comes back into the spinal cord, and activate large motor neurons that, contract the extra fusal muscle fibers, the large muscle fibers. It is a double cycle control system. There is also an amplification effect-the small motor system contracts little muscle fibers that the nactivate large sensory fibers that can then make a whole muscle contract. So there is a power increase in the system.
Unfortunately, people with brain damage often have too gruch activity of these small fibers, which makes the muscle spastic. There is too much muscle tone. The background activity onto motor neumns from the brain is largely inhibitory or suppressive. A person with a spinal injury is paralyzed initially, but after some days they develop’ too much tone. A spinal cord left by itself has too much tone. These reflex loops do not have the brain to suppress them, so they become overly active.
Effects of Rolfing on stretch reflexes
When Rolfers stretch fascia, the do more than just affect the muscles. The are also changing the “bias” of the stretch receptors, causing more or less slack in them. The muscle spindle is in series with large muscle fibers. So, if you change the length of large muscle fibers, you are also affecting muscle spindle length. There is a paradox in doing a fifth session. Rolfers stretch the abdomen, the rectus muscles, and then the next day the rectus may be shorter. Perhaps while stretching the muscle fibers, Rolfers also change the way the stretch receptors within the muscle spindles respond. If the small motor system were more active after Rolfing then the muscle spindles might fire faster, producing more tonic stretch reflex activity in the muscle, and more shortening. By working on the fascia, Rolfers affect the perception of the muscle by sense organs. Muscle tone can be changed the much various spinal reflexes or from the brain. The sensory information goes up to the brain and comes down even out of consciousness, which may also change the tone in a muscle. So, Rolfing has an impact on the nervous system.
Medial and lateral descending motor control There is a new concept that has finally appeased in medical textbooks. In stead of describing a dual motor control with the pyramidal system and the extra pyramidal system, there is the medial system and the lateral system. The nerve cells concerned are located medially and laterally in the spinal cord. The medial system descends from lower brain stem centers, producing motor control of postural and trunk muscles. The lateral system controls the peripheral part of the limbs, particularly the fingers and fine control. Rolfers really do not work very much beyond the elbows, perhaps because distal parts are less important for verticality. The cerebral motor cortex and red nuclei are used especially for the lateral control system that produces fine, discrete movements. The cerebellum, basal ganglia, and brain stem are more important for postural control by the medial system.
Figure1. A visualization of the body as an inverted pendulum, with the center of gravity (shown by an X) high above the small base of the feet.
Every movement requires posturale adjustments through out the body. Most I doctors and maybe even some Rolfers do not recognize that if you thrust your arm out, postural adjustments have to occur in the rest of your body in order to maintain balance. Movement is a series of postures; posture is movement stilled; posture follows movement like a shadow. Most of motor control by the brain is really postural control. Movements are more obvious, but ninety percent of the work of the nervous system is postural adaptations for those movements. So that when you thrust your arm out, there are all kinds of shifts and adjustments through your whole system to avoid falling over.
Stability of the body
Physically, the body is very unstable. Man is an inverted pendulum, as shown in Figure 1. He has a very small base and a center of gravity somewhere between the umbilicus and the pubes. Physiologically, man is stable because of stretch reflexes and motor control systems. The nervous system and muscle control systems compensate for shifts of the body away from the base.
Detecting linear and rotational acceleration, which is what the vestibular system does, may not be essential for stability. Tilting off center causes rotatory acceleration of fluid in the semi circular canals, which changes activity going into the central nervous system. You can predict imbalances because of this angular acceleration, which allows the body to compensate for such imbalances. You mainly use your eyes and contact with the ground for stability, but the vestibular system is also helpful.
Of course, there an occasions when vestibular input needs to be suppressed. I skaters and ballet dancers do not get dizzy when they are twirling. They avoid dizziness by fixating with their eyes at each rotation. The Whirling Dervishes twirl for hours to get high without using drugs; they learn to fixate on their outstretched hand. The rotatory input to the semicircular canals causes dizziness when you twirl, but it can be overcome by training via the visual system. If you put someone in ad rum with vertical white and black lines on it, a socalled optkinetic drum, you can twirl it around and see if they get dizzy. Then twirl them in a chair with eyes closed to see if they get dizzy, and then you do both simultaneously but in opposite directions. When you do that, you find the visual input has about twks the impact of the vestibular input.
Figure 2. A visualition of lateral inhibition. A- physical measurement of density change at an edge. B- appearance of density change at edge by neural processing of lateral inhibition.
Auditory input can contribute to body stability, too. If you stand on one leg with your eyes closed, you are more stable when you have a sound to focus upon. The sound may be an external reference point for the body. The ability of our ears to localize sound is quite elegant. When you hear a sound, you tend to look toward its source, because the auditory system can discriminate differences very sharply. When you turn your head to see the source of the sound, you are equalizing the intensity of both ears. For low frequencies, where the wavelength is long enough you detect amplitude differences that are slight phase differences. And at higher frequencies, you do it just by intensity differences and obtain good localization. Hearing a sound while standing on one leg may allow your body to balance itself with-out conscious intervention, which is probably the best way to do it.
Whether or not you pay attention to various stimuli will affect your responses to other stimuli. Your body has more sensory input than it can process. So, you select what you attend to, usually one channel at a time. The first experiment to show this was done thirty years ago. Sounding a click while recording from the brain of a cat causes an electrical response in nerve cells near the ear. If you put a mouse in front of the cat and do the click, then you see less response. The cat is distracted by the mouse, so its response to the sound is diminished. Blocking that activity can even occur right out in the ear. There are inhibitory nerve fibers going to the hair cells that are the original zeceptors in the ear. This inhibitory control can suppress input. Its more important role may be to enhance differences and give sharper discrimination.
An important mechanism for enhancing differences throughout the nervous system is lateral inhibition. When you look at the edge of a shadow on a wall, the wall may be brighter right next to that shadow than farther away. Physical measurement would show a simple step change of intensity, as in Figure 2A.Whatyouseewithyour retina is a darker region in the shadow right next to the light surface, and a lighter region on the light side right next to the shadow, as in Figure 28. The nervous system has processed the intensity information to enhance the difference and sharpen contrast. This phenomenon also happens in the ear to discriminate tones. One region of an inner ear vibrates more than nearby regions for a particular tone, but most of the length of the inner ear vibrates some. People can discriminate two tones that are only a few cycles apart, which is far superior to what the inner ear achieves. As sound is processed in the nervous system, there is enhancement of contrast by lateral inhibition at each relay.
A musical band can be used as an analogy for understanding lateral inhibition, with each instrument like one nerve cell.
Imagine every instrument telling every other instrument to be a little quieter. Then the horns are all going to tell each other to be slightly quieter, but the horns may be next to flutes, which are much quieter. The horn at the edge is going to tell the flute next to it to be quiet more than the other flutes beyond that flute do. In a group of flutes, the flute next to the horns will hardly be heard at all. In addition, this flute is not going to tell the horn at the edge to be as quiet at the other horns tell each other to be, so that horn is going to be much louder than the other horns. This explains the enhancement of the difference at the edge (Fig.2B), which is what goes on in the nervous system.
You can also think about lateral inhibition mathematically. Imagine that all the nerve cells in a group are firing, and they squelch one another by twenty percent of their rate. If half of them fire at double the rate of the other half, then they will squelch their slower firing neighbors by forty percent and only be ten percent squelched themselves. So there is going to be an enhancement of contrast where the two halves meet one another.
This lateral inhibition was first described in the eye of the horse shoe crab fifty years ago by Hart line, who got the Nobel Prize for his work.
Lateral inhibition happens through-out the nervous system. When images are relayed through the visual system, edges and corners are constantly enhanced. You hardly even see clear, open spaces, because the nervous stem does not notice open spaces and constant images. It is designed to detect differences and patterns and edges. If you think about auditory input, you have your attention drawn to something that changes in time, and then you pay attention to it. You turn on a channel and then you focus on a particular sensation.
There is also another phenomenon, habituation. When sounds are repetitive, you stop hearing them. If you live next to a railroad track, you often do not hear the trains. You habituate them, which is very efficient. The nervous system does not respond to repetitive events that lack new information.
Normally you focus your attention on only one thing at a time. Psychedelic drugs may act by lifting this control. People hear colors with psychedelic drugs like marijuana or see visual kaleidoscopes because multiple sensory inputs are allowed into the brain simultaneously, producing confusion. But normally we refine, define, sort out and focus sensory input from one or more modalities.
Finally, when you look at someone, there is going to be a contrast effect around them due to lateral inhibition. This may be confused with the aura of the person, if the aura is weak.
There are muscle stretch receptors in all the muscles of the body, except those that position the eyes.
The eye muscles are the only place that do not need information about their length, beam se the visual field tells where the eyes are. The visual control system shows a phenomenon that is important in control of all sensory systems; it is corollary discharge. This means a discharge that “goes along with”. When you send a signal to the eye muscles to contract and thereby move the eyes, that signal is sampled and goes to other neurons controlling eye position. So the nervous system first has an indication where the eyes are by how much it told the eyes to move.
Second, that information is compared with what the eyes see, and any differences can be corrected. What was expected is compared with what actualy happened. Such comparisons are typical of the whole nervous system.
When you make a movement, there is a corollary discharge from branches of motor nerve cells into regions of sensory input.
The nervous system predicts what sensory input will occur as a result of that movement and blocks the predicted input at the spinal level.
The nervous system filters out the expected and responds to the un expected events.
A similar sort of filtering system can block pain at the spinal level too.
The presence of small muscle fibers within stretch receptors allows two ways for the nervous system to tell a muscle to contract. It can fire the nerve cells to the large muscle fibers directly, or it can fire the small motor nerve cells to contractin transfusal muscle fibers and have them generate a stretch reflex. These are called the direct and indizect pathways. The nervous system probably does both simultaneously, which is called “co contraction”. The indirect pathway requires a fraction of a second longer between brain command and muscle contraction. The position of that limb may change slightly in the fraction of a second between when sensory receptors told the brain where the limb was kxated, and the brain command contracted the appropriate muscles. The indirect pathway will take into consideration any change of length in the muscle during this control processing time. Therefore, the indirect pathway takes longer, but the movements may be more precise, because they inerporate more recent sensory information from muscles.
How rapidly can you move precisely? For piano playing with trills, it is about twenty finger oscillations per second. This may reflect the natural frequency of the stretch reflex, which is about twenty per second.
When you initiate a movement, the motor cortex does not show the first sign of electrical activity. The decision to initiate a movement is probably in the motor cortex, but the first electrical signs of a movement come from lower motor centers-the basal ganglia and cerebellum. The cerebellum and the basal ganglia probably store certain patterns that are learned, and the motor cortex is calling upon a pattern. Then the generated pattern goes up to the motor cortex (the executive), which then sends information down to the muscle and also tells the basal ganglia and cerebellum what it has commanded, so they am then modify the commands.
The patterns for movement are learned not only in the motor cortex but also in the cerebellum and basal ganglia.
In order for man to stand vertically, he must have systems both for sensing the vertical and for achieving stability with motor control. Many sensory modalities contribute to detection of the vertical position. Vision is primary; contact with the ground secondary; and the vestibular system is tertiary. The nervous system must sometimes process conflicting sensory information and still maintain a vertical posture. The motor control system that achieves vertical alignment is a hierarchy of systems. Stretch reflexes at a spinal level establish muscle tone and prevent all apse by activating antigravity muscles. Neck reflexes with massive muscle stretch input stabilize the head on the spinal column.
Different part of the brain control axial and distal musculature, although posture and movement are aspects of one controlled system. Even sensory input is modified before it travels very far into the nervous system, by lateral inhibition and by descending output son to sensory input. The ability of the nervous stem to modify sensory input even at the receptor, such as the small motor system onto muscle stretch receptors, is a most peripheral control of input. This emphasizes that muscles in an intact body air intimately connected with the nervous system for all of their sensory, tone, and movement functions.
Former Rolfer and Associate Rolf Institute Member, Roger Thies is Associate Professor and Vice Chair of the Department of Physiology & Biophysics at the University of Oklahoma Health Sciences Center in Oklahoma City, Oklahoma. Dr. Thies is also a member of the Rolf Institute’s Research Committee.Which Way Is UP?