stretch reflex can be divided into two components: the dynamic stretch reflex and the static stretch reflex. The dynamic stretch reflex is elicited by the potent dynamic signal transmitted from the primary sensory endings of the muscle spindles, caused by rapid stretch or unstretch. That is, when a muscle is suddenly stretched or unstretched, a strong signal is transmitted to the spinal cord; this causes an instantaneous strong reflex contraction (or decrease in contraction) of the same muscle from which the signal originated. Thus, the reflex functions to oppose sudden changes in muscle length.
The dynamic stretch reflex is over within a fraction of a second after the muscle has been stretched (or unstretched) to its new length, but then a weaker static stretch reflex continues for a prolonged period thereafter. This reflex is elicited by the continuous static receptor signals transmitted by both primary and
Muscle contraction caused by a spinal cord signal under two conditions: curve A, in a normal muscle, and curve B, in a muscle whose muscle spindles were denervated by section of the posterior roots of the cord 82 days previously. Note the smoothing effect of the muscle spindle reflex in curve A. (Modified from Creed RS, et al: Reflex Activity of the Spinal Cord. New York: Oxford University Press, 1932.)
secondary endings. The importance of the static stretch reflex is that it causes the degree of muscle contraction to remain reasonably constant, except when the person's nervous system specifically wills otherwise.
"Damping" Function of the Dynamic and Static Stretch Reflexes
An especially important function of the stretch reflex is its ability to prevent oscillation or jerkiness of body movements. This is a damping, or smoothing, function, as explained in the following paragraph.
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