The Motor Unit

The functional entity for both relaxation and muscular activity is the motor unit, which is defined as a single motor neuron plus all the individuad muscle fibers it innervates (fig. 10.1). The fewer the muscle fibers in the motor unit, the more finely we can control the muscle. The fact that the smallest muscles of the fingers contain only 10-15 muscle fibers in each of their motor units is what allows us to perform precise and delicate work with our fingers. As motor units become larger, ranging up to 500-1000 muscle fibers in the individual motor units of the largest postural muscles, our capability for fine movement diminishes. This is the reason for establishing distal-to-proximal priorities in standing postures (chapter 4). If you first establish control distally, it is possible to keep awareness of the small muscles in the background while you attend more consciously to the bulkier and less easily controlled proximal muscles.

The motor unit, in short, is the sole link between the central nervous system and skeletal muscular activity. Every time a nerve impulse reaches an axon terminal, all the muscle fibers in the motor unit contract. And when many motor units in an individual muscle contract repetitively and in concert, the entire muscle becomes active. What is important to us here is that relaxation requires us to silence the individual motor units one o more at a time. And this is indeed possible. Studies with electromyograpl: t carried out with needle electrodes have demonstrated since the 1950s th. we can train motor units in most parts of the body to become totally silen

Although motor neurons exert absolute control over muscles, thi themselves are only agents of the body and mind as a whole. They a prisoners of our habits, addictions, and the willful decisions of the mit they are prisoners of our hearing, sight, taste, smell, and touch; and th y are prisoners of internal signals from stretched muscles, pain, or an ov loaded stomach. Data from all of these sources are integrated and funnel 1 into the final common pathway of the motor neurons.

FACILITATION. INHIBITION, AND RELAXATION

The key issue for someone trying to relax their skeletal muscles is to kr v something—knowledge is indeed power—about the specific form of t e marching orders that regulate the rate of firing of motor neurons. Th e orders take the form of signals from thousands of other neurons whose <_ II bodies are located throughout the nervous system (mostly in the brain a: d spinal cord) and whose axon terminals impinge on motor neuronal dendri s and cell bodies that are specialized for receiving this information. Sc e axon terminals signal the motor neuron to increase its number of ne e impulses per second (facilitation), and other axon terminals signal il o decrease them (inhibition). It is in this manner that orders to the mo .r neurons are translated into simple stop and go inputs. The motor neut 11 integrates the sum of these often conflicting signals to increase or dec re; e the firing rate of its axon, and in that way it dictates contraction or relaxat 1 of the individual muscle fibers of the motor unit (fig. 10.1). To relax we e 11 conceive of decreasing the firing rate of motor neurons in three wa by decreasing the rate of firing of the facilitatory neurons whose axe •> impinge on the motor neurons, by increasing the rate of firing of t inhibitory neurons, or by both in combination. Speaking simplistic® that's what is happening every time you do not respond to some desire sensory signal.

What we see recorded in our movements is the summed activity of mot units, but something more subtle happens in the central nervous syste 1 during deep relaxation: the motor neuron cell bodies in the brain ai spinal cord become so inhibited that a large facilitatory input is required fire them back into activity. This can have an unexpected result: if you ai in deep relaxation you may not be able to move on command. The telephoi may ring, and when you try to jump up and answer it—surprise. You can do it. You may experience several seconds of temporary paralysis, whit can be startling and even alarming. With practice, however, you can leai

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10 Ra AXA n()N AKIJ MtLHTATKW 545

axon of upper motor cell body of a facilitatory "upper" motor neuron motor region of cerebral cortex neuron (corticospinal)

cerebellum cell body of an inhibitory neuron axon of

Upper Motor Neuron

frontal lobe of cerebral hemisphere several motor neuron terminals (always facilitatory) in a hamstring muscle cylindrical segment ol the spinal cord in cross section motor neuron cell body in ventral horn of spinal cord

Figure 10.1. Two possible pathways for relaxation of skeletal muscles are shown here. In the cerebral cortex, if the activity of a facilitatory upper motor neuron is diminished, it will send fewer nerve impulses per second to spinal cord lower motor neurons (which in their turn drive the contraction of skeletal muscles). Conscious relaxation might also involve a pathway for stimulation of inhibitory pathways, as exemplified here by an inhibitory neuron in the brain stem. If such a neuron starts firing more nerve impulses per second than usual to the lower motor neuron, it could help silence the motor neuron independently of the reduced input from the facilitatory neuron in the cerebral cortex. In the spinal cord, the + indicates the corticospinal neuron's facilitatory synaptic effects, and the - indicates the brain stem inhibitory neuron's inhibitory synaptic effects (Dodd).

axon of upper motor inhibitory neuron-

dorsal hom of spinal cord ventral horn of cell body of a facilitatory "upper" motor neuron motor region of cerebral cortex neuron (corticospinal)

cylindrical segment ol the spinal cord in cross section motor neuron cell body in ventral horn of spinal cord cerebellum cell body of an inhibitory neuron axon of frontal lobe of cerebral hemisphere several motor neuron terminals (always facilitatory) in a hamstring muscle

Figure 10.1. Two possible pathways for relaxation of skeletal muscles are shown here. In the cerebral cortex, if the activity of a facilitatory upper motor neuron is diminished, it will send fewer nerve impulses per second to spinal cord lower motor neurons (which in their turn drive the contraction of skeletal muscles). Conscious relaxation might also involve a pathway for stimulation of inhibitory pathways, as exemplified here by an inhibitory neuron in the brain stem. If such a neuron starts firing more nerve impulses per second than usual to the lower motor neuron, it could help silence the motor neuron independently of the reduced input from the facilitatory neuron in the cerebral cortex. In the spinal cord, the + indicates the corticospinal neuron's facilitatory synaptic effects, and the - indicates the brain stem inhibitory neuron's inhibitory synaptic effects (Dodd).

to speed the process of facilitation and gear the system up for activity u less than a second. You can feel it happening. First you can't move, a hal' second later you feel the nervous system preparing itself, and a half-secon after that you can spring up like a grasshopper. This should never be don as a classroom exercise, of course, because it is too jarring. It's a physii logical experiment, not a yoga practice.

The temporary paralysis induced by deep muscular relaxation undi scores and documents the need for honoring the third axiom—not to oven > relaxation. It is tempting to be excessive, relaxing before and after meditatir before and after meals, and before and after a night's sleep, but doing tl , without the balancing effect of other hatha practices diminishes yo conscious control over the motor neurons. So get a good night's sleep, go > the bathroom, take a shower, and practice asanas enthusiastically for a hour. Then relax thoroughly while you are still full of energy.

Not everyone can will their muscles to relax. What goes wrong? We i a only make inferences. If the circuits from the cerebral cortex are int t (fig. 10.1), the potential for willed relaxation as well as willed movement s available, but if those higher circuits are not used regularly, they gradua y become dysfunctional and the unconscious input from other regions of 1 e brain gets bossier. And because these non-cortical circuits are not um r conscious control, their continuing activity prevents conscious relaxatii 1. More encouraging, as long as the circuits from the cerebral cortex e intact, hatha yoga can help train them.

Relaxation Audio Sounds Autumn In The Forest

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