On the nature of postural reflexes

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Abstract
Myograph records of a resting muscle being passively stretched show first a preliminary rigidity, then a phase of constant tension, and finally a viscoid reaction, i.e., the muscle acts first like an inelastic and later like an elastic body. Tendon shows only the viscoid reaction. Neither the rigidity nor the viscoid reaction of muscle are changed by complete nerve section. Slow muscles (soleus) have long after-reaction and summation in duration of contraction as well as in tension. Duration of relaxation varies directly with the duration of stimulation. In the tendon jerk each motor fiber conveys only one impulse before the rebound. Inhibition of the stretch reflex occurs completely and simultaneously in all parts of the muscle. Relaxation after the stretch reflex shows that the muscle behaves as it would after motor tetanus of a slow rate. To explain mechanical aspects of the stretch reflex nothing more than the usual phenomena of twitch and tetanus are required. Section of the sympathetic system or dorsal roots only modifies the process by increasing the rate of discharge of the motor units. Analysis of the action currents accompanying the stretch reflex shows that the rate of discharge is slow. Each group of extensor muscles contains a red (slow) fiber group. Both rapid and slow groups show the stretch reflex, but the red reveal it more easily and can produce a far greater tension. They are likewise of lower threshold for and respond more completely to a crossed stimulus than the pale ones. Usually flexor muscles show no strongly maintained stretch reflex. The development of the stretch reflex depends on the conditions in the central nervous system, being minimal in the fresh spinal preparation and after precollicular section, augmented after intercollicular section (decerebrate rigidity), and altered by labyrinthine and neck reflexes. The stretch reflex sets off two afferent impulses, (1) excitatory from the tendon-organs, (2) inhibitory from the muscle-spindle. Central excitation and inhibition are summat-ed and the relative amounts of excitation and inhibition at the motor unit determine its rate of discharge. Under special conditions the discharge is rhythmic and clonus results.

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