2 stretch sensor stimulates alpha motor neuron
The stretch reflex allows you to hold yourself in a fixed position, like keeping your trunk upright while standing, without having to constantly think about it. Although this automatic stabilizing function is absolutely essential in daily life, you have to use your conscious intention to override it whenever you move into a new position, especially an extreme one like a yoga asana. Two techniques for moving past the limits ordinarily imposed by the stretch reflex are the adaptation method and the anticipatory release method. If you apply them with diligence and focus, you can dramatically deepen both your yoga postures and your state of mind.
Before you apply these techniques, it's helpful to understand how the stretch reflex works. The stretch reflex is initiated by sensors, called stretch receptors, which are embedded within muscle. Whenever you stretch a muscle, you also stretch the sensors, which stimulates them to send nerve signals to your spinal cord. These signals electrically excite spinal nerve cells called alpha motor neurons. If the excitation is strong enough, the alpha motor neurons send return signals back to the stretching muscle. If the return signals are strong enough, they make the muscle contract, preventing it from lengthening any further and often bringing it back to its original length.
Both the adaptation method and the anticipatory release method work by reducing the strength of the signals coming from the stretch sensors. Reducing the signal strength makes stretching physically easier because it prevents or delays the reflex muscle contraction that limits muscle length. It also makes stretching feel less intense, because much of our conscious sensation of stretch arises when stretch receptor signals reach the brain.
A muscle's stretch sensors are housed in structures called muscle spindles. A spindle is composed of several very tiny muscle fibers with nerve endings attached to them. Pulling on the nerve endings sends electrochemical signals that travel along nerves all the way to the spinal cord.
There are two ways to pull on the nerve endings within a spindle. One way is to elongate the entire skeletal muscle, thereby lengthening the whole spindle and everything inside it. The other way is to contract the tiny muscles inside the spindle, which are arranged so that they tug directly on the nerve endings. The muscles within the spindle contract when spinal neurons called gamma motor neurons tell them to. Gamma motor neurons answer to commands from the brain. You activate them whenever you consciously choose to contract a skeletal muscle, and you inhibit them whenever you choose to relax the muscle.
Different stretch receptors respond to stretching in different ways. In some cases, the longer the receptors, the stronger their signals to the spinal cord. In other cases, the faster you lengthen them, the stronger their signals are. Once stretched, however, all receptors adapt, meaning that if you hold them at a fixed length, their signaling slacks off over time. This is the basis of the adaptation method of stretching.
Harness your mind to help you stretch deeply in Prasarita Padottanasana.
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