So far we have seen bow motor neurons drive the musculoskeletal system, how association neurons channel our will to the motor neurons, and how sensory input from muscles, tendons, and pain receptors participate with motor neurons in simple reflexes. But that's only the beginning. Many other sources of sensory input also affect motor function. Some of the most important are the vestibular sense, sight, and touch.
We have little conscious awareness of our vestibular sense even though it is critical for keeping us balanced in the field of gravity. Its receptors lie close to the organ of hearing- -the inner ear—in little circular tubes called serni circular canals and in a little reservoir called the utricle, all of which are embedded in the bony region of the skull just underneath the external ear. The semicircular canals and the utricle are all involved with maintaining our equilibrium in space, but within that realm they are sensitive to different stimuli—the semicircular canals to rotary acceleration, and the
utricle to linear acceleration and to our orientation in a gravitational field. They also participate in different reflexes: the semicircular canals coordinate eye movements, and the utricles coordinate whole-body postural adjustments.
Except for pilots, dancers, ice skaters, and others who require an acute awareness of equilibrium, most of us take the vestibular system Tor granted. We don't notice it because it does almost all of its work reflexly, feeding sensory information into numerous motor circuits that control eye and body movements.
Because the semicircular canals are sensitive to rotary acceleration, they respond when we start or stop any spinning motion of the body. One of their several roles is to help us maintain our equilibrium by coordinating eye movements with movements of the head. You can experience these if you sit cross-legged on a chair or stool that can rotate, tip your head forward about 30°, and have some assistants turn you around and around quickly for 30-40 seconds. Make sure you keep balanced and upright. Don't lean to the side or you will be pitched off onto the floor. Then have your assistants stop you suddenly. You eyes will exhibit little jerky movements known as nystagmus, and you will probably feel dizzy. Children play with this reflex when they spin themselves until they get dizzy and fall down. The sensation they describe as the world "turning" is due to nystagmus. The perception is disorienting at first but it slows down and stops after a while.
The receptors in the semicircular canals stop sending signals after about 30 seconds of spinning, which is why you have your assistants rotate yoi for that period of time. It is also why the reaction slows down and stops in 30 seconds after you are abruptly stopped. Third-party observers obvioush cannot observe nystagmus during the initial period of acceleration while you are being spun around. To observe these eye movements in a practical setting, we must rely on what we call post-rotatory nystagmus, the eyt movements that occur after you have been stopped suddenly.
The neurological circuitry for nystagmus is sensitive to excessive alcohol and this is why highway patrol officers ask suspected drunks to get out of the car and walk a straight line. If the suspect is suffering from alcohol induced nystagmus, the ensuing dizziness is likely to make walking straight impossible. Spontaneous (and continuing) forms of nystagmus that are not induced by drugs or alcohol may be symptomatic of neurological problems such as a brain tumor or stroke.
Occasionally students in hatha yoga classes are sensitive to dizziness when they do neck exercises. They may have had such problems from childhood or they may just not be accustomed to the fact that they are stimulating their semicircular canals when they rotate their head. And even otherwise heulthy students who are just getting over a fever may be
sensitive to dizziness. In any case, anyone who is sensitive should always do neck exercises slowly.
The second component of the vestibular organ, the utricle, detects two modalities: speeding up or slowing down while you are moving in a straight line, and the static orientation of the head in space. The rush of accelerating or decelerating a car is an example of the first case. As with the semicircular canals, stimulation ends after an equilibrium is established, whether sitting still or going 100 miles per hour at a constant rate on a straight road. The utricles also respond to the orientation of the head in the earth's gravitational field—an upright posture stimulates them the least and the headstand stimulates them the most. The receptors in the utricle adapt to the stimulus of an altered posture after a short time, however, which is why it is so important for pilots of small planes to depend on instruments for keeping properly oriented in the sky when visual feedback is absent or confusing. For example, a friend of mine was piloting a small plane and flew unexpectedly into a thick bank of clouds. Instantly lost and disoriented, and untrained in flying on instruments, he calculated that he would just make a slow 180° turn. Unfortunately, after having made the turn and exiting the clouds, he was shocked to see that he was headed straight toward the ground. Fortunately, he had enough airspace to pull out of the dive.
In ordinary circumstances on the ground, the receptors in the utricle do more than sense the orientation of the head in space: they trigger many whole-body postural reflexes that maintain our balance. This is the source of the impulse to lean into curves while you are running or cycling around a track. We also depend on the utricle for underlying adjustments of hatha yoga postures that we trigger when we tilt the head forward, backward, or to one side. Every shift of the head in space initiates reflexes that aid and abet many of the whole-body postural adjustments in the torso that we take for granted in hatha yoga.
The well-known righting reflexes in cats can give us a hint of how the vestibular system influences posture in humans. If you want to see these reflexes operate, drop an amicable cat, with its legs pointed up, from as little as a few inches above the floor. It will turn with incredible speed and land on all four feet, even if it has been blindfolded. Careful study reveals a definite sequence of events. The utricle first detects being upside down, and then it detects the falling sensation of linear acceleration toward the floor. In response to this the cat automatically rotates its head, which stimulates neck muscles that in turn leads to an agile twisting around of the rest of the body and a nimble landing on all four feet. The cat does all this in a fraction of a second. Comparable reflexes also take place in human beings, although they are not as refined as in cats.
When we are moving we are heavily dependent on vision, as anyone can attest who has stepped off a curb unawares or thought erroneously that one more step remained in a staircase. This is true to a lesser extent when we are standing still. If you stand upright with your feet together and your eyes open, you can remain still and be aware that only minuscule shifts in the muscles of the lower extremities are necessary to maintain your balance. But the moment you close your eyes you will experience more pronounced muscular shifts. For an even more convincing test, come into a posture such as the tree or eagle with your eyes open, establish your balance fully, and then close your eyes. Few people will be able to do this for more than a few seconds before they wobble or fall.
Visual cues are especially important while coming into a hatha yoga posture, but once you are stable you can close your eyes in most poses without losing your balance provided your vestibular system and joint senses are healthy. On the other hand, if you want to study your body's alignment objectively you can do it only by watching your reflection in a mirror. It is all too easy to deceive yourself if you depend purely on your muscle- and joint-sense to establish right-left balance.
the sense of touch
The sense of touch brings us awareness of the pleasure and luxury of comfortable stretch, and because of this it is the surest authority we have for telling us how far to go into a hatha yoga posture. The vestibular reflexes and vision help with balance, and pain tells us how far not to go in a stretch. But the sense of'touch is a beacon. It both rewards and guides.
The modality of touch includes discriminating touch, deep pressure, and kinesthesis. All three are brought into conscious awareness in the cerebral cortex, and along with stretch reflexes, vision, and the vestibular sense, they make it possible for us to maintain our balance and equilibrium. Discriminating touch is sensed by receptors in the skin, and deep pressure is sensed by receptors in fasciae and internal organs. Kinesthesis, which is the knowledge of where your limbs are located in space, as well as the awareness of whether your joints are folded, straightened, stressed, or comfortable, is sensed mostly by receptors in joints. If you lift up in a posture such as the prone boat and support your weight only on the abdomen, you can feel all three aspects of touch—contact of the skin with the floor, deep pressure in the abdomen, and awareness of extension in the spine and extremities.
Touch receptors adapt even more rapidly than receptors in the vestibular system, which means that they stop sending signals to the central nervous system after a few seconds of stillness. That's why holding hands with someone
1 MOIFMBNT AND POSTl'Rh: SI
gets boring in the absence of occasional squeezing and stroking. Without movement, the awareness of touch disappears. Rapid adaptation to touch is extremely important in hatha yoga postures, relaxation, and meditation. If your posture is stable, the receptors for touch stop sending signals back to the brain and you are able to focus your attention inward, but as soon as you move the signals return and disturb your state of silence.
touch and the gate theory of pain
If you bump your shin against something hard, rubbing the injured region alleviates the pain, and if your knee hurts from sitting for a long time in a cross-legged posture, the natural response is to massage the region that is hurting. There is a neurological basis for this—the gate theoiy of pain, according to which the application of deep touch and pressure closes a "gate" to block the synaptic transmission of pain in the spinal cord. Although it has not been possible to substantiate this theory as it was initially proposed, we all know experientially that somehow it works. So even though the mechanism is still uncertain, the general idea is widely accepted as self-evident—somewhere between the spinal cord and the cerebral cortex, touch and pressure pathways intersect with the ascending pathways for pain and either block or minimize its perception.
We use this principle constantly in hatha yoga. To illustrate, interlock your hands behind your back and press the palms together. Pull them to the rear so they do not come in contact with the back, and come into a forward bend. If you are not warmed up you may notice that you feel mild discomfort from the stretch. Now come up, press the forearms firmly against the back on either side of the spine, and come forward again. The contrast will be startling. The sensation of deep touch and pressure against the back muscles stops the discomfort immediately.
Is this good or bad? That is a vital question, and one of the challenges of hatha yoga is to learn how far this principle can safely be taken. If you underestimate the importance of the signals of pain, and diminish that pain with input from touch and pressure, you may injure joints and tissues. But if you baby yourself, you'll never progress. The answer, unfort unately, is that you may not know if you have gone too far until the next morning. If you are sore you know you misjudged.
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