So far our discussion has focused on neuronal connections from the top down—from our intention, to the cerebral cortex, to upper motor neurons, lower motor neurons, and skeletal muscles. But there is something else to consider, something much more primitive and elemental in the nervous system that bypasses our conscious choices: reflexes, or unconscious motor responses to sensory stimuli. In this context reflexes have nothing at all to do with the lightning-fast reactions ("fast reflexes") that are needed for expertise in video games or quick-draw artistry. These reactions refer to unconscious responses carried out at the spinal level.
/ MOX'EMONTAND POSTl'KF J7
/ MOX'EMONTAND POSTl'KF J7
half-brain, left side the final common pathway (the collective pool of all lower motor neurons) is still intact, but its conscious control is problematic half-brain, left side
[ igure 1.6. A hypothetical scheme illustrating how injury to a small region of the brain could interrupt pathways important for the precise control of skeletal muscular activity and cause spastic paralysis. The dotted line represents the systems that have been interrupted, and the solid line represents the remaining systems that cannot control muscular activity accurately by themselves, "a" - axon.
these 3 neurons represent motor systems that help control skeletal muscular activity, but less precisely than "upper motor neurons"
motor neurons on the right side of the spinal cord still receive input from the 3 neuronal systems represented by the solid line neuronal systems represented by these two neurons, as well as by the upper motor neuron above, are destroyed as a result of oxygen deprivation due to rupture of a small blood vessel at a site (arrowhead) through which the axons of all three systems course the final common pathway (the collective pool of all lower motor neurons) is still intact, but its conscious control is problematic
38 AAATOM) OF HATHA KX.A
Reflexes are simple. That is why they are called reflexes. They always include four elements: a sensory neuron that receives a stimulus and that carries nerve impulses into the spinal cord, an integrating center within the spinal cord, a motor neuron that relays nerve impulses back out to a muscle, and the muscular response that completes the action. More explicitly; the sensory neurons carry nerve impulses from a muscle, tendon, ligament, joint, or the skin to an integrating center in the spinal cord. This integrating center might be as simple as one synapse between the sensory and motor neuron, or it might involve one or more interneurons. The motor neuron, in its turn, innervates muscle cells that complete the action. By definition, the reflex bypasses higher centers of consciousness. Awareness of the accompanying sensation gets to the cerebral cortex after the fact and only because it is carried there independently by other circuits. There are dozens of well-known reflexes. We'll examine three, all of which are important in hatha yoga.
the myotatic stretch reflex
The myotatic stretch reflex, familiar to everyone as the "kneejerk," is actually found throughout the body, but is especially active in antigravity muscles (fig. 1.7). You can test it in the thigh. Cross your knees so that one foot can bounce up and down freely, and then tap the patellar tendon just below the kneecap with the edge of your hand. Find just the right spot, and the big set of quadriceps femoris muscles on the front of the thigh will contract reflexly and cause the foot to fly up. You have to remain relaxed, however, because it is possible to override the reflex with a willed effort to hold the leg in place.
The receptors for the myotatic stretch reflex are located in the belly of the muscle, where the dendrites of sensory neurons are in contact with muscle spindles—specialized receptors barely large enough to be visible with the naked eye. Named for their shapes, each of these muscle spindles contains a spindle-shaped collection of specialized muscle fibers that are loaded with sensory receptors (fig. 1.7).
The reflex works this way: When you tap the patellar tendon to activate the reflex at the knee joint, the impact stretches muscle spindles in the quadriceps femoris muscle on the front of the thigh. This stretch is as fast as an eyeblink, but it nevertheless stimulates the specific sensory neurons whose dendrites end in the muscle spindles and whose axons terminate directly on motor neurons back in the spinal cord. Those axon terminals strongly facilitate the cell bodies of the motor neurons whose axon terminals stimulate the quadriceps femoris muscle, causing it to shorten and jerk the foot up. The myotatic stretch reflex is specific in that it feeds back only to the muscle in which the spindle is located.
As with all reflexes, this one takes place a fraction of a second before you
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